One of the main benefits of your green roof is that it is a rainwater processor that automatically waters the green roof plants while slowing down or preventing roof runoff. According to the U.S. Environmental Protection Agency storm water runoff is a major polluter of rivers, lakes and streams. Especially in cities, roof runoff is a major problem that carries grease, oil, gasoline, chemicals and other pollutants into nearby bodies of water.
In studies done by the University of North Carolina, two extensive green roofs retained more than 60 percent of the precipitation during the study period. Penn State University studied the runoff from an extensive green roof, compared to a standard roof, and found the green roof only shed 25 percent of the water from a 1-inch rain. The rest remained in the roof to be used by the plants. Researchers concluded that the green roof “is thus a very effective storm water management tool and can be used to reduce the impact of new development on overburdened municipal storm water systems.”
While there is speculation that your green roof will filter rainfall and make the water cleaner, studies have shown the opposite may be true especially when it comes to nitrogen and phosphorous pollution. In the study referenced above at North Carolina University, higher concentrations of total nitrogen and total phosphorus leeched from shallow extensive green roofs than from standard roofs. The study concluded there was “no improvement of water quality in the green roof runoff when compared to the rainfall and the control roof runoff.”
In the study at PennState University referenced above, runoff from non-green roofs had nitrate concentrations in pounds per acre that was four times higher than green roofs, but the study cautioned that was most likely because the non-green roofs shed four times more water. These Pennsylvania research roofs were also four years old, while the roofs in the research at North Carolina University were a year old. PennState researchers assumed the nitrogen in the initial growing medium was used up or already leeched out and that explained the lower amounts in the green roof runoff.
Another factor is the amount of nitrogen naturally occurring in the rainwater in a given area of the country. Where the nitrogen is higher initially, higher concentrations are expected in the runoff. Other research in Germany suggests that controlling nitrogen and phosphorus runoff, and filtering other contaminants from the runoff, may simply be a matter of using the right growing medium and selecting the right plants.
That lines up with other research in the U.S. that addresses the growing medium as an integral part of storm water management and cleaner runoff. There is evidence that your green roof, when fitted with a cistern that will hold a water volume that is 5 inches higher than that of the total green roof area, can be a highly efficient storm water treatment system. Researchers at the University of Central Florida recommend using an expanded clay growing medium with a pollution control media below it, and then channel all runoff into the cistern. Not only will the runoff be reduced, but nitrogen and phosphorus loads in the runoff will also be reduced.
Mounting evidence confirms that, with the right plant-substrate mix, your green roof can be an efficient and clean rainwater processor.