At the building scale, the use of green roof has shown a positive impact on urban runoff (decrease and slow-down of the peak discharge, decrease of runoff volume). The present work aims to study the possible impact of green roof at a larger scale compatible with urban management issues (quarter, catchment). A methodology has been proposed to assess the green roof impact at such a scale. It combines a method to define green roofing scenarios by estimating the maximum roof area that can be covered and a modelling tool able to simulate the hydrological behaviour of green roof. This module has been integrated into the SWMM urban runoff management model. This methodology was applied to two urban catchments (Haut-de-Seine county, France) affected by flooding and combined sewage overflow hazards respectively. The results show green roof can be useful to reduce the frequency and the magnitude of such problems depending on the covered roof surface. Combined with other infrastructures, they represent an interesting solution for urban water management in the future.