AVRINNING FRÅN GRÖNA TAK / Runoff from green roofs
The runoff from an extensive green roof in Malmö was measured over more than a year. The roof, which is 4 m long sloping 2.5 %, is covered by 3-4 cm soil substrate with sedum vegetation. About 10 mm rain is required to generate runoff. The monthly and annual water balance of a green roof is much different from that of a hard roof. In spring and summer, the runoff can be 50 mm/month less than the precipitation. On annual basis the runoff is less than half of the precipitation. Rain falling on a roof, which is rather wet, generates hourly and daily runoff almost as large as the rain. On shorter time basis the peak runoff is attenuated relative the rain intensity and corresponds approximately to the 20 minute rainfall.