A Michigan State University study used test platforms to test the influence of roof slope (2 and 6.5%) and green roof media depth (2.5, 4.0, and 6.0 cm) on stormwater retention. For all combined rain events, platforms at 2% slope with a 4-cm media depth had the greatest mean retention, 87%, although the difference from the other treatments was minimal. The combination of reduced slope and deeper media clearly reduced the total quantity of runoff.
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.