Quantifying the effect of slope on extensive green roof stormwater retention

Impervious surfaces, such as rooftops, parking lots, and roads, increase runoff and the potential for flooding. Green roof technologies, which entail growing plants on rooftops, are increasingly being used to alleviate stormwater runoff problems. To quantify the effect that roof slope has on green roof stormwater retention, runoff was analyzed from 12 extensive green roof platforms constructed at four slopes (2%, 7%, 15%, and 25%). Rain events were categorized as light (<2.0 mm) (0.08 in.), medium (2.0–10.0 mm) (0.08–0.39 in.), or heavy (>10.0 mm) (>0.39 in.). Data demonstrated an average retention value of 80.8%. Mean retention was least at the 25% slope (76.4%) and greatest at the 2% slope (85.6%). In addition, runoff that did occur was delayed and distributed over a long period of time for all slopes. Curve numbers, a common method used by engineers to estimate stormwater runoff for an area, ranged from 84 to 90, and are all lower than a conventional roof curve number of 98, indicating that these greened slopes reduced runoff compared to traditional roofs.