Since 1991, the city of Zürich has had a law stating all flat roofs (unless used as terraces) must be greened roofed surfaces. The main advantages as a result of this policy include increased biodiversity, rainwater storage and outflow delay, and micro-climatic compensation (temperature extremes, radiation balance, evaporation and filtration efficiency). Roof biotopes are stepping stones which, together with the earthbound green areas and the seeds distributed by wind and birds, make an important contribution to the urban green infrastructure.