The United States Environmental Protection Agency cites rainwater runoff to be a major contributor to pollution in our lakes, rivers, streams, and ponds. In all areas, but primarily in cities, storm runoff carries pollutants to these waters, including chemicals, grease, and gasoline. A green roof will process rainwater and use it to water the plants on the roof, thus depleting the amount of runoff.
Studies at the University of North Carolina and at Penn State University have shown a substantial drop in runoff following heavy precipitation, with one study showing only 25% of the waters from a one-inch rain running off the roof. Researchers in both studies concluded that sedum roofs are an effective tool in controlling runoff from storm water and the use of the living roof would have a wide spread positive impact on the use of municipal water and drainage systems.
Initially, it was thought that the plants and substrate would serve to filter the water that does run off, diminishing even more the overall polluting effects. However, in the North Carolina study cited above, researchers actually found more phosphorous and nitrogen in the runoff than from runoff that would exist in a conventional roof surface. The overall conclusion is that there is no measurable difference in pollutants in the runoff from green roofs than that from standard ones.
The Penn State study showed substantially higher amounts of nitrates in water from traditional roofs than from green roofs, but researchers observed that they sampled runoff from heavier rains than those studied from the green roofs. It is also important to note that the green roofs in Pennsylvania were three years older than those in North Carolina, and in Pennsylvania, it is likely that there were very few nitrates left in the growing medium at the time of the study.
Another artifact to consider when comparing results between these two studies is that rain in some geographical areas will naturally have higher levels of nitrogen than in others, which will affect the findings of the tests. Researchers in Germany are working to establish that they can control the levels of phosphorus and nitrogen in rainwater runoff by using specific types of planting medium and the right combination of plants.
This is in concordance with a study in the United States that looks at the planting medium as an important part of the process toward cleaner runoff. University of Central Florida researchers recommend using a clay medium with a medium below with pollution controlling properties. All runoff would be channeled into a cistern installed on the roof. This would reduce overall runoff, as well as the level of pollutants in what is left.
Studies continue to confirm that living roofs are effective in processing rainwater, and that with continued improvement, a green roof will be designed that will transport rainwater with less pollutants as well.