Reflections on the Decay Mechanisms of Half-Timbered Walls in Traditional Spanish Architecture: Statistical Analysis of Material and Structural Damage

Abstract

Knowledge on the state of conservation and vulnerability of traditional techniques when faced with the most common degradation phenomena is vital in order to propose the most suitable conservation and maintenance actions. This article presents the systematic review of 1218 half-timbered walls found throughout Spain, enabling the identification of a total of 27 material lesions, classified by atmospheric, biological or anthropic origin, and 9 structural lesions due to stress or excessive deformation. Their qualitative and quantitative analysis has focused on the frequency of the individual lesions and the possible correlation with different constructive characteristics, such as the materials used, the geometry of the framework and the presence of plinths, eaves and protective rendering. Almost the entire sample presents some degree of material degradation, mostly atmospheric lesions of limited severity, such as superficial atmospheric erosion and chromatic alteration and dehydration of the timber. In terms of structural lesions, half-timbered walls are seen to be more vulnerable to this type of deformation. Considering the risk of loss affecting all traditional architecture, it becomes particularly important to promote the continued maintenance of half-timbered walls in order to reduce the influence of material lesions caused by atmospheric agents. Subsequently, suitable criteria for intervention are established in order to reduce the effect of anthropic lesions and structural degradation phenomena, particularly linked to a lack of maintenance and modifications of anthropic origin.

1. Introduction

Until the late 20th century, the degradation of traditional architecture was controlled by constant maintenance from the inhabitants who had access to the local knowledge and materials needed for conservation [1,2]. However, social factors such as rural exodus and the depopulation of towns and villages led to the abandonment of buildings and the disappearance of traditional maintenance activities [3,4]. Furthermore, industrialisation and the introduction of new materials for construction, in combination with the low value attached to this type of architecture, have brought about the development of dynamics for transformation and interventions that have not necessarily taken into account the conservation of traditional aspects or material and structural compatibility [5]. Knowledge on the state of conservation and vulnerability of traditional techniques in relation to the most common degradation phenomena is essential to the proposal of the most suitable conservation and maintenance actions.

Although generally associated with other techniques, Spanish traditional architecture features an extensive variety and the presence of half-timbered walls [6,7]. These techniques consist of the combination of a timber framework, mostly fulfilling a structural function, and other materials used for enclosure, insulation and finishes. However, these techniques have not been studied in depth as thoroughly as others, including rammed earth and adobe [8,9], or in as much detail as in other countries or regions [10,11,12,13,14]. Given the widespread abandonment and inappropriate spontaneous interventions affecting much of traditional architecture, it has become necessary to study degradation in this type of constructive technique, aiming to propose the basis for future restoration.

This publication is part of broader research whose main aim is to document the half-timbered walls found in Spanish traditional architecture while also analysing the state of conservation and transformation dynamics. This in turn should provide the necessary knowledge for the proposal of basic principles and guidelines for its valorisation and restoration [15]. This study is based on the qualitative and quantitative analysis of 1218 half-timbered walls documented throughout Spain (Figure 1). This has required an extensive bibliographical review of catalogues and studies on regional architecture [16,17,18,19,20,21,22,23,24,25,26,27], allowing areas of interest to be identified for the study, as well as developing fields of work and visually documenting case studies. Following this process, the database analysed finally included 1218 half-timbered walls documented in 950 buildings throughout 333 municipalities. A single building can feature several half-timbered walls, which, depending on their characteristics or location, can be analysed in different ways. In order to manage all this information and propose a sufficiently detailed study, a study fiche model is proposed to record general information on the building, on the specific half-timbered wall, and on the state of conservation and transformation, providing objective, scientifically valid statistics. Although the bibliographical review covers all of Spain, no evidence of the presence of this type of technique has been found in the outer walls in most of the south and northeast of the country [28].

2. Methodology

The study of such a diverse and complex constructive technique requires the development of a progressive classification system based on typological, geometric and material variants and finishes [29]. Although historically these have only been classified according to the materials used and the structural importance of the timber [30], it was felt that this classification should be expanded to include other aspects, such as the geometric layout of the timber framework, the location of the walls within the built complex, and the presence of protective rendering. Thus, different major characteristics were identified and grouped inductively to document 5 typological variants, 8 geometric variants, 35 material variants and 8 finish variants, a great architectural wealth of combinations and different specific characteristics.

In terms of the typological variants, considering the shape and position of the wall within the built complex, which conditions the general load distribution, a distinction is made between continuous typologies, such as façades and walls, and discontinuous typologies with eaves, porticos and galleries (Figure 2). In terms of construction and structure, continuous walls are simpler given that all the loads follow the same vertical plane, while discontinuous walls entail the concentration and redistribution of loads on different planes, so at times, it becomes necessary to add reinforcements.

The half-timbered walls are always made up of a main structure of uprights and beams but may also include intermediate elements meant to subdivide, make openings or triangulate the framework. Therefore, the classification of geometric variants was based on the possible presence and combinations of these intermediate elements, either vertical (V), horizontal (H), diagonal (D) or acting in multiple directions (X) (Figure 3). Thus, distinctions were made between simple geometries made up only of vertical and horizontal elements and complex geometries featuring diagonal and compound elements mainly designed for triangulation and bracing of the framework.

The materials used for infill or enclosure of the spaces outlined by the framework give rise to the wide range of these techniques within Spain, based on the raw materials locally available and the economic spending power of owners. The materials used can range from simple earth to masonry, brick, ashlar and more complex solutions of wattle-and-daub and lath-and-daub. It was felt to be timely to classify these according to the associated constructive logic, grouping them by heavy infill, which could be monolithic infill based on the use of formwork (Figure 4) or masonry infill using pieces that have been previously worked (Figure 5), and by lightweight screens using plant elements such as laths and boards to create rigid screens (Figure 6) or branches, wattles and interwoven reeds to generate flexible screens (Figure 7). Among the heavy infills found throughout much of the analysed territory, it is worth noting the high presence of adobe constructions and monolithic infill of a mix of earth, water, gypsum and rubble known as tapialete, which could be translated as coffered gypsum. In contrast, the presence of lightweight screens is far more limited and concentrated mostly in the north of the Iberian Peninsula.

Finally, for rendering and finishes (Figure 8), distinctions were made between continuous rendering in earthen mortar, gypsum or lime mortar and limewash and discontinuous rendering based on the use of elements such as roof tiles, boards or slate slabs. In addition, among the different types of rendering, it was considered important to distinguish between examples where the rendering completely covers the wall and examples where the rendering only covers the infill or screen, leaving the timber exposed. The cross-referenced analysis of all these variants and different geographic and climatic factors has made it possible to establish a correlation between the surroundings and the constructive techniques developed, highlighting both the diversity and the strategies for adapting to surroundings [29].

This text aims to provide an analysis of the state of conservation of the 1218 half-timbered walls documented through the information recorded in the last section of the study fiche. Thus, efforts are made to understand the frequency of the different degradation phenomena and the possible correlation with different constructive characteristics, which could increase or reduce this frequency. As the main characteristic of half-timbered walls is the combination of a timber framework with other materials, their material degradation therefore combines mechanisms linked to timber and the infill type in question. Furthermore, structural behaviour is particularly complex and is conditioned by different factors, such as the different rigidity and resistance of materials, geometric layout, bracing, contribution of infill and traditional joints [31]. Therefore, mechanisms relating to the timber and the different infills, as well as mechanisms relating to the interaction of both materials, were identified.

Firstly, it was necessary to carry out a comprehensive bibliographical review of degradation mechanisms for the identification and classification of possible lesions [31,32,33,34,35,36,37,38,39,40,41,42]. Following this, as case studies were gradually visited, documented and photographed, the possible lesions have been progressively adapted to the specific context of half-timbered walls. Thus, a total of 27 types of material lesions have been identified and classified according to whether they were caused by atmospheric, biological or anthropic agents, while a total of 9 structural lesions were grouped based on whether they were associated with excessive stress or excessive deformation (Figure 9). Once the data had been collected, the relevant section was completed in fiches, and statistics were obtained on the frequency of the lesions and their correlation to different constructive characteristics. The frequency of each lesion was determined by comparing the number of cases that present those damages with the total number of documented half-timbered walls and with the number of different material and geometric variants. The percentage of cases with specific constructive attributes that could have increased or diminished its development has also been studied in some specific degradation mechanisms. Specifically, the height ranges of plinths or lower walls, the overhang ranges of eaves, the presence or absence of coatings, and the materials used in these coatings. Concurrence between some specific lesions has been analysed in the cases of degradation mechanisms that could contribute to developing other mechanisms.

