A roof that has been covered with a dense mat of growing plants is called a Green Roof or a Living Roof. With a Green Roof, a waterproof membrane is installed on the roof, which then is covered with about 4 inches of soil and planted tightly with low-growing succulent and alpine plants. The effect is astonishingly beautiful, especially when the plants flower. Succulents are the plant of choice for this technologically because they can tolerate hot, dry conditions, wind, and can absorb water from the air, which is an asset in D.C.'s sultry summer climate.
The
benefits of a living roof are great both for the environment and the wallet.
Having plants on your roof provides three major environmental benefits: First, a
Green Roof greatly reduces the heat island effect; the footprint of natural
space that a concrete, heat-trapping building removes from the ecosystem can be
essentially replanted—on the roof. 
Second,
the texture of the plants captures particles of pollution in the air, which then
wash down into the soil where they can be broken down and even reabsorbed as
fertilizer! In a sense a Green Roof scrubs the air. Finally, the water that hits
the roof in a storm will be largely absorbed and purified before it can run off
and pollute some nearby body of water, in this case, the Anacostia River.
The
economic benefits of a Green Roof are impressive too. First and foremost, the
waterproof barrier, soil and plant material on the roof provide considerable
protection to it, often extending its life two or more times over. To a certain
extent, a Green Roof can also serve as fire protection, since any hot ember that
lands on the roof from a nearby fire will be extinguished amid all the green
succulent plants and moist soil. 
The
Green Roof's insulation benefits and attendant cost-savings in heat and cooling
are quite promising, though as of yet unquantified in this region.
Green Roofs are extremely beautiful and can provide a needed, relaxing greenspace for apartment dwellers with access to the roof. Because they are can be an attractive common space, much like a park, they can facilitate community by bringing neighbors together. Green Roofs also dampen sound, which can provide urban residents a needed respite from noise pollution. Green Roofs are also relatively easy to install, so they lend themselves to the development of businesses that can provide jobs to relatively unskilled workers.
The Green Roof is also a technology that is perfect for the urban setting and addresses specifically urban environmental and economic ills. Our demonstration project at Matthew Henson Park sets a precedent in the city so that others can adopt the technology on their own roofs.
See photos from the community installation of D.C.'s first green roof!
More information about green roofs!
Rooftops: The Last Urban Frontier - DC Greenworks Green Roof Demonstration Proposal - In high-density commercial areas such as Downtown Washington, rain barrels, storage tanks, and other forms of storm water management and non-point source pollution prevention are neither structurally nor economically feasible. Green roofs are currently the only Best Management Practice applicable to high-density commercial urban zones. Check out why...
Growing Green on Top - Washington Times, December 5, 2002 - Interview with Dawn Gifford of DC Greenworks
Green Roofs: Stormwater Management from the Top Down - Environmental Design and Construction Magazine, Jan 15, 2001
Gardener on the Roof - E-Architect, The American Institute of Architects, October, 2000