Passage IV

A green roof on a building is a cover of living plants growing on several other layers of various materials (see typical green roof cross section below). Green roofs help control runoff during rain events.

Five 2.5 m x 2.5 m platforms (1 gravel platform, 1 media platform, and 3 vegetated platforms) were constructed at a location on a building's roof. The gravel platform had a 2 cm layer of gravel over a waterproof fabric (a typical roof covering) and was unsloped. The media platform had all the layers as shown in the cross section except for the plants and was unsloped. The vegetated platforms had all the green roof layers as shown. One vegetated platform had a slope of 2.0%, another had a slope of 6.5%, and the third was unsloped (0% slope).

A runoff collection device was attached to each platform and a rain gauge was mounted on the roof. Two studies over a 2-month period measured the runoff from the platforms after each of 21 rain events, all lasting 60 min or more.

Study 1

For each of the 3 unsloped platforms, the amount of runoff in the collection device was recorded every 10 min for 180 min following the start of each rain event. Figure 1 shows the cumulative runoff from the platforms for a selected representative light, moderate, and heavy rain event. The 3 rain events produced a total rainfall of 0.5 mm, 4 mm, and 10 mm, respectively, over the 180 min.

Study 2

For the 2 sloped vegetated platforms, the amount of runoff in the collection device was recorded every 10 min for 180 min following the start of each rain event. Figure 2 shows the cumulative runoff from the platforms for the same 3 representative rain events selected in Study 1.