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FEATURE ARTICLE - JULY/AUGUST 2007

Is a Green Roof Right for You?

We give you tips for deciding to go green with your roof, plus more ways to put your roof to work for the planet if a "green roof" isn't right for you.

The sound of the rain pattering on your roof during a summer storm may also be the sound of a missed opportunity—for you and for the planet. Increasingly, homeowners are discovering the benefits of “greenroofing”—or covering a flat section of their rooftops with an expanse of small plants growing in a few inches of engineered soil. In addition to reducing household cooling and heating costs and extending the life of the roof, green roofs also assist with a host of urban environmental problems—they filter stormwater, help to cool and clean city air, and help prevent flooding. And, they add a cheerful touch of greenery that
can be appreciated by those who look down on the roof from taller homes and buildings.

Green Roofs Defined
The design of green roofs are much more than meets the eye—from above, you might see an expanse of growing media or pebbles dotted with patches of small plants. But the real action takes place in the layers between the leafy surface and the roof. (We’re referring here to an “extensive” green roof, with two to six inches of growing media and small plants, as opposed to an “intensive” green roof, which has several feet of growing media and much larger plants.)

An extensive green roof is placed like a rug over a swath of roof on which people don’t walk much. Moving from bottom to top through a slice of green roof, you’d find a special membrane covering the roof itself. This bottom layer is either a hot-applied rubberized asphalt or a cold-applied layer of synthetic rubber, which, in combination with a root-repellant material, is designed to block moisture and roots from damaging the roof. A drainage layer of pebbles or a geocomposite drain mat lies on top of the membrane; a filter cloth lies atop the drainage layer. The top, visible layer of a green roof is the several inches of a growing medium, which hosts a crop of hardy low-lying plants, like sedum, chives, talinum, and delosperma. These sandwiched materials provide a natural sponge and filter for rainwater, and protection for the rooftop itself.

An extensive green roof is super low-maintenance. The drought-resistant plants used on these green roofs do fine with rainwater and don’t need supplemental watering after establishment, except in extreme conditions. They usually require weeding once or twice a year.

Benefits for You
Green roofs can save homeowners on cooling and heating costs. The leafy cover of a green roof helps cool the air through evaporation, by providing shade, and by forming a more lightly colored surface than the dark roof underneath. In the summer, a house wearing a green roof can keep cooler than a house with heat-absorbing black roof tiles—thereby using less energy on air conditioning. During winter, the insulation provided by the green roof can also help lower heating costs.

A green roof can help to reduce noise in your home, and the protection offered by a green roof may more than double the life of your home’s existing roof. Some homeowners with new roofs topped with green roofing have been able to negotiate especially long 20-year warranties for this reason, says Linda Velazquez, editor of Greenroofs.com.

Benefits for the Planet
Green roofs are also a great solution for the environment. Urban waterways become polluted in part because falling stormwater runs off nonporous sidewalks, roofs, and parking lots and directly into area waterways. Green roofs can absorb up to 90 percent of the rain that falls onto them. Their layers filter that water, removing pollutants before the water continues on its way to streams and rivers. By delaying the rush of stormwater into sewers after a rainfall, widespread green roofs can also help prevent flooding. The plantings on green roofs help absorb airborne toxins and carbon dioxide as they photosynthesize, and can provide welcome habitat for birds.

In the summer, a home with a green roof can keep city air cooler. The expanse of dark surfaces in heavily-developed areas are to blame for the “urban heat island effect,” in which many cities are two to ten degrees Fahrenheit warmer than the surrounding countryside, exacerbating smog (which forms more readily at higher temperatures) and driving up air conditioning costs and energy use.

The City of Chicago has undertaken a massive investment in greenroofing, beginning with a 20,000 square foot rooftop garden on Chicago City Hall, with the goal of reducing the heat island effect in America’s third largest city. An energy study estimated peak demand would be cut by the equivalent of a small nuclear power plant if all of Chicago’s roofs were greened, according to Weston Design Consultants, because more Chicagoans could give their air conditioners a rest.

Is a Green Roof Right for You?
Would your home or building be a good candidate for a green roof? If your roof is flat or no more than 30 degrees sloped, and in a sunny location relatively unshaded by trees, then you might be able to greenroof your home. Because green roofs weigh more than conventional roofing, you will also need to ensure that your home can support the added weight of the soil after a rainstorm—about 20 pounds per square foot. (Check out the checklist offered by DC Greenworks, to explore whether your home is “green roof ready.”) The cost of residential greenroofing generally ranges from $12–$35 per square foot, and should be installed by a professional. Look for an installer who brings both green roof experience and training from Green Roofs for Healthy Cities, which is currently developing an “Accredited Green Roof Professional” credential.

If you’ve been thinking about going solar at home, you may be wondering which is better—greenroofing or solar panels? While green roof plants obviously cannot grow in the shade of a solar panel, some homeowners still might be able to do both, installing a greenroof on a flat section of roof and solar panels on an angled section.

But if you have to make a choice, solar panels are better for the planet, since they can help you encourage the green energy future and cut down your global warming emissions by a third. Solar systems can cost from $25,000 to $45,000, so for those who don’t have that kind of money, greenroofing is a good second choice. Green roofs generally cost around $2,000.

Even If You Can’t Greenroof ...
If the place you currently live isn’t a good candidate for a green roof, there are a number of things you can do to “green” your rooftop—see the box below for some ideas. And you can promote the benefits of greenroofing in your community by asking your neighborhood library, school, or congregation to consider greenroofing.

Next time the summer rain is drumming on your rooftop, decide to put those raindrops—and that space—to better use: cool the air, clean the water, and support plant life with a living
green roof.

—Joelle Novey