Green roofs’ soil and vegetation can catch and hold precipitation. This means that green roofs can be an important aid to reduce the amount of stormwater that runs off onto city streets.
Factors that affect the ability of a green roof to retain stormwater include
- the duration of the precipitation;
- how soon the precipitation occurred after the previous one; and
- the thickness of the growing substrate or soil.
Many studies show small to moderate storms are very effectively managed by green roofs. But as you can imagine, there are challenges. Shallow substrates (1 to 2 inches) can only hold so much water. If there is irrigation before a rain event, or even a recent storm, the substrate can be challenged. The roof will capture much less precipitation, leading to more stormwater runoff.
Green roofs with substrates of moderate depths (4 to 5 inches) are very effective tools for managing stormwater. In fact, the depth of growing media has been shown in most studies to be the most important factor in determining how well a green roof intercepts rainfall. On average, green roofs retain about 45 to 60% of rainfall that hits them, with intensive roofs holding more rainfall than extensive roofs.
There are also variations between monolithic (continuous) and modular (contained) green roof systems. Modular systems that are expressly designed to retain stormwater can be very effective. However, it’s usually monolithic green roofs that excel at stormwater management. Water moving through a monolithic green roof is in contact with plants all along the drainage path. In contrast, with modular green roofs, once water drains out of an individual modular container it passes underneath adjacent containers and is unavailable to other plants before leaving the roof.
Simply by decreasing the volume of urban runoff, green roofs can also reduce the level of contaminants carried in stormwater. A drop in runoff volume means a reduction in the mass of contaminants that the stormwater can transport.
In addition, green roofs can improve stormwater quality. The soil or growing medium may be able to hold back certain contaminants in urban air and precipitation. For example, heavy metals such as zinc, copper, and lead can be removed from rainwater to some degree by the soil’s natural ability to retain them. In one study, a green roof made of a lightweight, expanded shale and composted biosolids performed well. Research continues to find ways to build better green roofs that reduce stormwater volume and also improve its quality.
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