During a rainstorm, the roofs of city buildings act as funnels, channeling water down to the street and into the nearest storm drain. But it doesn't have to be this way. Roofs can serve as enormous sponges, too. This 3,000-square-foot garden atop Philadelphia's Fencing Academy absorbs up to a half-inch of rain, then filters and slows down the flow of runoff that does reach the street. The plants also insulate the building, reducing energy costs, and protecting the roof from wear and tear. Overall, this "green roof" is expected to cut total annual runoff from the building by more than half.

Back to introduction | Next page

Photo: Charlie Miller