Shallow substrate layers on flat or moderately sloped roofs are usually planted with droughttolerant perennials, especially Sedum species. But it is also possible to establish wetland vegetation when the substrate surface is permanently supplied with water. Even synthetic fabric mats are sufficient to support wetland or swamp-vegetation. The main reason for installing such a vegetation system is the effect of cooling the building efficiently without too high an energy input (Ziepke 1992) (Figure 8.23). The permanently moving water allows the use of wetland plants from Table 8.2 (in particular, monoculture species), even if it has got a low nutrient content. Lightly contaminated water can be purified by running through such a large area of suitable substrate and water plants. On small roofs—mainly in private gardens—a covering with low growing oligotrophic fen or bog vegetation (see Table 8.2) is feasible to gain an exciting visual and botanic diversity, by saving roof-loading.