It is important to consider the geographical limitations of this research, as it is physically impossible to visit all Spanish municipalities to ascertain the existence or absence of this type of technique. However, this aspect was countered with an exhaustive bibliographical review, thanks to which areas of interest were identified and a large number of examples were statistically analysed. In order to establish the statistical reliability of the conclusions obtained, a formula (1) was used to define the sample error e of an infinite population [43]. The examples studied were considered as n individuals from an infinite population (N > 30,000), the confidence interval was set at a standard value of 95% (α = 5%, z = 1.96) and the variability expected was set in the responses from the population and a maximum value (p = q = 0.5). In general, the sample studied presents a sample error of less than 6%, except in the case of lightweight screens, where this figure increases to 10.3%, and flexible and rigid screens, which account for 12.2% (

Table 1. Sample error from the database analysed in general and classified according to the different variants identified in this research.

		n	e
Overall sample	Locations	333	5.4%
	Buildings	950	3.2%
	Walls	1218	2.8%
Typological variants	Continuous walls	951	3.2%
	Discontinuous walls	267	6.0%
Geometric variants	Simple geometry	742	3.6%
	Complex geometry	476	4.5%
Material variants	Monolithic infill	402	4.9%
	Masonry infill	714	3.7%
	Rigid screens	91	10.3%
	Flexible screens	65	12.2%
Cladding and rendering variants	Coated	608	4.0%
	Uncoated	610	4.0%

).

e = √((z² · p · q)/n)

(1)

In addition, the assessment of the state of conservation was limited to the exterior visual analysis of the façades and party walls, as obtaining access to the buildings would have further complicated the documentation work without necessarily guaranteeing satisfactory results, given the frequent presence of interior rendering. It should also be added that a high percentage of the buildings documented currently seem to be unoccupied or used only in summer periods.

3. Material Degradation Phenomena

Material degradation refers to the physical and chemical changes occurring in the materials due to the action of atmospheric, biological and anthropic agents (Figure 10), from which 27 different pathologies have been identified. These phenomena have been observed throughout almost the entire sample, in different combinations and different degrees of severity, specifically in 1194 cases of the 1218 documented.

Atmospheric exposure is the main cause of deterioration of half-timbered walls, and as it is unpreventable for the most part, its influence depends greatly on the existence and correct maintenance of traditional protection elements such as eaves, plinths and rendering. Among this type of lesion, the processes related to damp, infill erosion and the chromatic alteration and dehydration of timber stand out. Damp can also encourage other processes linked to biological agents, affecting approximately half of the sample, including rot, mould, lichens and attacks from xylophagous insects. Finally, anthropic agents refer to the lesions caused by the direct and indirect action of humans, documented in approximately two-thirds of the sample. This type of lesion, often caused by repairs with incompatible materials and the installation of improper elements, illustrates the process of abandonment observed in this type of traditional architecture and the transformations that have occurred in constructive culture (Figure 11).

3.1. Caused by Atmospheric Agents

Direct exposure to atmospheric agents such as rain, wind and damp found in the terrain are the main causes of degradation detected, affecting almost the entire sample, 93.5%, in varying degrees (Figure 10). To a great extent, this exposure is inevitable, although the influence and development of associated lesions are mostly conditioned by the existence and proper maintenance of traditional protection elements such as eaves, plinths and rendering. The danger of many of the lesions linked to other agents, such as those caused by lack of maintenance or unsuitable interventions, is the elimination or transformation of these elements. For instance, the elimination of traditional rendering leaves materials completely exposed, while adding a new cement mortar rendering can lead to problems in breathability, damp accumulation and efflorescence.

Within the analysis of the lesions caused by atmospheric agents (

Table 2. Presence of material lesions caused by atmospheric agents in relation to the total sample and individual groups of material variants.

Material Lesions Caused by Atmospheric Agents	Total	Monolithic Infill	Masonry Infill	Rigid Screens	Flexible Screens	Simple Geometry	Complex Geometry
Damp stains due to capillarity	10.8%	13.4%	9.8%	6.6%	3.1%	11.2%	10.3%
Damps stains due to water runoff	46.5%	50.0%	46.8%	40.7%	35.4%	45.6%	47.9%
Efflorescence	6.3%	9.0%	5.6%	2.2%	0%	6.1%	6.7%
Erosion due to atmospheric action	72.6%	73.1%	77.7%	52.7%	55.4%	31.9%	33.0%
Erosion due to capillary damp	5.3%	8.0%	6.2%	4.4%	1.5%	6.2%	3.8%
Erosion due to washing	19.4%	16.4%	22.5%	8.8%	10.8%	17.9%	21.6%
Material loss	32.3%	35.6%	28.3%	52.7%	52.3%	31.9%	33.0%
Chipped rendering	40.6%	35.3%	42.3%	46.2%	56.9%	39.9%	41.8%
Chromatic alteration and dehydration	82.3%	85.1%	78.4%	98.9%	93.8%	84.0%	79.8%
Deterioration of timber cracks	53.8%	56.7%	49.7%	78.0%	64.6%	54.4%	52.7%
Rust of metal elements	6.9%	4.2%	6.0%	27.5%	4.6%	7.4%	6.1%
Carbonation and soot stains	1.3%	0.5%	1.3%	1.1%	0.0%	0.8%	1.1%

), the most frequent is the erosion of infill, which at times can even evolve into a loss of volumetry and chromatic alteration, dehydration and deterioration of cracks in timber elements. However, the presence of damp in the sample studied is also important, especially because, in the long term, it favours the disaggregation and erosion of the infill as well as the surface deterioration of the timber and the proliferation of xylophagous insects and rot.

3.1.1. Damp Stains

Both the timber of the framework and the different infills are hygroscopic materials that absorb water to varying degrees, either due to capillarity from the ground (Figure 12), rainwater infiltration through walls (Figure 13) or condensation from atmospheric humidity [44].

Damp stains due to capillarity was only documented in 10.8% of cases studied (Table 2), probably due to the frequent presence of plinths or lower walls in more moisture-resistant materials. However, a slight predisposition to its development is observed in heavy infills, possibly because they are used more frequently in contact with the ground. In contrast, the occurrence in screens drops to almost half, perhaps because this type of solution is mostly used in agricultural buildings, which are better ventilated, but also because the damp absorbed is more easily eliminated as they are thinner. The occurrence of damp due to capillarity depends directly on the height of the plinths or lower wall sections on which the half-timbered walls are built and is gradually reduced as the height of these protection elements increases (Figure 14).

In contrast, the occurrence of stains caused by water runoff is considerably greater as these are situated in upper sections of the building. This pathology has been documented in 46.5% of the case studies analysed in this research (Table 2). This is generally the result of the intense action of rainwater during specific periods of time and can easily be prevented through eaves, suitable ventilation and solar exposure of façades. However, in north-facing façades, prolonged water accumulation can occur, favouring the appearance and development of mould and lichens.

As was the case with damp due to capillarity, the distribution in the different material variants also displays relatively better behaviour of walls with lightweight enclosures. Damp stains due to water runoff were specifically documented in 40.7% of rigid screens and 35.4% of flexible screens analysed, compared to 50% in monolithic infills and 46.8% in masonry infill (Table 2). Eaves are the traditional protective element for the prevention of damp stains due to water runoff, whose presence decreases as the overhang of the eaves increases (Figure 15).

3.1.2. Efflorescence

The constant presence of damp can entail the migration of salt found in the ground or the actual infill, which, when the water has evaporated, is deposited and crystallises, causing the formation of superficial efflorescence (Figure 16) or internal crypto-efflorescence. The cyclical process of crystallisation or dissolution causes the formation of internal forces due to the increased volume of the salts, which, in the long term, can bring about the disaggregation of the material [44].

Although it can cause major damage, such as the loss of cohesion and detachments in the infill, efflorescence was only detected in 6.3% of the walls analysed (Table 2). However, there is a slight tendency for heavy infill to develop this type of lesion. Although these salts may originate from the infill mass, the ground, or the atmosphere, the main origin appears to be cement mortars used in different interventions, such as rejointing, rendering, filling in of cavities and filling in of cracks. Although the occurrence of efflorescence depending on plinths does not indicate that the ground is the main source (Figure 17), analysis of its occurrence in relation to the presence of interventions with cement shows a high correlation with cement mortar rendering (Figure 18).

3.1.3. Erosion

Erosion caused by rain and wind is the second most common lesion, found in 72.6% of cases analysed (Table 2). However, this is generally a superficial material loss on the volume of the wall, either the infill or the coating, but it does not affect the structural behaviour of the elements (Figure 19). Its distribution in relation to material variants is relatively homogeneous, with heavy infills showing erosion due to atmospheric causes in 73.1% of monolithic infills and 77.7% of masonry infills, and lightweight enclosures, where it is found in 52.7% of rigid screens and 55.4% of flexible screens.

To begin with, coatings are a layer protecting the infill from the impact of rain and air-driven particles, which can be easily repaired and replaced, so the occurrence of erosion should presumably be lower in rendered walls. However, the percentage of cases with no rendering displaying erosion is slightly lower than in cases protected with rendering (Figure 20). In relation to this, it is important to note that some very frequently used material variants, such as coffered gypsum or lime and earth concrete, do not include rendering, but their configuration intrinsically presents a resistant surface with a high ratio of gypsum or lime. In contrast, earth rendering and limewashes are slightly more susceptible than lime or gypsum mortars and cement mortars.

Another possible cause of erosion is the softening resulting from the constant damp absorbed by capillarity from the ground. In the case of earth, when the material reaches a certain point of saturation due to this damp, it softens and becomes more susceptible to the action of wind, the impact of rain, the impact of people or animals and the recurring beating of vegetation. If these conditions persist, erosion may develop, resulting in a loss of section, which can compromise the stability of the element. In half-timbered walls, this effect is limited to infill, and even if instability occurs, failure is generally limited to the infill of a panel but does not compromise the overall stability of the wall (Figure 21), which is more dependent on the timber framework and its state of conservation.

