Shaw Library in Washington, D.C. is one of many buildings in the city that has a green roof.
Up a flight of utility stairs at Sibley Memorial Hospital in Washington, D.C., and out through a normally dead-bolted door, is a 14,000 square foot green roof.
"It's very much like a garden on a roof," says Mike Lucy, with the Anacostia Watershed Society. "These plants are different types of sedums. They evolved and grew in high alpine conditions so they do well in this kind of roof environment where they're planted in expanded stone, basically slate and shale."
The plants are all very pretty, kind of like smooth cacti, but they serve a very real purpose.
"Green roofs can actually control storm water," says Milind Khire, a professor of geo-engineering at Michigan State University.
When rain hits a parking lot or a regular roof, it's funneled down to a drain, then a gutter, and then some creek somewhere where it scours the sides of soil and dumps all kinds of motor oil and grease and even air pollution into someplace, such as the Potomac. When water hits a meadow, the water just gets absorbed. A green roof is more like a meadow, and it's good insulation.
"A second purpose it can serve is a cooling effect because the material has good insulation properties, and basically absorbs a lot of heat," says Khire.
There are 2 million square feet of green roofs in D.C., more than anywhere in the country besides Chicago. Lucy says they're significantly more durable than regular roofs.
"In general, they say it extends the life two to threefold," he says. "There are green roofs in England that are 100 years old."
Once they're established, they're largely maintenance free, he adds. The roofs don't come cheap — they cost between $10 and $28 per square foot — but in D.C., the District Department of the Environment's green roof rebate program will pay up to half the cost.
Sibley Hospital is undertaking its green roof to make its construction as green as possible, says Sibley's Greg Ward.
"I think in this day and age, it's more, why wouldn't you do it, especially with the rebate programs and the environmental impact that you have," he says. "We do care about making this a nice place to live."
Map: Green Roofs and Combined Sewers in D.C.