Currently widespread in new urban projects, green roofs have shown a positive impact on urban runoff at the building scale: decrease and slowdown of the peak discharge, and decrease of runoff volume. The present work aims to study their possible impact at the catchment scale, more compatible with stormwater management issues. For this purpose, a specific module dedicated to simulate the hydrological behaviour of green roof has been developed in the distributed rainfall-runoff model (Multi-Hydro). It has been applied on a French urban catchment where most of the buildings roofs are flat and assumed to accept the implementation of green roof. Catchment responses to several rainfall events covering a wide range of meteorological situation have been simulated. The simulation results show green roof can significantly reduce runoff volume and the magnitude of peak discharge (up to 80%) depending on the rainfall event and initial saturation of the substrate. Additional tests have been made to assess the susceptibility of this response regarding both spatial distributions of green roofs and precipitation. It appears that the total area of greened roofs is more important than their locations. On the other hand, peak discharge reduction seems to be clearly dependent of spatial distribution of precipitation.