Given that low percentages are seen for cases in direct contact with the ground and cases with damp stains due to capillarity, there is also a low presence of erosion due to capillary moisture, limited to only 5.3% of cases analysed in this study (Table 2). As was the case with damp stains due to capillarity, a slightly greater presence is detected in heavy infill, with these lesions being found in 8% of monolithic infills and 6.2% of masonry infill, compared to 4.4% found in rigid screens and 1.5% in flexible screens. As was the case of damp stains due to capillarity, the occurrence of this type of lesion decreases as the height of plinths or lower walls in more moisture-resistant materials such as stone or brick increases (Figure 22).

Erosion can also occur due to water runoff, mainly when the roof and eaves designed to prevent this phenomenon are too small or in a poor state of conservation (Figure 23).

Erosion due to washing has been documented in 19.4% of the cases analysed in this study and is found more frequently in heavy infill (Table 2). A total of 16.4% of monolithic infills and 22.5% of masonry infills display erosion due to washing in the infill, while this figure drops to 8.8% in rigid screens and 10.8% in flexible screens. In terms of the correlation between types of mortar and the presence of erosion due to water runoff, percentages are again higher in earthen mortars and limewashes than in lime or gypsum mortars (Figure 24).

3.1.4. Material Loss

The combined action of erosion caused by wind and rain and possible mechanical impacts from people, animals or vegetation transported by air currents generate disaggregation, followed by the surface erosion of infill. If these conditions persist, a greater loss is progressively generated, potentially endangering structural stability. While the constructive discontinuity resulting from the combination of a timber framework and different infill materials implies that these can become detached, collapse or fall off with more ease, it also means that the loss is delimited by the framework itself (Figure 25).

This condition has been detected in general in 32.3% of the half-timbered walls analysed, with a relatively homogeneous distribution between heavy infills and screens (Table 2). On the one hand, heavy infills seem to display better behaviour with this type of degradation, documented in 35.6% of cases with monolithic infill and 28.3% of masonry infill. On the other hand, enclosures show a slightly greater propensity for this type of lesion, found in 52.7% of rigid screens and 52.3% of flexible screens. This may be due to their being thinner and the fact that they are made of timber elements such as laths or branches, which can easily lose resistance due to rot, attacks from xylophagous insects, loss of jointing with the main framework, etc.

The way in which this loss of volumetry occurs depends on both the materials and the form of execution of infills and screens. In the case of monolithic infill, it starts in specific isolated spots, generally at the corners, which continue to erode after exposure to the elements, increasing the loss of volumetry. In contrast, in masonry infills, the loss of volumetry depends on the resistance of pieces and on the bonding mortar. In the case of adobe and brick, these erode gradually until viewed as lost, although complete detachment can occasionally occur, as is the case with ashlar, which leaves a staggered form. In rigid screens, the loss can be limited to a few laths, which may have been partially lost, becoming detached at the joints, whereas in flexible screens, the loss can greatly depend on the erosion of rendering, which leaves the branches or plant elements exposed. They thus start to suffer other degradation processes, such as rot and attacks from xylophagous insects, until they eventually break up, fall or slide down (Figure 26).

3.1.5. Lacunae and Chipped Rendering

Traditionally, certain maintenance tasks were carried out to ensure that rendering, conceived as layers for protection from exposure to the elements, continued to preserve their integrity, fulfilling their objective of protection. However, for a number of reasons, these tasks are no longer carried out, and rendering has been left particularly vulnerable to the processes described earlier, eventually leading to a loss of adherence and the appearance of lacunae and chipping (Figure 27).

This phenomenon was documented in 40.6% of the walls analysed, with a slightly greater presence in screens (Table 2). It was specifically identified in 56.9% of flexible screens and 46.2% of rigid screens, compared to 35.3% of monolithic infills and 42.3% of masonry infills. This increase may be due to the laths and branches of the elements that could experience losses in volumetry and movements, which further favour this type of lesion.

Furthermore, it is important to remember its presence and scope in the number of cases with rendering. The percentage of cases affected in general is 66.2% when analysing only the walls with rendered infill or enclosure and up to 87.9% of cases with rendering covering the entire walls (Figure 28). This increase can be directly linked to the discontinuities in the construction of the walls and the difference in rigidity between the framework and the infill and occurs in all material variants. However, as stated above, in the case of rigid and flexible screens, there is less variation due to the internal discontinuities characteristic of laths and branches.

Remembering their protective function from the erosion phenomena previously analysed, it should be stated that the percentage of cases with coating that is chipped and eroded simultaneously is 7.6% lower than in cases with chipped rendering in walls with cladding covering only the infill and 6.0% lower in cases with rendering covering the entire wall (Figure 29). This shows the protective function of rendering while also demonstrating the great ease with which erosion develops once chipping has occurred, leaving the infill exposed.

3.1.6. Dehydration and Chromatic Alteration

The exposure of timber to solar radiation causes it to dry, bringing about the cellular degradation of the lignin within it, firstly causing the appearance of a darker brown, which in time takes on a greyer hue when cellulose is left exposed [34]. However, this phenomenon is a slow process, with increases estimated at a rate of 1 to 13 mm per century depending on the type of timber and exposure, a speed that may also increase due to the combined action of solar radiation with water [39]. Although this is the most widespread lesion, affecting 82.3% of the cases analysed in this study (Table 2), this phenomenon does not constitute a great risk for the conservation of timber or its resistance. However, it is a major indicator of the degree of abandonment and lack of maintenance observed in this type of architecture.

Nevertheless, a slight tendency to the development of this type of lesion can be observed in lightweight screens. Whereas 98.9% of the rigid screens and 93.8% of the flexible screens analysed display deteriorated cracks, this percentage drops to 85.1% in monolithic infills and to 78.4% in masonry infills (Table 2). This may be due partly to the fact that screens generally incorporate a greater percentage of timber than heavy infill, and some variants in particular are rigid screens completely made up of timber, therefore providing a larger exposed surface where this type of lesion can develop.

As this pathology is characteristic of timber, its condition depends less on the material variant than on the type of timber, its surroundings, the maintenance of buildings where it is used and, above all, the presence and conservation of a cladding protecting the material (Figure 27). If occurrence is observed in relation to the presence and scope of rendering (Figure 30), it is greater in cases with no rendering and in cases where the rendering only covers the infill. It has also been observed that in cases where rendering covers both the structure and infill, this alteration is limited to occasional areas where the rendering has become detached, leaving the timber exposed. In any case, this pathology reflects the importance of correctly maintaining rendering, as well as possible protective treatments applied to timber.

3.1.7. Deterioration of Cracks

Cracks are fissures that can appear in the timber in both axial and radial directions. However, it is the latter that can lead to structural collapse to the extent that the material should be ruled out as a constructive element [45]. Regardless of whether they were caused by unequal stress from growth, an unequal drying process or excessive loads, they have been included in this section for their capacity to degrade due to exposure to atmospheric agents, potentially becoming a point of entry for damp, rot fungi and the attacks of xylophagous insects (Figure 31).

Cracks were identified in 53.8% of the case studies in this research (Table 2) and are the third most common pathology after atmospheric erosion and the chromatic alteration and dehydration of timber. A slightly higher tendency to develop this type of lesion is observed in screens, where it is found in 78% of rigid screens and 64.6% of flexible screens compared to the 56.7% of monolithic infills and 49.7% of masonry infills documented. In this respect, the explanation may again be that, due to its nature, a higher percentage of timber is susceptible to this type of degradation.

3.2. Caused by Biological Agents

In addition to atmospheric agents, other natural factors can influence the conservation of half-timbered walls. This is the specific case of the biological agents, including the lesions relating to the presence of living beings (animals and plants) around the building. Although not as common as those caused by atmospheric agents, lesions caused by biological agents were documented in 57.9% of the sample (Figure 10).

It is thus important to remember that the constant presence of damp encourages the development and proliferation of fungi, mould and lichens that favour the disaggregation of surfaces and the accumulation of water. At the same time, this transforms infills and earthen rendering into an ideal substrate for the germination of seeds transported over time, eventually leading to germination. Once grown, the roots of these plants will make their way through the construction (Figure 32), disaggregating and potentially breaking it [32]. In timber, the constant accumulation of damp above a given percentage due to infiltrations, poor ventilation or lighting leads to the proliferation of xylophagous insects and rot fungi [46].

Among these types of lesions (

Table 3. Presence of material lesions caused by biological agents in relation to the total sample and the individual groups of material variants.

Material Lesions Caused by Biological Agents	Total	Monolithic Infill	Masonry Infill	Rigid Screens	Flexible Screens	Simple Geometry	Complex Geometry
Vegetation	7.6%	11.2%	5.5%	4.4%	6.2%	7.5%	7.6%
Mould and lichens	38.2%	45.5%	34.2%	44.0%	36.9%	38.0%	38.4%
Xylophagous insect attacks	22.0%	25.6%	18.9%	20.9%	41.5%	21.2%	23.3%
Rot	21.8%	22.9%	17.1%	38.5%	50.8%	22.2%	21.2%
Biological degradation	4.2%	0.7%	6.2%	4.4%	1.5%	3.5%	5.3%

), the most widely found in half-timbered walls is the presence of mould and lichens, which could affect both the infill and the timber framework. Equally, it is also common to encounter other processes specifically affecting timber, such as attacks from xylophagous insects and rot. In spite of this, the damage caused by mould and lichens is relatively superficial, with a slight disaggregation of walls and increased damp, which could also encourage the appearance of other more serious phenomena. In contrast, the damage caused by rot and xylophagous insects can affect the mechanical capacity of timber and pose a greater risk to the material and structural conservation of this type of wall.

