It wasn’t long ago that the idea of planting a rooftop garden was dismissed as expensive, impractical, and even silly. Now, more than a generation after European pio-neers began making so-called green roofs fixtures of the German and Scandinavian landscapes, North Americans are finally tuning into the benefits of going green.
Green roofs, alternately called eco-roofs or vegetated roofs, have begun to sprout atop residential and commercial buildings across the continent. Chicago’s City Hall has one, as does the Gap headquarters in San Bruno, California. Guinness World Records recognizes the 10.4-acre green roof at the Ford Motor Co. truck plant in Dearborn, Michigan, as the world’s largest, a title soon to be claimed by the expanded Javits Center in Manhattan, where a 22-acre green roof is in the works. Escher GuneWardena Architecture incorporated a green roof into its winning design for the Dwell Home II in Los Angeles.
Advocates say that the flurry of green-roof construction has as much to do with the roofs’ practical benefits as with their aesthetic appeal. Equal parts roof garden, insulation system, and sponge, green roofs absorb storm water, outlast conventional roofs, and help make build-ings more energy-efficient, according to Alison Empey, a spokeswoman for Green Roofs for Healthy Cities, a Toronto-based advocacy group.
"Green roofs absorb pollutants in the air and release oxygen," Empey says. "They help improve air quality and help combat urban heat island effect," which occurs when asphalt and rooftops absorb and radiate heat. The concept isn’t a new one. The Hanging Gardens of Babylon, completed around 600 B.C., is perhaps the earliest example. Sod roofs have been common in Iceland for hundreds of years. Modern green-roof technology originated in the late 1960s and early 1970s in Germany, where they remain popular.
Tom Liptan, an environmental specialist with the city of Portland, Oregon, became an advocate in the mid-1990s after the city received a state mandate to significantly reduce the amount of raw sewage in its storm water runoff. Liptan, who had attended a lecture on green roofs given by a British environmentalist, installed one on his single-car garage as an experiment. He began with a waterproof membrane, added a thin layer of soil, and, ultimately, perennial sedum plants and grass. Tests that Liptan conducted revealed that the plants absorbed most of the rainfall, allowing the rest to drain slowly.
Liptan’s garage experiment eventually gave rise to Portland’s green-roof incentive program. The city offers grants of up to $5,000 for green-roof installation (from time to time the city also gives grants of up to $50,000) and gives "floor-area bonuses" to developers who incor-porate green roofs into their designs, allowing them to build additional square footage.
Green roofs generally fall into two categories, exten-sive and intensive. Extensive roofs, like Liptan’s, are lightweight and thin, usually weighing between 10 and 50 pounds per square foot. Self-regenerating plants, such as sedums, mosses, and meadow flowers, work well because they require little irrigation, fertilization, or maintenance. A typical extensive green roof costs between $14 and $25 per square foot to install.
Intensive green roofs are more elaborate. Soil depths typically start at six inches, allowing for a wider variety of plants, shrubs, and trees. Intensive green roofs can weigh 120 pounds or more per square foot and, because structures often have to be engineered to support the extra weight, can cost $40 or more per square foot to in-stall. In both cases, a root barrier, often a layer of dense polyethylene, is used to keep roots from penetrating the roof, while lightweight engineered soils help ward off unwanted insects.
Architect G. Mackenzie Gordon made an intensive green roof a focal point of the three-bedroom, 3,300-square-foot concrete house he built for himself in Lakeville, Connecticut. "Most of the roof plantings are evergreen shrubs and spreading junipers," says Gordon, a trained landscape architect. "I think it’s more interesting to have more variety. The shrubs that I picked are mostly frag-rant ones with varied blooming times. It’s a richer landscape than if you just put down one crop there."
Breathe Architects, a Toronto-based firm that specializes in renewable energy systems, is in the process of installing an extensive green roof on top of a 3,700-square-foot house for a family of four in Mono Mills, Ontario. The roof, which sweeps at a 12-degree angle over a carport, was engineered to carry up to 115 pounds per square foot, anticipating heavy snowfalls. Native prairie grass and wildflowers will provide the cover. Essential to the roof’s appeal, explains Breathe Architects’ Martin Liefhebber, is that it insulates the house, reducing the need for air-conditioning. Also, the soil and plants shield the roof from the elements, giving it a longer life expectancy than a conventional roof.
"Shingles are carbon- and oil-based and, of course, every ten years they’re essentially junk," Liefhebber says. "With a green roof, the membrane keeps the water out, but it’s underneath the soil where it stays shaded and cool, so you’re looking at a 40-year roof instead." Liptan says the long life span is appealing, but isn’t reason enough to make the installation of a green roof worthwhile. Instead, he prefers to think of green roofs as being greater than the sum of the benefits they confer.
"A green roof doesn’t give you enough bang for your buck as it relates to any one particular issue, when it comes to energy savings and things like that," Liptan says. "But when you quantify the benefits and add them all together, green roofs start to become very attractive. If you have the ability to do those calculations, you start to see that when you put it all together it really pays off."