Green roofs can absorb precipitation, allowing water that would otherwise flow off a roof and contribute to urban runoff to be captured and taken up by plants. In this way, green roofs help both to reduce stormwater quantity and improve stormwater quality, and can be especially beneficial in dense urban areas where land isn’t available for detention ponds, bioswales, and other stormwater treatment structures.
Because green roofs contain soil and vegetation that infiltrate and intercept precipitation, they can reduce the amount of stormwater that runs off a roof, into roof gutters, and onto city streets.
Factors that affect the ability of a green roof to retain stormwater include the duration of the storm, how soon the storm occurred after the previous one, and the thickness of the growing substrate or soil. Small to moderate storms are very effectively managed by green roofs in many studies. However, when the substrate is shallow (1 to 2 inches) and a large rain event is preceded by rain or irrigation of the plants on the green roof, the roof will capture much less precipitation, leading to more stormwater runoff.
When green roofs have growth substrates of moderate depths (4 to 5 inches), they become very effective tools for managing storm water. But research shows that green roofs must also be designed to respond to seasonal rainfall patterns in different regions. If a region experiences wet winters and dry summers, for example, a different substrate composition may be needed than in regions with wet summers and cold winters during which plants are dormant.
There are also variations between monolithic (continuous) and modular (containerized) green roof systems. Some modular systems have been found to be less useful than monolithic systems at capturing stormwater. But modular systems that are expressly designed to retain stormwater can be very effective.
One reason why monolithic green roofs do a better job of capturing stormwater is that the water moving through the roof is in contact with plants all along the drainage path. With modular green roofs, in contrast, once water drains underneath an individual modular container, it passes underneath adjacent containers and isn’t available 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. This is because a drop in runoff volume also means a reduction in the mass of contaminants that the stormwater can transport.
In addition, green roofs can improve stormwater quality because the soil or growing medium may be able to adsorb certain contaminants in urban air and precipitation. For example, heavy metals such as zinc (Zn), copper (Cu), and lead (Pb) can all be removed from stormwater to some degree by the soil’s natural ability to retain them. A green roof under study in Connecticut, for instance, retained 65% of the Zn and 100% of Pb in precipitation over a period of five months. The scientists attributed the retention to soil processes in the green roof growing medium, which was made of lightweight, expanded shale and composted biosolids.
The plants in green roofs can also take up contaminants—especially some nitrogen-bearing contaminants common in urban atmospheric dust and precipitation—since nitrogen is an essential plant nutrient.
Green roofs as potential sources of contaminants
While green roofs can remove contaminants from urban stormwater, they can also be possible sources of contamination. This occurs when the growing medium includes chemical contaminants that are easily picked up by rainwater. It can also happen when the roof is fertilized and excess fertilizer makes its way into runoff. Several studies suggest, for example, that green roofs can be a source of the nutrient, phosphorus, in runoff. This effect is most likely associated with the phosphorus in composts or biosolids used in growing media, or in fertilizers. Copper is also found in green roof runoff if copper sulfate-bearing fertilizers are used.
Research continues on ways to improve the ability of green roofs to remove contaminants and improve stormwater quality. For example, research has shown that use of expanded shale in the growing medium can increase contaminant removal from precipitation that falls on the roof.