3.2.1. Vegetation

The wind can transport seeds, which mostly germinate in the infills and earthen mortars when the correct hygrothermal conditions occur, although there are other types of plants that, growing in the ground, find in the walls a structure on which to grow. This vegetation, which includes bushes and larger plants, entails the existence and progressive growth of roots through the mass or construction of the wall, generating internal forces that favour disaggregation and breakage, potentially causing losses in volumetry and structural stability problems (Figure 33). Furthermore, the presence of vegetation nearby can favour the erosion of walls due to the repeated effect of branches shaken by the wind, and at the same time, it can offer protection from erosion caused by atmospheric agents such as rain and wind.

Vegetation has been detected in 7.6% of the cases analysed in this research (Table 3), with a slightly higher presence detected in monolithic infills. This may be due to the monolithic variants presenting a high percentage of earth that can absorb the damp, either in its own mass, as is the case of tapialete, or the lime and earth concrete and the bonding mortar of the different masonry variants.

3.2.2. Mould and Lichens

The constant presence of damp also encourages the proliferation of fungi, mould and lichens in both the timber and the materials for infill, enclosure and rendering. These phenomena frequently occur in north-facing walls or in walls that receive few hours of direct solar exposure, concentrating especially in zones where damp accumulates due to their geometric and constructive configuration. At the same time, the presence and spread of damp increases its accumulation in materials, potentially damaging the cohesion of walls and causing superficial disaggregation. Some specific types can secrete organic acid, affecting the chemical composition of clay particles [44]. In the case of timber, mould and chromogenous fungi only cause a change in colour, given that, by feeding on reserves, they do not affect their structural function [47]. However, at the same time, they lead to an accumulation of damp, which can favour other phenomena, such as rot and xylophagous insects.

This pathology, the most frequent one of biological origin in half-timbered walls, is found in 38.2% of the case studies analysed in this research (Table 3). The occurrence of this type in relation to material variants is relatively homogeneous, although showing a slightly greater predisposition in monolithic infills and rigid screens. This high percentage may be linked to the presence of these techniques in urban settings where surrounding buildings could limit the solar radiation received by façades, as well as the presence of large eaves projecting shade, favouring the accumulation of damp and the proliferation of fungi and lichens.

Equally, the occurrence of this type of lesion is greater in cases with continuous rendering, as 38.3% of earthen mortars, 49.4% of lime or gypsum mortars and 45.5% of cement mortars have developed mould and lichens on their surface, compared to the percentage of non-rendered cases displaying these phenomena, which drops to 33% (Figure 34). The possibility of some mortars, especially lime, gypsum and cement, limiting the capacity of infill or enclosure to dissipate the damp content may have led to an increase in these lesions, which are particularly visible on this type of surface (Figure 35).

3.2.3. Attacks from Xylophagous Insects

As they are made of timber, half-timbered walls are susceptible to attack from xylophagous insects, particularly in different areas where geometry, exposure and lack of ventilation favour the accumulation of damp. Detecting this type of lesion through exterior visual analysis of the buildings means that it has only been possible to document their presence through gaps on the surface caused by larvae of insects such as anobids and cerambicides and occasional internal galleries made by termites in broken or split timber elements (Figure 36). The former mostly cause damage to the outer layers of the timber without endangering their structural resistance. However, the latter are photosensitive insects that feed on the interior nucleus of the timber and are harder to detect, thus posing a greater risk to conservation [46]. Consequently, their occurrence may be greater than observed, especially considering that the underground termite species Reticulitermes lucífugus Rossi is quite widespread throughout the Iberian Peninsula [34].

Nevertheless, it is the second most common pathology after mould and lichens and has been documented in 22% of cases analysed (Table 3). The vulnerability of timber to attacks from xylophagous insects mostly depends on the species used or the presence and efficiency of traditional protection methods such as varnish, paint and other finishes [48], so generally, timber has a high damp context. Therefore, these lesions tend to appear in areas such as the ends of the elements that are left exposed to the weather or in those embedded in other elements, retaining higher levels of damp.

As this pathology is characteristic of timber, analysing its occurrence in the different material variants is relatively homogeneous, except in flexible screens, where the percentage of cases affected is 41.5%, probably due to the use of less durable species than those used in the frameworks [49]. In general, the percentage of case studies affected by damp stains, where the presence of xylophagous insects is detected, increases to 29.4% and is doubled, reaching 60.9%, in the case of flexible screens (Figure 37).

3.2.4. Rot

The constant accumulation of damp, along with poorer conditions for ventilation and solar exposure of the timber (Figure 38), also favours the proliferation of rot fungi, which, by feeding on certain components of the timber, cause the destruction of its anatomical structure, greatly reducing its loadbearing capacity. The most frequent types are brown or cubical rot, which feeds on cellulose, causing the timber to crack and darken; white or fibrous rot, which feeds on lignin, giving it a clear, fibrous appearance; and finally, soft rot, which affects certain cellulosic components, making the material weak and soft [37].

On average, this type of lesion has been documented in 21.8% of cases analysed (Table 3), although its presence could be greater in parts of walls that are not visible, so that study would require a thorough inspection of individual cases. Occurrence based on material variants is greater in lightweight screens, with 38.5% found in rigid screens and 50.8% in flexible screens. In contrast, this lesion has only been detected in 22.9% of monolithic infills and 17.1% of masonry infills. The higher occurrence in monolithic infills may be linked to a higher level of damp retention due to the characteristics and thicker materials, while in screens, it may be due to the use of types of timber particularly susceptible to rot despite their thickness, allowing more breathability and evapotranspiration of the water contained.

The frequency and extent of this type of lesion are directly dependent on the type of timber used as well as the presence and maintenance of possible protection finishes such as varnish and paint. However, damp and its accumulation increase this frequency and scope. It therefore usually appears in uprights in direct contact with the ground due to damp absorption by capillarity and on beam ends directly exposed to rain, although these lesions are often observed in the upper beams with some sort of infiltration from roofs in poor condition.

3.2.5. Biological Degradation

In addition to the animals mentioned earlier, there are others that can lead to biological degradation by nesting in buildings. Although this phenomenon is not as serious, some earthen infills have a consistency that allows insects to nest, forming internal galleries in the same way that xylophagous insects do in timber (Figure 39) [50].

In addition, other animals, such as birds and rodents, can find refuge in these buildings without perforating walls but using gaps or existing volumetric losses to build nests (Figure 40). Their continued presence leads to the accumulation of organic secretions, which can worsen damp in some areas and cause alterations due to acidity, although the greatest risk is the effect on the health of inhabitants. Livestock should also be considered a biological agent of degradation, although due to the presence of half-timbered walls on plinths and lower wall sections in more resistant material, no related traces such as blows, erosion or scratches have been detected. In any case, these biological degradation phenomena have only been documented in 4.2% of the case studies in this research (Table 3).

3.3. Caused by Anthropic Agents

This type of lesion includes those deriving from human actions, either direct actions with active participation, such as vandalism or the installation of unsuitable elements, or indirect actions resulting from the withholding of the maintenance needed for the correct conservation of traditional architecture. Maintenance is the best guarantee for the conservation of half-timbered walls, although this is not always the case in the current situation, where there has been a loss of associated ways of life and a change in constructive culture due to industrialisation. Even when certain tasks or interventions are executed, they may not be suitable or compatible due to a lack of means, knowledge or interest.

Lack of maintenance is very serious when it affects the traditional protection elements as their degradation results in the walls being left exposed, with a point of entry for other degradation agents. In addition, the use of incompatible industrial materials could speed up degradation; for example, new cement rendering and plastic paints may prevent breathability and favour the accumulation of damp. Equally, the installation of unsuitable elements such as cables creates points of entry for water and encourages degradation as the metal elements used rust, altering their traditional appearance.

This type of lesion has been documented in 69.1% of cases (Figure 10), mostly the result of the addition of elements and unsuitable materials (

Table 4. Presence of material lesions caused by biological agents in relation to the sample and each group of material variants.

