**1. Introduction**

Due to its multiple benefits, biological coatings on buildings (i.e., green roofs and walls) were used by different human civilizations [1,2]. According to the scientific community, there is testimonial evidence of the use of biological coatings on Babylon's Hanging Gardens and Babel Tower [2–4]. Similarly, religious buildings called "Ziggurats" were constructed in Mesopotamia using plant growth on their building surfaces most probably to reduce heat interchange with the environment [3]. Biological coatings were also part of vernacular architecture in Scandinavia. For instance, in Norway, green roof systems are still used as a thermal isolation system [5].

Even when green roofs were present on the major buildings of the ancient world, their use on contemporaneous architecture reappeared only in the 20th century with the Swiss architect Le Corbusier who included them among modern architecture principal elements [6]. In fact, he stated that "the garden terrace will be the reunion place preferred by citizens and it will also mean the recovery of the built surface from the city". In this sentence, the preoccupation that exists since then about the accelerated urban development is synthesized. The truth is that the construction of urban infrastructure with non-permeable materials such as traditional concrete and asphalt significantly reduces rainwater flow to the soil layers and water table [7]. In a basin, the water cycle alteration can produce floods, river and lake disappearances, and consequently ecosystem extinction. Biological coatings for buildings such as green roofs and walls are used to mitigate urban threat and other environmental problems. In general, the benefits of green roofs and walls can be classified into three categories: environmental, economic, and social. Aspects such as thermal, moisture, noise, and electromagnetic protection in buildings, heat island effect reduction, sewage system load reduction, runoff water quality and air quality improvement, habitat development, and natural landscape reconstruction are among the major environmental benefits from green roofs and walls. From an economic viewpoint, green roofs and walls on buildings increase the property's commercial value, increase waterproof membrane lifespan, increase fire resistance, and reduce energy consumption. Regarding social aspects, these biological coatings improve the occupant's health and wellbeing, while they also help to generate employment for maintenance and even for food production at the urban scale (urban agriculture).

Recently, the use of green roofs on buildings increased significantly around the world due to different government incentives and due to pressure exerted by a new market that seeks products and processes that are more environmentally friendly. However, the lack of a detailed characterization and guidelines for their design resulted in the implementation of these biological coatings on buildings being currently based on experience, making them inefficient and even risky. In order to improve this situation, this review of the state of the art presents a proper green roof classification, which is based on their components. Similarly, a detailed description of the factors that control the hydraulic and thermal performance of green roofs is given. Based on these, an estimation of the impact of green roof systems in sustainable construction certifications is included. Finally, research challenges for the correct implementation of these biological coatings on buildings are addressed.

#### **2. Components and Classification**
