In last decades, urban population increase led to the displacement of green spaces and natural infiltration areas, with impermeable surfaces, harming the urban water cycle. Furthermore, and due to the increased frequency and intense extreme precipitation events that are happening more frequently due to global climate change effect, traditional urban drainage systems could not effectively manage precipitation-generated surface water in such intense events, straining the city's water drainage infrastructure. In this regard, Green Roofs (GR) have been considered a sustainable engineering solution that can help to minimize drainage systems stress, due to their retention and detention capacity into their multilayer system structure, and therefore decreasing the amount of drained water that runs-off to the pluvial urban drainage network. The present work developed a simulation procedure, encompassing four roofs of different areas (150 m², 300 m², 500 m² and 1000 m²) to assess the impact of the implementation of an extensive GR with expanded clay in its composition (drainage layer and substrate), on downstream drainage systems sizing process. Characteristic runoff coefficients of different types of roof structures and also a runoff coefficient from an experimental GR tested were selected, to determine the rainwater inflow rate to drainage pipes downstream, Q. Simulation results obtained showed that expanded clay extensive GR allows lowest diameters for the drainage system downstream when compared to other roof types, leading to a potential decrease in the costs associated with the drainage network installation.