Material Lesions Caused by Anthropic Agents	Total	Monolithic Infill	Masonry Infill	Rigid Screens	Flexible Screens	Simple Geometry	Complex Geometry
Modification of structural geometry	8.9%	12.4%	8.5%	3.3%	1.5%	7.4%	11.1%
Modification or addition of openings	12.0%	15.7%	12.5%	2.2%	1.5%	9.2%	16.4%
Replacement of infill	11.6%	9.5%	15.1%	4.4%	6.2%	9.6%	14.7%
Replacement of structural elements	4.1%	5.7%	4.1%	1.1%	0.0%	3.2%	5.5%
Installation of unsuitable elements	35.6%	37.8%	38.8%	14.3%	13.8%	30.7%	43.3%
Incompatible material repairs	41.3%	50.5%	42.2%	15.4%	12.3%	36.9%	48.1%
Transformations of the surroundings	3.2%	3.5%	3.9%	0,0%	0%	3.4%	2.9%
Partial or complete loss of roof	8.1%	6.5%	7.7%	12.1%	9.2%	8.4%	7.8%
Wall heightening	2.5%	3.0%	1.5%	0.0%	0.0%	1.5%	2.3%

). However, to a lesser extent, other types of intervention have been identified, causing changes in the structural system, for example, the replacement of panels, the addition of unsuitable openings or the modification and replacement of structural elements. There are other less common actions affecting the conditions of the outline of the wall, including urban modifications, the loss of roofs and the heightening of walls. The complete lack of vandalism may be due to the presence of this type of technique in urban settings and the upper areas of buildings, which are harder to access, compared to more accessible buildings in peripheral or rural areas with better accessibility and less human presence.

3.3.1. Replacement of Infill

The combination of a timber framework with other materials acting as infill or enclosure entails a discontinuity in construction associated with some problems of adherence. Consequently, when serious lesions of atmospheric or biological origin appear, compromising the structural stability of these infills or enclosures, there are losses in volumetry, detachments and ultimately partial or total collapse of the infills or enclosures. On other occasions, when the design of the support of the roof and foundations is unsuitable and there is an outward rotation of the wall, the ductility of timber and traditional joints will enable the framework to adapt to these new outlined conditions. However, the infill or enclosures could also rotate and eventually become detached due to the absence of suitable adherence.

Whether due to lack of maintenance or deliberate demolition, it is common for these traditional infills to be replaced with typical industrial brick masonry with cement mortar joints, generating a notable visual contrast with the appearance of the rest of the wall. Even in rendered walls, examples have been detected where industrial brick can be sensed in walls due to the differences in damp absorption between the brick and the bonding mortar. The use of these industrial materials can cause material and structural compatibility issues, favouring the appearance of other lesions, such as efflorescence, and occasionally increasing in weight and rigidity compared to the traditional solution (Figure 41).

The replacement of infill is a relatively common direct anthropic lesion, which has been documented in 11.6% of cases (Table 4). It is more frequent in heavy infill, appearing in 15.1% of masonry infill and 9.5% of monolithic infill compared to 4.4% of rigid screens and 6.2% of flexible screens. This slight difference may be due to the monolithic infills being based on the execution or pouring of earthen mixes that adapt, connect and adhere slightly better to the timber structure. In contrast, masonry infills are made up of pieces where the bonding and joints can lead to weak points in terms of stability and consistency if they are poorly executed, increasing the probability of them becoming detached and later replaced with industrial bricks.

3.3.2. Modification of Openings

During the construction process for half-timbered walls, the position of openings should be considered from the outset in order to configure the geometry of the framework, positioning the suitable elements for the definition of openings. The incorporation of openings after the construction of the structure can be carried out by simply eliminating the infill or enclosure of one of the spaces delimited by the framework. However, current comfort and habitability conditions have led to the need to add larger openings, possibly making it necessary to cut or eliminate some timber elements and also involving a modification of structural geometry. The same standards have driven the replacement of joinery with new elements, improving thermal and acoustic insulation, although in stark contrast with the traditional image of the building (Figure 42).

The modification of openings was identified in general in 12% of cases (Table 4). The analysis of occurrence according to material variants shows greater frequency in heavy infills, with 15.7% of monolithic infills and 12.5% of masonry infills compared to 2.2% of rigid screens and 1.5% of flexible screens. This may be due to a wider use of heavy infill in residential buildings where improved comfort conditions are sought, particularly in terms of lighting, insulation and ventilation. Furthermore, screens tend to be used in auxiliary buildings or rooms requiring suitable ventilation, an issue solved by the intrinsic configuration of the screen with separate laths or leaving the wattle unrendered.

3.3.3. Geometric Modifications

Whether due to functional needs, the addition of new openings or material degradation compromising the structural function, a modification of structural geometry was identified in 8.9% of cases studied (Table 4).

At times, these are alterations of the original position or the elimination of one or more timber pieces to generate larger openings, leaving the marks of the traditional union to bear witness to the element previously found in that position (Figure 43). At other times, several uprights are eliminated from a specific part of the wall and replaced with a brick wall, keeping the wallplates to bear witness and contrasting greatly with the original geometry, which is still found in other parts of the wall (Figure 44).

On other occasions, the structural geometry is not modified. Instead, the damaged elements are replaced with other quite similar linear pieces (Figure 45). These new elements may be timber and show a degree of compatibility with the structural operation of the wall, although industrial elements such as metal pieces, prefabricated concrete pieces and brick pillars have also been documented. This type of replacement was clearly identified in 4.1% of the cases analysed in this research (Table 4). Although this does not imply a geometric transformation in itself, it can lead to a modification in the resistance of the elements and, therefore, of the wall’s structural behaviour.

Moreover, the heightening of the walls also involves a modification of the structural scheme as there is an increase in loads, often associated with the elimination and replacement of upper wallplates. These extensions tend to be linked to the functional needs of inhabitants, but their construction with industrial materials like brick or concrete blocks generates both visual impact and material and structural incompatibility with the pre-existing structure (Figure 46). Building heightening executed so that it affects a half-timbered wall has been documented only in 2.5% of cases (Table 4), although the frequency with which other parts of the building are extended upwards is much greater.

3.3.4. Incompatible Repairs and Unsuitable Installations

The use of industrial materials that are incompatible with repairs can speed up the degradation of half-timbered walls by encouraging other associated processes. As this is the most frequent pathology of anthropic origin, detected in 41.3% of cases (Table 4), it is a considerable risk factor affecting heavy infill to a greater extent. Although, in principle, these lesions are not as serious as those previously described, these actions are carried out without taking into account material, structural or aesthetic compatibility, also depending on the resources and needs of inhabitants. These tend to be patches, rendering, reintegration and rejointing with cement mortar in the most damaged sections of the wall (Figure 47). The use of this industrial material hinders the breathability of the wall, favouring the accumulation of damp inside, and can eventually develop efflorescence and problems of chipping and lacunae due to differences in rigidity. Industrial brick is also a common material, both in the replacement of infills detailed above and in the filling in of holes, in the configuration of jambs when openings are created or modified inappropriately or in the structural replacement of part of the half-timbered walls with new brick walls.

Equally, the installation of unsuitable elements is the second most frequent anthropic lesion, documented in 35.6% of the cases analysed, also with a higher occurrence in heavy infills (Table 4). These are generally smaller elements, although they have a considerable aesthetic impact on the buildings, such as cables, signs, downpipes, antennae, clotheslines, blinds, railings or modern balconies, etc. But beyond this visual alteration, these interventions are usually carried out carelessly, breaking constructive elements and introducing industrial materials for anchoring or fixing (Figure 48). When these elements rust, they increase in volume and can potentially cause fissures in the wall materials.

The lower occurrence of lightweight screens, both in repairs with incompatible materials and the installation of unsuitable elements, may be partly due to the fact that these techniques are often used in agricultural buildings or secondary areas of dwellings where owners do not consider it necessary or essential to repair and update elements. In contrast, heavy infills tend to be associated with dwellings where the mere fact of living there has prompted a higher number of actions or interventions from the owners.

3.3.5. Modification of Surroundings and Loss of the Roof

Modifications in surroundings can consist of a change in ground level and asphalting of the streets surrounding the building, where the use of waterproof materials or an incorrect design can encourage unsuitable evacuation and absorption of rainwater. This problem is particularly serious when the increase in ground level has partially concealed the plinths or lower walls, shortening the height of these elements and causing the water absorbed by capillarity to reach the half-timbered walls more easily. Another possible modification of surroundings is the demolition of neighbouring buildings, resulting, on the one hand, in the exposure of party walls to inclement weather, and on the other, in a loss of rigidity and containment of possible horizontal movements and rotation of the structure. This type of lesion has only been documented in 3.2% of case studies (Table 4), although, as can be deduced from the marks and evidence, which may or may not have been conserved, occurrence may be greater than that recorded in this study.

The loss of volumetry of eaves and roofs is one of the most serious manifestations of an overall lack of maintenance observed in traditional architecture. Roofs can be considered essential to the conservation and durability of this architecture, whose construction and elements are more likely to suffer degradation than the plinths built with masonry, ashlar or brick. These losses in volumetry facilitate the entry of water into the building and constructive elements, favouring the development of lesions associated with damp described above. This is a continuous degradation process where small losses evolve, potentially causing the complete loss of the roof and affecting the remaining constructive elements.

In the sample studied, the complete or partial loss of the roof affects 8.1% of buildings analysed (Table 4), a percentage which increases when linked to the development of other lesions resulting from rainfall. A total of 61.7% of walls with eaves or roofs in a poor state of conservation also develop damp stains due to water runoff, and 40.2% develop erosion processes caused by washing (Figure 49). In the specific case of half-timbered walls, in addition to exposing materials to the degradation caused by atmospheric and biological agents, the structural collaboration of the roof in bracing the system can be compromised, also encouraging the appearance of structural lesions.

