Modern approaches in green roof design have begun including a detention component which allows for more temporary storage during large rain events and has the potential to significantly reduce peak flow rates compared to retention alone. Because this detention based approach is relatively new in green roof design, understanding the proper way to quantify performance is critical to their success. To determine sufficient rainfall-runoff response, Green Roof Diagnostics’s (GRD) rainfall simulator was used to evaluate performance of various green roof profiles and configurations, under a number of different conditions including rainfall intensity, storm type, initial soil moisture, and storm duration.