Green roofs are increasingly being used as part of primary urban strategies to improve stormwater management and reduce energy costs. Different methods for green roof design are based on different assumptions and parameters. However, the input parameters are uncertain, which influences design accuracy. A mathematical model was developed to simulate the water movement across and through a green roof. This model was used to assess the sensitivity of the hydrological response of a green roof to roof and rainfall characteristics. Peak discharge rates and depths of runoff mainly depend on the rainfall characteristics. Green roofs can significantly reduce both the peak discharge rates and runoff depths for small storms but they have little effect on large storms. Furthermore, roof characteristics mainly the roof slope is an important design criteria. The model benefits from using the NRCS infiltration equation and curve number, as well as the specific yield of the soil.