Green roofs are all the rage, and for good reason—they trap and filter storm water, provide habitat for birds, insects and small animals and bring color to urban neighborhoods. Covered with vibrant grasses and plants, green roofs provide insulation in winter and reduce heat during warm summer days. Chicago has ambitious goals to cover over 2.5 million square feet with greenery on over 200 rooftops; in Germany, they account for an estimated 10 percent of all flat roofs. Installation varies from laying out simple pre-planted modules to setting up more elaborate multi-layered systems. While it can be twice as expensive as a conventional roof, a green roof is cheap to maintain. Homeowners may even be eligible for utility rebates and state- or city-government incentives. What's more, they may double your roof's lifespan by shielding it from sunlight, wind and extreme temperatures. A quick note: Your roof will need to be able to support at least 30 pounds per square foot, depending on the system used, and have a slope of no less than one percent and no greater than seven percent. "From the perspective of increasing biodiversity, green roofs are a success," says Amy Lesen, assistant professor of biology at New York's Pratt Institute. But if it's measurable energy savings you're after, you might consider solar panels. "Studies show that payback [from a green roof] in terms of energy savings can take up to 200 years," notes Chris Benedict, an architect specializing in energy-efficient design. Electricity-generating photovoltaic (PV) solar roofs significantly reduce household carbon-dioxide emissions, and typical systems can last up to 30 years, adding to your home's value. Installation is easiest when putting on a new roof and should be done by a licensed electrician with experience putting in solar panels. Your service provider can help set you up with "net metering" to offset any energy you buy with that you produce. PV roof panels are sold in "per kilowatt" increments, with four to five kilowatts covering about half the average home's energy needs. While installations run from $8 to $10 per watt, costs and payback time will depend on electric rates, state incentives and climate. Federal tax credits of up to $2,000 are now available, along with a variety of local credits; to find out what's available in your area, see www.dsireusa.org. However, breakthroughs in panel efficiency that promise to reduce costs aren't yet available to consumers, and, given the current high prices, you may want to concentrate on personal energy conservation before investing in photovoltaics. But if you do have the resources, consider combining PV panels with a green roof. According to the Scandinavian Green Roof Institute, shade provided by the panels can even encourage more biodiversity. You and the environment just may find yourselves in a win-win situation. Resources Solar Living Institute (www.solarliving.org) Solar Estimator (www.findsolar.com) www.Greenroofs.com