Extensive green roofs* are basic, light weight systems, characterized by minimum maintenance and management (artificial irrigation, fertilization) after establishment of the system. According to the NBS catalogue, a minimum performance of 25 l/m² storing capacity and at least 95 % of vegetation coverage after three years is needed. The installation and management/maintenance of extensive green roofs is less expensive than that of intensive systems. Extensive green vegetation is often established on roofs that are not accessible or with limited access for public or recreation purposes (but annual maintenance) and partially characterized by steep slopes. Appropriate plants for extensive green roofs are low growing, rapidly spreading and shallow-rooting plants/hardy perennials (succulents such as sedums, herbs, wildflowers, grasses, mosses) that are able to survive with minimum nutrient uptakes and without additional nutrient supply. The selected plants for extensive green roofs are generally well adapted to alpine environments/climate and tolerate different climate conditions (e.g. drought) and temperature fluctuations. The number of different plant species is limited on extensive roofs, yet the biodiversity on extensive green roofs is generally greater than on other (intensive) green roof types. Through the establishment of (extensive) green roofs on rooftops, different services of natural vegetation layers are replicated. As a result, the potential to mitigate the urban heat island effect is increased compared to sealed surfaces without any vegetation. Extensive green roofs provide limited services and benefits for the surrounding environment. As described above, it is characterized by a low vegetation surface that covers the buildings surface. Although the surface covering is the main service of extensive roofs, it also leads to positive effects on microclimate: Evaporation is increased in comparison to black roofs and leads to a heat reduction of the surrounding air temperature (=air cooling). Furthermore, the vegetation binds particular matter. The growth medium is relatively thin compared to intensive green roofs. As a result the service of water buffering, temporary storage, retention and filtration albeit lower than for intensive green roofs, yet exists.
*different terms for extensive green roofs used in literature are low-profile/Ecoroofs (source: "Green Roofs." Provided n: Reducing Urban Heat Islands: Compendium of Strategies. Draft. https://www.epa.gov/heat-islands/heat-island-compendium)
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