Green Roof Resource

In natural, undisturbed systems, approximately thirty percent of the rainwater reaches shallow aquifers that feed plants, another thirty percent percolates and nourishes deeper aquifers, and approximately forty percent returns to the atmosphere through evaporation and plant transpiration [1]. In urbanized areas, seventy-five percent or more becomes stormwater runoff. It is well known that green roofs are one type of porous surface that can correct this rainwater distribution reducing the demand on municiple sewer systems.

Depending on the climate of a region, green roofs can retain as much as fifty to seventy percent of water that falls onto a roof. Retention varies according to climate. Hutchinson et al (2003) reported that a 10 to 12 cm vegetated roof in Portland, Oregon retained 69% of the total rainfall with peak flow reductions of 80% during a 15-month monitoring period [2]. Investigations in East Lansing, Michigan compared a gravel roof and a vegetated roof with the mean percent rainfall retention ranging from 48.7% for a gravel roof to 82.8% for a vegetated roof [3]. A similar range was seen on two roofs of different slopes in Goldsboro, North Carolina. Total rainfall retention was 55% for the sloped roof and 63% for the flat roof [4]. Peak flow was reduced by 57% for the sloped roof and 87% for the flat roof [4].

Other studies have investigated the dependence of stormwater retention on slope. VanWoert et al (2005) evaluated the effect of roof slope and substrate depth on stormwater retention. Two slopes were tested, 2 percent and 6.5 percent at 3 green roof substrate depths (2.5, 4.0, and 6.0 cm) [3]. The greatest retention occurred on a 2% slope roof with a media depth of 4 cm [3]. Further studies on the effect of slope were conducted for the 4 cm roof by Villarreal and Bengtsson (2005). The slopes evaluated were 2, 4, 8, 14 degree under dry conditions and wet conditions (field capacity) located in Lund, Sweden [5]. They observed no effect on runoff from slope variation, but did observe an effect on runoff by the moisture content [5]. Under dry conditions the green roof retained 6 to 12mm of rainfall prior to runoff initiation while under wet conditions the green roof initiated runoff toward the beginning of a rainfall event [5].

Evidence suggests that both slope and moisture content affect retention and peak flow reduction. However, due to the variability of climates, further modeling of the hydrology of the green roof system is needed to better understand and anticipate performance.

References:
[1] Scholz-Barth, Katrin. 2001.
[2] Hutchinson et al. 2003.
[3] VanWoert et al, 2005.
[4] Moran et al. 2005.
[5] Villarreal and Bengtsson. 2005.

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