4. Structural Degradation Phenomena

In general, timber structures are systems with the correct measurements for needs in terms of resistance, but their rigidity and bracing do not always provide them with the necessary stability [51]. In the original structural scheme of half-timbered walls, the structural function was mostly concentrated in the timber framework, but the infills collaborate with the system’s rigidity and can be under load in specific situations. The material degradation of timber elements can compromise their resistance but can also cause breakages and deformations in the framework, which can transmit loads to infills.

Equally, the constructive technique of these walls combines a timber framework with different material solutions to fill in or close the spaces defined, involving a discontinuity in construction that could favour differential movements between both elements. Along with these discontinuities, the presence of traditional joints, which are not completely rigid, grants a degree of flexibility that is beneficial in seismic events, although it also makes them particularly susceptible to deformations and movements (Figure 50).

This section presents the analysis of the degradation processes linked to the structural movement of half-timbered walls in terms of the presence of stress and excessive deformations, which can compromise their stability, ultimately causing the structure to collapse. These structural lesions alter the original scheme for load distribution, generating second-order stress and evolving to cause the structure to collapse. In the specific case of half-timbered walls, the structural system presents a degree of flexibility that enables it to move and adjust to these new schemes of load distribution before collapsing completely. In any case, intervention is required on the causes of these structural movements, as well as on the effects observed on the structure, which could continue to evolve. In this research, structural lesions have been detected in 695 of 1218 cases, a high percentage generally linked to deformations and movements such as fissuring and the lack of union between the timber framework and the infill or enclosure. The lesions caused by excessive stress are present in almost a quarter of the sample, while those linked to excessive deformations account for slightly over half of the sample (Figure 51). However, the former are mostly linked to breakages resulting from other material lesions compromising the resistance of the material, such as rot or attacks from xylophagous insects. Equally, although the structural lesions associated with excessive deformations are common, the majority correspond to fissuring and a lack of adherence between the timber framework and the infill or enclosure.

4.1. Caused by Excessive Stress

This group, caused by the failures occurring when structural elements show very concentrated efforts or efforts greater than their work capacity, is documented in 23.6% of total cases (Figure 51). Although these situations may have originally been caused by unsuitable dimensions, poor structural design or an incorrect execution, they are more frequently associated with the material degradation of elements or with modifications in loads derived from the transformations and expansions during their useful life. Among the different lesions derived from the presence of excessive stress (

Table 5. Presence of structural lesions caused by excessive stress for the entire sample and individual groups of material variants.

Structural Lesions Caused by Excessive Stress	Total	Monolithic Infill	Masonry Infill	Rigid Screens	Flexible Screens	Simple Geometry	Complex Geometry
Timber compression	1.2%	1.2%	1.4%	0%	1.5%	1.3%	1.1%
Infill compression	4.4%	3.2%	3.1%	2.2%	32.3%	4.6%	4.0%
Infill fissuring	2.5%	1.5%	3.6%	0%	0%	2.6%	2.3%
Infill shear	2.3%	2.0%	2.9%	0%	0%	2.7%	3.8%
Timber breakage	11.4%	8.7%	9.1%	46.2%	36.9%	17.3%	17.0%

), the breakage of timber elements is the most common, although it generally appears as a consequence of problems for the material conservation of the elements, which compromise their resistance. In addition, some structural lesions have been documented, which are linked to the infill withstanding the load, generally due to problems in the material or structural conservation of the timber framework. The least frequent lesion is the compression of timber perpendicular to the fibres, which is not usually the result of small dimensions but of the material degradation of the element itself.

4.1.1. Breakage and Compression of Timber

Half-timbered walls are usually sufficiently squared off to meet the resistance requirements for structures, although their design is not always correct in relation to the stability and rigidity of the system [13]. In this regard, the breakages of timber parallel to the fibre, which have been detected in 11.4% of cases (Table 5), are mostly related to material degradation processes that limit their original resistance or to the presence of growth defects intrinsic to timber including knots, cracks, ring shakes, etc. The percentage of cases with rot and cases with broken timber elements increase to 29.1% and 21.8% when considering cases with the presence of xylophagous insects (Figure 52).

Although in half-timbered walls this is the most common structural lesion derived from excessive stress, a large part of these cases corresponds to the breakage of laths or branches used in lightweight screens (Figure 52). Specifically, while 8.7% of monolithic infills and 9.1% of masonry infills with structural degradation present breakages on some element of the framework, the percentage of breakages increases to 46.2% in rigid screens and 36.9% in flexible screens (Table 5). This lesion does not appear to depend on the structural geometry used in the half-timbered wall but rather on the state of conservation of the timber elements and the loads they have to withstand.

This may be due to the general use of small boards and branches from less resistant timber species, which, in the event of an attack from xylophagous insects, rot or rusting of anchoring elements, can break more easily. When this type of lesion is observed in the framework, it appears mostly in the upper beams, where the load from the roof is combined with the effects of rot or attacks from xylophagous insects favoured by runoff or the possible poor condition of eaves. However, it can also occur in lower beams where the loads are greater, which in turn can lead to more serious consequences in structural terms (Figure 53).

Timber elements can also display crushing when subjected to compression perpendicular to the fibre, although this pathology was only detected in 1.2% of cases, and there does not appear to be any particular vulnerability dependent on material or geometric variants (Table 5). This can occur in the upper beams when the load on the roof is excessive. However, there is a higher probability of this happening in the beams situated below the uprights. The upright transmits a load that, when the section of the beam is not enough or if a material degradation compromises its structural capacity, the timber fibres will be crushed until the mechanism stops [51]. In principle, this does not involve serious stability problems, although when the upright is lowered due to the crushing of the beam, it can lead to the infills beginning to bear the loads of the upper beam and developing other types of structural lesions. Equally, the differential movement of the upright in relation to the neighbouring materials can cause the rendering to fissure.

4.1.2. Crushing, Fissuring and Punching of Infill

Crushing in infill and enclosures, documented in 4.4% of cases, in 3.2% of monolithic infills and in 3.1% of masonry infills (Table 5), can be identified by a slight bulging of the central section of the infill. Heavy infills are relatively slender compared to their equivalents in other constructive systems, such as rammed earth, adobe and masonry walls, which tend to be thicker. Although a large part of the loads should be transmitted by the timber framework, a part of this load may be transmitted to these infills, which are so slender that they are crushed. This occurs, for example, when the upright has descended due to crushing from the lower beam or to problems in the union preventing the correct transmission of loads throughout the framework. This differential movement between the framework and the infill finds relatively little resistance due to the lack of union and reduced adherence between both elements.

However, it occurs much more frequently in flexible screens, which can develop crushing in 32.3% of cases, compared to 2.2% in rigid screens (Table 5). While the rigid screens are anchored or fitted into the main timber framework and therefore tend to move as a unit, the movement of the branches of the flexible screens is not as coordinated. In this case, the mechanism is due mostly to the material degradation of the lower horizontal branches, which cannot withstand the weight of the upper branches, making them slide down and be crushed (Figure 54). The substructure is maintained, and a partial loss of rigidity, provided by the branches, has made it easier for these movements to occur.

The high concentration of loads on a specific point can favour the punching of the infill, especially when its resistance is exceeded, for example, when the roof beams rest directly on the infill with no intermediate elements such as wallplates or footing to help distribute the loads (Figure 55). Wallplates can also favour this concentration of loads on a particular point, whether due to the use of irregular axes or because they have suffered material degradation or structural deformation. For example, the breakage of the upper wallplate due to the effect of rot or attacks from xylophagous insects brings about a loss of material resistance at that point so that the infill now bears the roof loads, contributing to punching.

The excessive accumulation of stress at a specific point of the infill leads to the appearance of a series of fissures that fan out. These are particularly visible in the masonry infills as they occur in the mortar joints, which are the weakest points [31]. However, the occurrence of this type of fissure is minimal and is observed in only 2.3% of cases (Table 5). According to the analysis of occurrence based on material variants, 2.9% of masonry infills display infill punching, compared to 2% of monolithic infills. This may be the result of the monolithic behaviour slightly increasing resistance and rigidity, while the presence of joints in masonry infills favours the appearance of these fissures. However, the same mortar joints with low resistance provide half-timbered walls with a mechanism to liberate energy during seismic events, in combination with the ductility of timber, the flexibility of traditional joints and the areas of contact between framework and infill [52].

Infills can also present fissures when the wallplates on which they rest undergo excessive buckling, especially in the case of large porticos or structural modifications carried out for the addition of openings (Figure 56). In these cases, eliminating infills or uprights involves eliminating elements that limit the flexion in the wallplates and contribute to the development of this type of fissure. Another possible cause of this flexion is the differential settling of the foundations, which involves the movements of the plinth or lower walls and, in turn, the deformation of wallplates (Figure 57). In these cases, a discharge arch is generated in the infills, and the loads move to the sides that are still in contact with the wallplate, transmitting the weight of the infills. At the same time, the central area of the infill is left unsupported and slides down slightly, being separated from the rest of the element. This mechanism has been identified in 2.5% of documented cases (Table 5) and is slightly more frequent in masonry infills, as already observed with punching. This increased vulnerability may be due to the presence of mortar joints, which tend to break as a result of the tensile stress created at these points.

