Green roof stormwater retention: effects of roof surface, slope, and media depth

• Published in: Journal of environmental quality 2005-05, Vol.34 (3), p.1036-1044
• Main Authors: VanWoert, N.D, Rowe, D.B, Andresen, J.A, Rugh, C.L, Fernandez, R.T, Xiao, L
• Format: Article
• Language: English
• Subjects: agricultural runoff; Agronomy. Soil science and plant productions; Applied sciences; Biological and medical sciences; Cities; duration; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environment Design; Exact sciences and technology; Facility Design and Construction; Fundamental and applied biological sciences. Psychology; Gravel; Green infrastructure; Green roofs; Plants; Pollution; pollution control; Pollution, environment geology; Rain; Retention; Roofing; roofs; Runoff; slope; soil depth; Storm runoff; Stormwater; stormwater management; Sustainable materials; Urban areas; urban runoff; Vegetation; Waste Disposal, Fluid - methods; water holding capacity; Water Movements; water pollution
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq2004.0364

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 roof-tops, 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.