As cities become more densified , a number of problems come in: there is a loss of biodiversity and green space; stormwater and the risk of flooding become heightened; and the widespread use of heavy construction materials such as concrete and dark coloured materials such as asphalt for roads contribute to the Urban Heat Island . One solution is to use the roofs of buildings to reintroduce vegetation. A green roof at the University of the Witwatersrand Green roofs can either be extensive, as shown in the photo, with a fairly shallow layer of soil over the whole extent of the roof-space, or intensive with much deeper soil to support larger plants such as shrubs or even trees. Intensive roofs are quite heavy, so usually they can only be used where they have been designed for in the original construction of the building. By contrast, extensive roofs are quite lightweight and can be installed on a variety of existing flat roofs without the need for structural modification. The green roof i...
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