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%).