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TECHNICAL REPORTS

Landscape and Watershed Processes

Green Roof Stormwater Retention

Effects of Roof Surface, Slope, and Media Depth

^a Department of Horticulture, Michigan State University, East Lansing, MI 48824
^b Department of Geography, Michigan State University, East Lansing, MI 48824
^c Department of Crop and Soil Sciences, Michigan State University, East Lansing, MI 48824
^d College of Agriculture and Natural Resources Statistical Consulting Center, Michigan State University, East Lansing, MI 48824

^* Corresponding author (rowed{at}msu.edu)

Received for publication September 27, 2004. Urban areas generate considerably more stormwater runoff than natural areas of the same size due to a greater percentage of impervious surfaces that impede water infiltration. Roof surfaces account for a large portion of this impervious cover. Establishing vegetation on rooftops, known as green roofs, is one method of recovering lost green space that can aid in mitigating stormwater runoff. Two studies were performed using several roof platforms to quantify the effects of various treatments on stormwater retention. The first study used three different roof surface treatments to quantify differences in stormwater retention of a standard commercial roof with gravel ballast, an extensive green roof system without vegetation, and a typical extensive green roof with vegetation. Overall, mean percent rainfall retention ranged from 48.7% (gravel) to 82.8% (vegetated). The second study tested 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. For both studies, vegetated green roof systems not only reduced the amount of stormwater runoff, they also extended its duration over a period of time beyond the actual rain event.