4.2. Caused by Excessive Deformations

This section examines the lesions resulting from the movement of elements, which can modify the distribution of loads and influence the appearance of lesions detailed earlier. The lack of union between infill or enclosure and framework, the relatively slender elements and the lack of rigidity of the unions make these structures somewhat susceptible to experiencing movement despite the bracing provided by the union with floors, roof and perpendicular walls. In fact, 53.2% of all the cases in the sample display lesions derived from the excessive deformation of the elements (Figure 51).

Among the deformations identified (

Table 6. Presence of structural lesions caused by excessive deformations concerning the entire sample, each material variant group and each geometric variant group.

Structural Lesions Caused by Excessive Deformations	Total	Monolithic Infill	Masonry Infill	Rigid Screens	Flexible Screens	Simple Geometry	Complex Geometry
Lack of bonding	45.8%	48.5%	46.2%	39.6%	50.8%	45.4%	46.4%
Excessive buckling	8.4%	8.2%	9.2%	6.6%	6.2%	7.3%	10.1%
Instability of infill	17.1%	17.4%	17.1%	18.7%	18.5%	17.0%	17.2%
Collapse of timber	3.0%	3.0%	3.1%	5.5%	6.2%	3.1%	2.9%
Wall collapse	5.3%	6.5%	5.3%	4.4%	6.2%	4.2%	7.1%
Shift or breakage of joints	13.1%	15.2%	12.7%	17.6%	16.9%	11.9%	15.1%

), the vast majority are caused by issues inherent to the technique, such as the hygrothermal variations of materials, which encourage fissuring between the framework and infill or the displacement of unions. The lack of adherence between the framework and infill also contributes to different levels of collapse of infills and risk of instability, either due to the presence of horizontal stress, to the rotation of some timber element or even to the erosion of the infill. However, it may also be due to modifications in the structural geometry of the framework or the inclusion of new loads in the structure, altering the stresses and distribution of loads, as well as alterations in the surrounding conditions, such as the demolition of adjacent buildings or differential settlements in the ground. In any case, it is important to analyse whether these movements are still active or if these are past actions (Figure 58), in relation to which the structure has found a new situation of balance that does not require intervention [13].

4.2.1. Fissuring and Lack of Union

Half-timbered walls are built following a technique that incorporates several materials with different rigidity coefficients and structural behaviours where the absence of a method of union combines the infill and the timber framework and can bring about differential movements due to hygrothermal causes or general structural movements. This can generate fractures between both elements with different degrees of severity, clearly visible in some cases. As this cause is intrinsic to the constructive techniques, it appears more frequently than the remaining structural lesions and is observed in 45.8% of the walls analysed in this study (Table 6). This percentage refers to cases in which there is a visible separation between the infill and the timber framework, although taking into account the high number of causes showing signs of repair at these points (Figure 59), it is likely that the occurrence and frequency of this structural pathology is even greater. The widespread use of cement mortar to fill in these cracks may be detrimental given its hygrothermal behaviour and different rigidity [37]. However, interventions in these types of cracks are needed to prevent thermal breaks, potentially resulting in points of entry.

A greater occurrence is observed in heavy infills, where 48.5% of monolithic infills and 46.2% of masonry infills have developed this problem (Table 6). In contrast, rigid screens are the material variant that least frequently displays a lack of union, as can be seen from it only being documented in 39.6% of cases. This improved behaviour may be due to the laths of the lath-and-daub being nailed to the framework [37], although they can also display problems in terms of lack of union when the nails used rust and fracture the timber. In the case of flexible screens, with a slightly higher percentage than heavy infills, the branches used are not anchored to the structure but are forced in and can therefore suffer variations in dimension due to hygrothermal factors, causing the appearance of fissures in the union with the framework and even a general cracking of the mortar. The different geometric variants do not appear to have an influence on the development of these lesions (Table 6), which suggests that it is a factor intrinsic to the constructive technique and dependent on the union between the timber framework and the infill or enclosure in question.

4.2.2. Excessive Flexion of Timber Elements

The elimination of infills to create larger openings can lead to the wallplates flexing freely, a movement that was partially limited prior to intervention. This can also happen in walls with timber porticos due to the elimination of the occasional pillar to increase accessible space, making room for vehicles (Figure 60). On the one hand, the replacement of the original infill with industrial brick or the construction of heightened walls results in an increased load for which the wallplates may not have the correct dimensions. Even when no modifications have been carried out, the presence of material degradation or the displacement of knots can lead to excessive flexion in horizontal elements.

This type of pathology has been detected in 8.4% of cases, with a slightly higher occurrence in heavy infills and complex geometries (Table 6). In terms of material, this increase may be due to monolithic infills and masonry infills resulting in a greater load than lightweight screens, which, in the event of situations of material degradation or modification of the structure, may generate excess deflection with greater ease. In this respect, when the relationship between flexion processes in the timber wallplates and the modification in structural geometry is analysed, 21.8% of cases with excess flexion of the timber have also experienced some sort of modification. In terms of the geometry, this may be due to this type of wall having undergone more frequent modifications in structural geometry.

The development of excessive flexion in wallplates is directly related to the load transmission scheme, determined by both the typology and geometry of the walls. Continuous walls are based on the transmission of loads over a single plane, where the flexion of wallplates is limited by the presence of uprights, infills or plinths on the same plane. While only 6.2% of continuous walls display this lesion (Figure 61), the percentage increases considerably for discontinuous walls where the flexion of wallplates is less limited. In the case of overhangs, with an occurrence of 16.9%, the upper wallplate rests on a series of joists whose dimensions may be insufficient to withstand the weight or where a different state of conservation between elements could result in excessive flexion at a given point. In porticoed arcades, where this lesion has been detected in 15.6% of cases, the occurrence depends on the distance between the pillars of the porticos and possible modifications experienced. In contrast, in porticos between walls, the percentage falls to 12.1%, possibly because embedding the wallplates in the side walls slightly limits the rotation of the wallplates.

4.2.3. Instability and Collapses

In most cases, this lesion takes the form of the isolated collapse of infill or enclosure of one or several panels, as documented in 17.1% of the examples analysed in this research, and no increased tendency is observed depending on the material or geometric variants (Table 6). Although very occasionally these collapses are due to a serious erosion process at the base of the infills (Figure 21), these movements tend to be associated with the rotation of the timber element supporting the infill or enclosure, so this movement is transmitted with practically no opposition due to the lack of adherence of the rest of the framework. If the collapse occurs from a greater height, this can progress until the infill or enclosure is completely lost. This may be the reason for the complete replacement of panels with brick constructions. In any case, the timber framework can remain standing despite these losses of infill, even withstanding the loads from the roof, but this involves a loss of spacing, increasing the movement of traditional unions, which are not completely rigid [52].

In contrast, the isolated collapse of timber elements has only been documented in 3% of case studies analysed (Table 6). These collapses are usually due to the combination of a movement perpendicular to the wall in relation to a union that allows the timber element to rotate, either due to its intrinsic geometric configuration or because it presents some process of material degradation (Figure 62).

The infill or enclosure limits movement parallel to the plane of the timber elements, but there is no impediment to transversal rotation to the wall. In terms of material, greater vulnerability is observed in half-timbered walls with lightweight screens (Table 6). This may be due to the fact that heavy infills have sufficient rigidity and consistency to aid the confinement of the timber framework and slightly hinder the collapse of timber elements, whereas lightweight enclosures lack this constraining capacity.

The collapse of a façade, either fully or partly (Figure 63), can essentially be attributed to a possible settlement of the ground and foundations or to a modification that has caused a redistribution of forces [13]. Equally, the complete movements of the half-timbered wall may be the result of movements in other elements of the building, including the roof, floors or perpendicular walls. This hampers the extraction of general conclusions. Even so, this lesion has been documented in 5.3% of cases studied, with a higher occurrence in complex geometries (Table 6).

These deformations do not necessarily lead to a risk of structural collapse, as the flexibility of half-timbered walls allows slight deformations, enabling them to balance this out. In order to establish the severity of these deformations, it is important to analyse whether movements continue to occur or whether they are a historical event that the structure has managed to overcome, achieving stability.

4.2.4. Displacement and Breakage of Knots

Traditional unions are not completely rigid and can suffer displacements due to the general movement of the structure or to variations in the volumetry of timber elements as a result of hygrothermal factors [51]. They are particularly problematic as they reduce the efficiency of the support and bracing of the system, favouring the appearance of other structural lesions and generating a point where damp and associated biological attacks can accumulate. For example, the opening generated between uprights and braces reduces the efficiency of triangulation between elements, the separations between bridges and enclosing uprights generate lesions around the openings or the longitudinal sliding in the joins between wallplates can lead to increased buckling or movements (Figure 64).

The influence in relation to material variants is relatively homogeneous, although analysis of the frequency of displacements or breakages in the knots based on geometric variants shows a slightly greater presence in complex geometries (Table 6). This may be due to complex geometries presenting a greater number of joints subjected to stress in different directions, which can cause these lesions more easily as a result of movements of the structure or expansions and contractions of elements.

