In order to estimate the detention effect (storage and peak flow attenuation) of a sedum green-roof, a comparison between the response of an impervious roof (runoff coefficient C = 1) and a green-roof has been carried out. Precipitation and runoff data from several controlled experiments (with wet initial conditions, i.e. approximately at field capacity) on a sedum green-roof were used for the analysis. For events with variable rain intensity, the green-roof's attenuation effect was as high as 65% for design storms. In contrast, events with constant intensity had increases in peak flows of up to 5%. The lowest detention volumes were observed for the experiments with constant rain intensity, for which a maximum of 29% of the rain volume was detained, whereas for rain events with variable intensity the detention volume was up to 52%. Simulations (using PondPack), based on a real case study, showed that green-roofs can be implemented in combination with other structural best management practices (BMPs) to obtain a desired detention. They can be used to reduce the size of other structural BMPs, and can occupy unused space that is readily available.