At the basin scale, green roofs can represent an efficient tool to manage stormwater in urbanized areas. Widely implemented and/or completed with additional sustainable urban drainage systems, they show a positive impact on urban runoff: decrease and slowdown of the peak discharge and decrease of runoff volume. To assess green roof performances at this scale, a specific module dedicated to simulate their hydrological behaviour has been developed in the Multi-Hydro rainfall-runoff model. Its distributed structure gives the opportunity to test the susceptibility of green roof response regarding spatial distributions of precipitation. Based on radar rainfall fields, an ensemble of 50 realistic downscaled rainfall fields with a resolution of 10 m in space has been generated by using multifractals downscaling technique. Simulations have been conducted on a small urban catchment close to Paris (France) where most of the buildings roofs are assumed to easily accept the implementation of green roof. Although green roof confirm their ability to reduce urban runoff, these results illustrate that peak discharge reduction seems to be clearly dependant of spatial distribution of precipitation. Implementation of green roof can also produce concomitance situations and higher peak discharges than those produced by impervious roofs.