In the Netherlands, flat roofs are generally calculated to be able to carry a load of 1 kN/m 2 for the gravel layer. Roof of the European Patent Office in Rijswijk © Copijn Several providers have developed software with which the influence of a green roof on the water system can be determined. Green roofs have been further developed by the various suppliers and are on offer as a total kit including planning, advice, construction, maintenance and all applicable guarantees. Standard packages are also available for the various purposes such as rainwater retention, biodiversity and/or a garden roof. There is good quality literature available on the various producers, with main details, implementation guidelines and technical data. Sloping green roof, De Vouw Hoofdkantoor Afvalzorg NV © Optigroen In order to make green roofs walkable or to create a standing spot, a paved surface must be installed. For rainwater retention, a slope of up to 7 o is most efficient. If the additional provisions to counter sliding tear through the waterproofing roof membrane this will increase the risk of leaks. Steeper roofs dry out faster due to the faster runoff of rainwater. At more than 35 o extra provisions are required to prevent sliding. Green roofs can be installed on roofs with slopes ranging from 1o to 35 o. Intensive green roofs vary from watered grass/herb roofs to walkable city parks on buildings. Extensive green roofs are easier to realise on existing buildings. Extensive green roofs are thinner and lighter in construction and generally less costly. The difference lies in the intensity of the required care as well as the different way of constructing. In principle two kinds of green roofs can be discerned: extensive green roofs and intensive green roofs. ‘Green roofs’ is a collective term which includes moss/sedum roofs, grass/herb roofs, and retention roofs, and is used for walkable planted roofs and sloping roofs. California Academy of Sciences, architect: Renzo Piano, San Francisco © atelier GROENBLAUW Besides being attractive, their capacity for rainwater retention, their capture of particulate matter, and the roof and the surroundings heating up less are often a consideration in favour of constructing green roofs. Green roofs also contribute to a larger biodiversity in the city. Green roofs are increasingly used as a building block towards attaining sustainable urban development. Green roofs do not attract the same amount of heat as bitumen or EPDM roofs.
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