This case study is located in the city of Geneva, Switzerland. It describes how green roofs have been implemented on the roof of a hospital (0.55 ha) in order to decrease flood risks in urban areas. Indeed, this green roof is designed to retain about 30.25 m3/day water in the soil and plants and prevents it from running off. Compared to a traditionnal roof, the green roof retains about 60% of the water quantity that would run off otherwise. Moreover, green roofs are seen as a good way to recreate habitat in urban areas.