Growing plants on top of buildings can be a good thing
How can a residential or commercial roof be modified to provide environmental benefits?
Most buildings are designed to shed rain and so are built with hard, impenetrable roofing surfaces. As a result, rainwater bounces off and collects as runoff, picking up impurities — including infectious bacteria from animal waste, as well as harmful pesticides and fertilizers — on the way to municipal storm sewers, which in turn eventually empty out into local bodies of water.
Minimizing this run-off means that more impurities will remain in local soils, where they can be broken down more easily into their constituent elements than if they are concentrated downstream. To achieve this goal, landscape architects have developed so-called green roofs, which involve the placement of living plant matter and soil on top of a building absorb, collect, and reuse rainwater while preventing run-off. There are abundant uses for the water green roofs collect, from watering exterior plantings at ground level to flushing toilets inside.
According to Steven Peck of the Toronto-based nonprofit Green Roofs for Healthy Cities, green roofs can play an important role in maintaining ecological integrity within otherwise paved-over areas. "The roofscapes of our cities are the last urban frontier — from 15 percent to 35 percent of the total land area — and the green roof industry can turn these wasted spaces into a force for cleaner air, cleaner water, energy savings, cooling, beauty, and recreation," he says.
The U.S. Environmental Protection Agency encourages the creation of green roofs to mitigate the urban "heat island effect," whereby temperatures in crowded cities can soar some 10 degrees Fahrenheit higher than in less developed areas nearby. Other benefits, says the EPA, include providing amenity space for tenants (in effect, replacing a yard or patio), reducing building heating and cooling costs through the buffering effect of the plant matter and soil, filtering pollutants such as carbon dioxide out from air and heavy metals from rainwater, and increasing bird habitat in otherwise built-up areas.
In addition to going all out to build a "living" green roof, builders can use certain inorganic materials to make an existing roof greener. The nonprofit Cool Roof Rating Council, for instance, suggests roofing surfaces that reflect the sun's heat so as to reduce the urban heat-island effect while improving residential energy efficiency. According to the group, "a cool roof reflects and emits the sun's heat back to the sky." Builders can check out the CRRC's Web site for a database of information on the radiative properties of various roofing surfaces to help them make the smartest choices for their clients and for the environment.
Another quality that makes certain roofs greener than others is how long they last. Metal roofs are known to be relatively maintenance free and last longer than shingles in most situations. Slate roofs also have an excellent reputation for long life, although getting work done on them can be expensive when they do need repairs. The Slate Roofing Contractors Association reports that sea-green slates can last anywhere from one to two centuries, depending on where the slate is quarried and how well it's eventually installed.
For more information: Green Roofs for Healthy Cities, www.greenroofs.org; CRRC, www.coolroofs.org.
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