If the term "green roof" evokes an image of a few potted plants arranged tastefully on the top of a building, then the time seems ripe to rethink that definition. Green roofs may be the next hot trend to cool the urban landscape and lower the cost of controlling temperatures in the average suburban home.
Green roofs are generally categorized by one of two forms. Extensive green roofs, also known as eco-roofs or low-profile roofs, are made with a few thin layers of soil, are lightweight, relatively less expensive and require little maintenance. Extensive green roofs are the correct choice, the experts say, when the primary desire is for an ecological cover with limited human access.
On the other hand, intensive, or high-profile, green roofs look like traditional roof gardens because a much wider variety of plant material is usually included. They have soil depths ranging from 8 to 12 inches, with growth that can extend upward of 15 feet. They can include such architectural features as waterfalls, ponds and gazebos. Their construction and maintenance is much more costly.
New to the U.S.
Tristan Roberts, an executive with BuildingGreen in Brattleboro, Vt., says that though interest in green roofs within the U.S. has taken root only in the past decade, green roofs as a phenomenon have a long history.
"They go back for hundreds, if not thousands, of years," he says, "as sod houses in this country and as living roofs in Europe. The modern equivalent has been around for quite a few decades and is much more prevalent in Europe, particularly Germany."
BuildingGreen, which bills itself as an independent operation dedicated to distributing information to building-industry professionals and policymakers, sees green roofs as one way to improve the environmental performance and reduce the adverse impacts of traditional buildings.
Bill Retzlaff, the chairman of biological sciences at Southern Illinois University-Edwardsville and coordinator of research cooperative Green Roof Environmental Evaluation Network, or GREEN, says the green-roof industry in the U.S. is very new. "While Europeans have been doing them for a long time, our climate is different, so there's not much useful data on their performance here."
The GREEN cooperative was established two years ago to evaluate green-roof technology in the Midwest, Retzlaff says. In addition to the university, members include several commercial suppliers and Greenroofs.com, an international resource and online information portal for the green-roof industry.
"Our longest-running project has been on the ground since September 2005," says Retzlaff. "We now have seven specific green-roof projects at our field site. We're evaluating storm-water runoff and temperatures, comparing those results with the results we get with a flat membrane roof."
Though green roofs don't necessarily require a lot of care, it's a misconception to suppose that they will just take care of themselves. GREEN researchers are working with systems that don't require a lot of maintenance, but there's no such thing as a no-maintenance green roof.
"You must water them for the first 10 weeks or so," Retzlaff says. "They have to get established, just like any other garden. And at minimum, you need to add some plants, fertilize and weed a couple of times a year."
Before establishing a green roof, consumers need to have some sense of what they want from it beyond the vague notion that they're good for the environment.
"Everything from roof materials to plant varieties has to be picked for the issue you want to address – storm-water retention, more green space or thermal control," says Retzlaff. "Every green roof provides some of each of these things, but to decide you want a green roof because it's a good thing is not enough."
Benefits of lofty gardens
For example, Retzlaff cites a green roof in St. Louis on the seventh floor of a children's hospital that has a pond and large walkways.
"It costs $200,000 a year to maintain," he says. "But the benefit is that when those children go through those double glass doors leading to the green roof, they can forget everything inside."
Retzlaff says Ford Motor has a 10-acre extensive green roof in Dearborn, Mich., that remains the largest in the U.S. "It has a 2-inch depth, so they have to irrigate it. They catch storm water in retention ponds and irrigate with that."
In Chicago, Mayor Richard M. Daley is interested in lowering the urban center's heat-island effects that drive up temperatures. "He has been told by various research facilities that if about 65% of downtown buildings had green roofs, that would lower the heat by about 10 degrees," Retzlaff says. "So the city issues grants to offset the costs of installation and will fast-track building permits if a green roof is included."
Retzlaff expects GREEN to become a resource for would-be green roofers around the country. "Eventually, people will be able to contact us to find out exactly what growing medium and plants are best for their climate and purpose," he says.
BuildingGreen's Roberts says he sees more incentives -- and so more green roofs -- for commercial buildings than homes right now, but there are several benefits that could make them appealing to homeowners. They include increased insulation and the fact that the garden materials make a roof more durable.
"For homeowners, a green roof on a low slope or flat roof can extend the life of that roof many years by shielding it from rainwater and ultraviolet sun rays, which degrade roofing materials," says Roberts.
Green roofs also filter out some air pollutants, he adds.
Green roof solutions
In addition to collecting storm water, reducing urban heat and acting as insulation to cool buildings, green roofs are seen by advocates as opportunities to increase food production, beautify cities and provide sound insulation by absorbing or deflecting noise by machinery, planes and traffic.
For the average homeowner concerned with rising energy costs, it's likely the insulation qualities of a green roof would prove most appealing.
Dennis Yanez, the national marketing manager for Chicago's American Hydrotech, says his company offers waterproofing and all the components for garden-roof assembly, including plastic foam, soil and plants. American Hydrotech also offers a single-source warranty, which Yanez says is unique in the industry. Clients are primarily architects and developers.
"We have not done a lot in the consumer market," Yanez says. "We do have some homes that have our systems in them, but they're all higher-end, in the 7,000- to 10,000-square-foot range."
That's because a homeowner who wants a green roof, Yanez says, would have to start by hiring an engineer or architect to design it. He estimates the price of building a green roof from "the high teens to the low $20s" per square foot.
Greenroofs.com estimates costs at $9 to $25 per square foot for extensive green roofs and $25 to $40 or more for the intensive variety.
"We've seen they have a growing appeal," Yanez says. "In general, sustainability and green building has taken off in the past five or six years. Putting together buildings that disrupt the environment as little as possible is becoming a real concern."
By Cliff Bowden, Bankrate.com
