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)

Performance

Evapotranspiration none none
Shading none 1
Reflection (Albedo) none
Water Conveyance 1
Water Infiltration none
Water Retention 1
Water Storage none
Water Reuse none
Water Filtering none
Water Bio-remediation none
Deposition none
Bio-filtration none
Habitat Provision 1
Connectivity 1
Beauty / Appearance 1
Usability / Functionality none
Social Interaction none
Role of Nature / Mode of Action: 
As illustrated in Fehler! Verweisquelle konnte nicht gefunden werden. the model for a green roof is atural soil with its vegetation cover. Through the establishment of green roofs on buildings, different services of natural vegetation layers are replicated.
Technical & Design Parameters: 
Different greening systems for extensive green roofs - and therefore no uniform technical/design construction - exists. a) direct  vegetation grows direct on concrete (special “biological concrete”) b) textile systems  vegetation is established on synthetic fibre mats c) textile-substrate-systems  vegetation is precultured on organic fibre mats + underlying substrate d) substrate fill  substrate mix that varies in height on drainage layer - plants: less variety (moss, sedum, herbs, grasses) - water requirement: low - growing medium: 2-6” (~ 5-15 cm); reservoir board for extensive roofs is needed - slope gradient: 0-35° (a-d) (steeper slopes up to 85° (a-c)/ 45° (d) are possible with technical devise) - weight: 20 kg/m (b); 30-90 kg/m² (c); 50-190 kg/m² (d) - water retention capacity: up to 20 l/m² (a, c), up to 24 l/m² (b); 30-50 l/m² - investment: low (a); 45-60 Euro/m² (b), 55-70 E/m²; 15-35 Euro/m² (d)  low to medium maintenance: low (a), 0,50 Euro/m²a (b), 1 Euro/m²a (c); 1,50-3,00 Euro/m²a
Conditions for Implementation: 
- site characteristics often depend on project objectives  e.g. objective = improving aesthetics surrounding buildings - solid, stable concrete buildings/bearing capacity - flat or relatively flat concrete rooftops and underground concrete structures - artificial irrigation but at least (rainwater) watering facility in critical/dry periods - in some cases special plates are needed to distribute pressure on rooftop (for planters)
Benefits & Limitations: 
Benefits: - human health and quality of life - storm water/rainwater management and quality - improved air quality - aesthetic value/visual attractiveness - thermal performance/temperature reduction - energy reduction for buildings (heating/cooling) - reduction of noise/sound transmission - habitat provision for urban wildlife Potential limitations/disservices: - limited development of undisturbed habitats because of human activities/public purposes - limited spread of flora and fauna because of regular maintenance and management limited space for roots

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 730052 Topic: SCC-2-2016-2017: Smart Cities and Communities Nature based solutions