With the recent heat wave, Philadelphia's urban landscape has been baking hot, leaving everyone desperate for the slightest hint of relief. A handful of buildings around the city and region are a little cooler than their neighbors, because they use a sun-sheltering technique known as "green roofs."

One example is PECO's Center City headquarters, shown here, where the lower part of the building roof is covered with vegetation instead of heat-absorbing asphalt. The greenery helps keep the surface cooler – some 60 to 90 degrees lower than the scorching 150 degrees commonly found on summer rooftops. On a recent day when the temperature hit 95, visitors to the green roof at PECO found it was noticeably more comfortable than the street below.

Even more important, a green roof helps absorb storm water before it cascades into the streets and overloads Philadelphia's outdated storm drainage system, much of which steers the runoff into the sanitary sewer lines. A big rain or melting snow can flood sewage treatment plants and flush pollutants into local rivers.

Each acre of green roof cuts water runoff by about 1 million gallons a year, according to the Philadelphia Water Department. Green roofs are a small but important part of the city's plan to avoid those pollution episodes without spending billions of dollars to install separate storm drainage lines throughout the city. Green roofs also help capture airborne pollutants we'd otherwise breathe in.

PECO shows off its green roof on regular tours, in hopes other Philadelphia building owners will follow suit. Its investment wasn't just an exercise in corporate do-goodism – a green roof can more than double the useful life of a standard covering, as plants and soil shield the roof from the elements. Philadelphia businesses can also get a break on the city's standard storm water discharge fee.

Green roofs are definitely catching on. At last count, there were 52 in the city, covering 10.6 acres. They can be found at St. Joseph's University, the Philadelphia Free Library, the Society of Friends Center, Urban Outfitters headquarters, and Drexel University dorms, among other sites. Pedestrians can see a ground level example recently installed on the bus shelter at 15th and Market. (See video.)

Green roofs are a building technique that harks back to sod roofs on the American prairie. They have been used for more than 100 years in places like Germany. In the quest to save energy, clean up water and air pollution, and green up urban environments, this old idea has new promise.