The article presents a method to study the changes of urban catchment runoff behavior from green roof implementation. The proposed method takes into account the thresholds in green roof response and abrupt changes in areal characteristics, which allows the method to correctly scale various types of green roofs, and ultimately other BMPs, for a range of storm characteristics and BMP implementation. We used the Illinois Urban Hydrologic model (IUHM) as a base model for urban sewer systems, with the incorporation of a green roof model that was developed to simulate the major processes affecting the hydrologic response of green roofs. The modified IUHM model, herein named Combined IUHM-Green Roof Model (CIGM) was used to test green roof impact on hydrologic response at a catchment scale. Results indicate that the implementation of green roofs has a distinct effect on decreasing the volume and peak of urban storm water runoff and postponing the time to peak runoff. The volume and peak discharge reduction depends on the temporal distribution of rainfall and the volume of rainfall and green roof coverage.