Perhaps the most important benefit of green roofs is their ability to retain stormwater. On an annual basis, a 3.5-4" deep sedum green roof in central Pennsylvania can retain approximately 50-60% of the annual rainfall. Retention occurs primarily in the summer months from May through September when as much as 100% of the rain can be retained. In the cool season months less water is retained, 20-30% on average (Figure 1). During the summer the plants actively transpire, and evaporation from the media surface is high so the roof can effectively store and release most of the rain. In the winter, evaporation is slow and the plants are dormant so less retained water is returned to the atmosphere. In effect the green roof restores the evapotranspiration component of the hydrologic cycle to the otherwise impervious roof surface. In any rain event the amount retained by the green roof depends on the water holding capacity of the media and the plants and how much is still retained from the last rain event. An average 4" roof can usually retain most of a 1 inch rain during the summer if the last rain was at least a week before (Figure 2).

In addition to retaining a considerable percentage of the annual rainfall, a green roof can reduce peak runoff rates and delay runoff from many storms, acting as a roof top detention basin. A typical summer rain produces a runoff hydrograph like that shown in Figure 3 and 4. Note that runoff from the green roof is delayed and peak rates are reduced compared to the rain or runoff from a non-green flat roof. Even in an extreme event the green roof will reduce total runoff volume and delay the runoff (Figure 5 and 6). In this rain the green roof adsorbed the initial rain completely, midnight- about 2:20am , and delayed and reduced the peak flows, 2:20 - 3:00am, and further delayed and retained the late peak (~7:10am). The green roof is thus a very effective stormwater management tool and can be used to reduce the impact of new development on overburdened municipal storm water systems.

Figure 1: Cumulative monthly runoff and retention as percent of received rain on green roofs in Rock Springs, PA in 2005.
Figure 3: Runoff from green and non-green roofs from a rain of about one inch on August 16, 2005. The last major rain event occurred about two weeks earlier.
Figure 4: Hydrograph from the rain in Figure 3.
Figure 5: Runoff from green and non-green roofs in an extreme event (August 30, 2005) where nearly one inch of rain fell during a very short period.
Figure 6: Hydrograph from the rain in Figure 5.