Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century?

During the last two decades, a large amount of research has been published in German on the reduction of rainwater runoff for different types of roof greening. This paper analyzes the original measurements reported in 18 publications. Rainfall–runoff relationships for an annual and seasonal time scale were obtained from the analysis of the available 628 data records. The derived empirical models allowed us to assess the surface runoff from various types of roofs, when roof characteristics and the annual or seasonal precipitation are given. The annual rainfall–runoff relationship for green roofs is strongly determined by the depth of the substrate layer. The retention of rainwater on green roofs is lower in winter than in summer. The application of the derived annual relationship for the region of Brussels showed that extensive roof greening on just 10% of the buildings would already results in a runoff reduction of 2.7% for the region and of 54% for the individual buildings. Green roofs can therefore be a useful tool for reducing urban rainfall runoff. Yet in order to provide a greater effect on overall runoff they should be accompanied by other means of runoff reduction and/or water retention.