Rooftops

In dense urban situations rooftops also receive a high level of solar radiation, paralleling the heat of ground surfaces. Rooftops constitute a large proportion of the city’s upperlevel environment in downtown areas and are, from taller structures, often highly visible parts of the urban landscape. Rooftop vegetation functions in the same way as it does at ground level. Rooftop gardens therefore, can, in addition to reducing stormwater drainage, enhance water quality and create bird habitat (see Chapter 4 on Deptford Creek), and thus perform a multi-functional role in climate control. The limitations to creating this kind of landscape relate to structural support for soil and plants which, while they can be overcome in new projects, may constitute a limitation for the large areas of existing roof spaces in the city if deep soils are a requirement. In cold climates there are also drainage, irrigation, nutrient and frost problems to be overcome. Green roofs, however, present strategies that combat one of the major climatic and visual problems of many urban areas. An examination of many old rooftops will reveal that fortuitous plant communities often gain a foothold. Mosses, grasses and, in places where a small amount of humus and water can collect over time, even adventitious shrubs and small trees colonize these unattended and forgotten places. The issues of fortuitous plant communities have been discussed in detail in Chapter 3 (pp. 112-17), but it is relevant here to pursue their role in climate amelioration. A research programme undertaken by the Parks Department in Berne, Switzerland, on a concrete garage roof, showed that

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minimum soil depth or humus content.30 The city of Dusseldorf, Germany, has regulations requiring large, flat-roofed areas to have roof gardens and Stuttgart actively encourages them (see pp. 215 and 217). These and other examples in Europe suggest that hardy plants adapted to city conditions can survive in hostile environments at very little cost, without adding to the weight of existing or new building structures. The creation of an economical landscape at roof level can contribute to the climatic conditions of the city.