A Michigan State University study used test platforms with three different roof surface treatments to quantify differences in stormwater retention over a 14-month period with 83 measured rain events. The typical extensive green roof with vegetation retained 337 mm of the 556 mm of cumulative rainfall (60.6%). The extensive green roof system without vegetation retained 281 mm (50.4%) and the standard commercial roof with gravel ballast retained the least rainfall, 151 mm (27.2%).
VanWoert, Nicholaus D., D. Bradley Rowe, Jeffrey A. Andresen, Clayton L. Rugh, R. Thomas Fernandez, and Lan Xiao. (2005). Green Roof Stormwater Retention: Effects of Roof Surface, Slope, and Media Depth. Journal of Environmental Quality, 34(3), 1036-1044.