5. Discussion

The analysis carried out shows that direct exposure to atmospheric agents, such as solar radiation, wind and especially water, is the main cause of material degradation in half-timbered walls. However, the frequent positioning of this type of wall above plinths reduces the occurrence of lesions relating to damp absorbed by capillarity, while the overhang of eaves reduces the occurrence of lesions resulting from water runoff. Accordingly, the correct maintenance of these elements becomes particularly important for preventing the acceleration of the process of material degradation of half-timbered walls. Equally, the correct maintenance of protective rendering reduces or slows down other major material degradation processes, such as the erosion of infills or the dehydration and chromatic alteration of timber elements. Furthermore, it is necessary to note both the vulnerability of this type of constructive technique to losses in volumetry due to discontinuities in construction and the advantage it provides by delimiting losses, preventing them from spreading throughout the wall. However, this also leads to some incorrect intervention dynamics, including the partial replacement of infills with industrial brick, altering loads and the rigidity of the wall and causing considerable visual impact. Therefore, in addition to the correct maintenance of elements that make up half-timbered walls, it is necessary to promote the maintenance and dissemination of associated traditional trades as a strategy to ensure the availability of the resources, materials and elements necessary for a respectful reconstruction.

The development of lesions caused by biological agents is also favoured by the accumulation of damp in walls, encouraging the proliferation of fungi, mould, lichens, vegetation and xylophagous insects. In this respect, it is worth remembering that the visual identification of some of these processes is limited and that the occurrence might be greater and, in the case of rot and xylophagous insects, particularly serious. In this type of lesion, it again becomes particularly important to carry out periodic reviews and correct maintenance, as preventive actions can reduce the impact of these lesions and facilitate action when the scope is limited.

The danger of many of the lesions caused by anthropic agents lies in the elimination or alteration of protective elements or in the transformation of the half-timbered wall, leading to the development of other material lesions. Apart from the lack of maintenance associated with the abandonment of many of the buildings, in cases where direct anthropic actions have been carried out, the main causes for anthropic degradation are the imposition of industrial materials when executing repairs or interventions and the installation of unsuitable elements when adapting to current needs. Therefore, in addition to the maintenance and preventive actions previously described, it is again necessary to remark on the need to conserve associated traditional trades and promote new, more respectful and compatible forms of intervention and repair.

In general, the dimensions of timber structures are suitable for covering resistance needs, although the material degradation of the elements that make up the half-timbered wall and the discontinuities in the construction of the technique and lack of rigidity of the unions can favour structural degradation. In this respect, lesions generated by excessive stress are limited and mostly correspond to the breakage of timber elements due to material degradation processes limiting their resistance. In contrast, lesions caused by deformations occur frequently and are favoured by issues intrinsic to the technique, such as the lack of union, the relatively slender elements and the lack of rigidity of the unions, as well as external aspects, such as the modification in structural geometry, the elimination of infill, the inclusion of new loads and altered conditions in the surroundings. In most cases, these deformations are fissures in the framework or infill and the displacement of joints as a result of the variations in dimensions caused by hygrothermal issues. The lack of adherence, along with possible horizontal stresses, the rotation of support elements or severe erosion processes, may also contribute to varying degrees of collapse. However, it is common for collapse to be limited to infills or enclosures, affecting timber elements or, less frequently, the whole wall.

Analysis of the frequency of the different material and structural degradation phenomena has highlighted the risk faced by this heritage, mostly caused by lack of maintenance and the development of unsuitable spontaneous interventions. Given this risk situation affecting all traditional architecture, which is exposed to processes of cultural homogenisation, rural exodus, disappearing associated ways of life, loss of traditional constructive knowledge and, above all, the lack of value attached by owners and specialists, makes it necessary to preserve and promote the tangible and intangible materials still conserved. This is of particular importance in the specific case of half-timbered walls, especially given their location in increasingly depopulated areas [53,54,55], the general population’s distrust of the structural capacity of timber, its behaviour in the event of fires [56,57] and the limited technical knowledge relating to its intervention, conservation and potential. Whereas in other countries with a presence of this type of constructive technique, measures are being developed for their enhancement, dissemination and conservation [58,59,60], along with the publication of specific restoration articles and manuals [36,37,38,40,61,62,63,64,65,66], in Spain, there have been few instances of initiatives for ensuring the survival and valorisation of this type of heritage [67,68,69]. Generally, these have been geographically limited studies whose valuable information has not been transmitted to the general public and does not influence decision making.

At best, norms, tools and comprehensive actions have been developed in certain municipalities where traditional architecture and half-timbered walls have been conserved to some extent, such as Covarrubias (Burgos), La Alberca (Salamanca) and Calatañazor (Soria). However, neighbouring towns with the same type of heritage but without those measures have seen alterations, transformations and demolitions that have brought about considerable losses, as in the case of Retuerta (Burgos), Cepeda (Salamanca) and Talveila (Soria) (Figure 65).

In this respect, 64 of the 333 municipalities studied in this research have been declared Historical Artistic Complexes, a distinction that was first awarded in Spain in 1940 to La Alberca (Salamanca) [70]. Although this declaration entails the obligation to draft a Special Protection Plan or other planning mechanism features in urbanistic legislation, the degree of compliance with this is variable and depends directly on the resources available to individual town councils [71]. The diversity of criteria, categories, interpretations and definitions found in the different regional legislations linked to heritage, the lack of definition of municipal norms and the absence of protection bodies in a large proportion of vernacular buildings [72,73] have also brought about the abandonment of numerous buildings and the proliferation of a wide range of interventions that do not always favour the conservation of this type of heritage.

Therefore, it is still necessary to develop legislation and tools for protection that take into account traditional constructive techniques. However, while this type of heritage is mostly found in private residential buildings, its dissemination and valorisation are equally important. In this respect, knowing the general frequency with which half-timbered walls develop certain lesions can help to set out priorities for their maintenance, thus helping owners to organise the actions with the highest priority. Considering the data obtained in this research, the maintenance of protective elements such as plinths, eaves and rendering is a priority, and the execution of modifications on structural geometry must be analysed in depth and suitably designed to prevent lesions.

In any case, the actions aimed at their valorisation and conservation should not be relegated solely to academic research but should also include the authorities in charge of their legal protection, specialists such as the agents who intervene in its conservation and transformation and, finally, the wider population, who are generally the ones inhabiting this type of heritage. The simple implementation of a legal framework for protection is not enough to ensure its survival since it also requires the development of the necessary technical knowledge to ensure its survival, financial collaboration from the authorities in terms of subsidising the interventions, training specialists, developing traditional crafts and industry, and showcasing its value among the general population, who ultimately are those who decide to invest in its conservation. It is thus also considered necessary to strike a balance between heritage conservation and the adaptation of buildings to modern standards to ensure and facilitate continued use and, therefore, maintenance. This use must also ensure respect for the heritage value of these buildings, making it necessary to reflect on the frequent reconversions of buildings into cultural or tourist buildings, which can ultimately affect the ways of life associated with this type of heritage and part of its cultural value [74].

Equally, given that part of the value of popular architecture lies in the presence of traditional constructive techniques, which represent collective knowledge developed over centuries to adapt to a specific geographical context and culture [75], the promotion of detailed knowledge and recovery of traditional constructive techniques both for its valorisation and for application in interventions must be continued. The conservation, recovery and valorisation of traditional constructive techniques and systems and related artisan trades are associated with the possible regeneration of an artisan industrial fabric, enabling the improvement of the economy and the labour market in rural settings. Thus, it is possible to collaborate in the promotion of the fight against depopulation, which is so widespread in the areas where these traditional techniques have been documented, along with greater sustainability in the field of construction thanks to the reuse of existing buildings, local labour and materials obtained from local surroundings.

6. Conclusions

This analysis has made it possible to establish the frequency of the different material and structural lesions identified, as well as the possible influence of the constructive characteristics of the different variants documented and the correlation between the development of certain lesions. It should be noted that while any intervention on a traditional building requires prior diagnostic studies adapted to each specific case [31,76], having access to global information on the frequency with which each of the types of half-timbered walls develops certain degradation phenomena can be of help when proposing the basis for conservation and restoration. The main cause of degradation of this type of wall is constant direct exposure to the elements, making it necessary to promote the correct maintenance of the walls and protective elements. Furthermore, as these are privately owned residential buildings, it is also relevant to draft detailed protection norms and promote technical knowledge in order to reduce the number of spontaneous interventions that favour degradation.

Given the access to the information in a GIS geolocalised database in the future, it will be possible to cross-reference the state of conservation and climatic conditions to establish the influence of factors such as wind, rainfall and solar radiation in certain degradation processes. As with studies carried out on Spanish earthen architecture [77], it will then be possible to establish a correlation between constructive characteristics, development of material and structural lesions and geographical conditioning factors, which provide information on the vulnerability and resilience of half-timbered walls. These are indispensable aspects, bearing in mind climate change and the consequent variation in the climatic conditions for which traditional architecture was developed and adapted.

However, it should also be noted that, as this type of wall has mostly been documented in privately owned residential buildings, the needs, tendencies, preferences and, particularly, possibilities of their residents should also be considered. Therefore, further research must analyse the dynamics for intervention and transformation, which have generally been carried out by the occupants themselves, in order to learn about and understand these needs, tendencies, preferences and possibilities. Once the vulnerabilities characteristic of the constructive technique and the conditioning factors imposed by users are known, it will be possible to propose more detailed general recommendations, guidelines and intervention techniques, ultimately aiming to valorise the technique, promote its restoration and, eventually, show its suitability for residential purposes.