Think of it as a toupee for your home: the green roof.
Instead of shingles, metal or tile, a green roof is covered with soil and low-growing plants, such as flowers and drought-resistant grasses—or for the very ambitious (with a very sturdy roof), vegetables and herbs.
Green roofs, which are common in Europe and are growing in popularity in the U.S., can reduce water runoff and contribute to the neighborhood's biodiversity, inviting butterflies, bees and birds to stop by.
Plants also make great insulation. For example, when the outside temperature is in the 80s, a green roof on a one-story building can lower the temperature inside the house by as much as 10 degrees.
However, before you break out the ladder, there is more to a green roof than throwing down some sod and grass seed. (In fact, you may want to consider hiring a licensed contractor, which has the economic benefit of pumping up the green jobs industry.)
There are two primary types of green roofs: extensive and intensive. Extensive roofs are low-maintenance, with up to 6 inches of soil and short, horizontal growing plants, like carpet. Since these roofs are not intended for vegetable gardening, they can be sloped.
Intensive roofs are almost like a separate floor of the home, and are best built on flat roofs. The soil is much deeper—at least a foot—and it is expected that people will walk on the roof to weed, harvest vegetables, etc.
You'll need to install layers of waterproofing, insulation and drainage material; we're talking about a green roof, not a green living room ceiling. It's also best to consult with a structural engineer or architect to make sure the roof can handle the weight of the soil and plants.
Initially, like any landscape, the roof will need to be watered to establish the plants. Then it will need watering using drip irrigation only in extreme drought, especially if you use drought-resistant plants. (It makes little environmental sense to have a green roof if you have to waste water to keep it alive.) And obviously, avoid flammable plants, those that are especially waxy or twiggy.
Now the moment you've all been waiting for: the cost. An extensive roof costs $8-$20 a square foot; an intensive roof, $25 a square foot or more, depending on the complexity. Some states offer tax credits to help offset some of the costs. If passed, federal legislation would offer federal tax incentives. Information on green roofs is abundant on the Internet. See Great Lakes WATER Institute Green Roof Project for general info and OpenCongress for the bill introduced in the U.S. Senate, SB 320.