As a heat wave blankets much of the nation, the combination of concrete and asphalt can make urban jungles hotter than rural areas by 2 to 10 degrees. But cities around the country are trying to cool off by replacing their concrete and planting gardens on their roofs.

Green roofs -- roofs partially or completely covered by soil or vegetation -- have long been popular in Europe and are increasingly cropping up on buildings in major cities across North America.

It takes only a few inches of soil to grow an extensive green roof of ground cover and grasses. An intensive green roof, though, where shrubbery and trees grow, requires a minimum of 1 foot of soil depth.

The rooftop gardens are mostly credited with reducing the pollution carried in storm water runoff and limiting the heat island effect that stems from urban areas emitting heat, but some environmentalists claim the roofs can also cool down buildings in the summer and save money on energy bills.

The thinking is that the vegetation helps insulate a building in both summer and winter, alleviating the stress on air conditioners and heaters.

"It's a small contribution, but it's real," says Steve Skinner, Garden Roof Product Manager at American Hydrotech Inc., a roofing and waterproofing manufacturer in Chicago. He says the savings only amount to about 10 percent of an energy bill during the hot summer months.

Putting in a green roof can typically cost 2½ times more than a regular flat roof, Skinner says. For a roof with 1 to 5 inches of soil depth, though, the cost is more -- an estimated $8 per square foot, while a traditional roof costs about $1.25.

Still, he says the demand for green roofs has ballooned in the past three years, and the company is considering almost 900 different projects across the country.

Jennifer Sprout, director of local market development at the group Green Roofs for Healthy Cities in Toronto, says there was an 80 percent jump in green roofs from 2004 to 2005.

Some cities have developed policies that encourage new buildings to be constructed with green roofs and existing buildings to be retrofitted with them. Chicago leads North America in green roof implementation, according to a survey by Green Roofs for Healthy Cities. Washington, D.C., New York City, and Des Moines, Iowa, are not far behind, the group says.

David Beattie, director of the Center for Green Roof Research at Pennsylvania State University, says the increase in green roofs is part of a current green revolution. He says the benefits of green roofs go beyond saving money on a summer energy bill.

Because green roofs are just catching on in the United States, he says there's still a lot of research necessary before anyone knows for sure how beneficial they are in conserving energy. Instead, he looks at the other, more obvious benefits such as absorbing air pollution, mitigating the effect of rainwater on sewage systems, and the cooling of urban heat islands.

Green roofs absorb more rainwater, which he says eases stress on cities' sewage systems and reduces water pollution. Green roofs are also credited with absorbing carbon dioxide and reducing some city noise, as well as providing an urban habitat for birds.

No overarching body yet exists that can estimate just how many roofs have gone green, and it's still considered a new technology in the United States. The major U.S. green rooftop groups and contractors have been operating only since the late 1990s, and experts say more research needs to be done about the potential benefits of green roofs.

"Green roofs are not the be-all and end-all. People need to understand that this isn't going to make your paycheck bigger," Beattie says. " Green roofs are about creating a better environment, a better quality of life."