Bob Ehmer, National Geographic magazine's My Shot

Green roofs are an environmental and economical win-win: These aboveground gardens absorb the greenhouse gas carbon dioxide, combat the urban heat island effect by deflecting the sun's rays, divert harmful storm-water runoff, and can help lower a building’s heating and cooling costs.

Living roofs are gaining serious traction in big cities like Chicago, where city hall wears a green roof. But homeowners and building managers anywhere can also join the green-roof revolution—without forking over too much dough.

First, you'll likely need to hire a landscape contractor for the project—nearly all green roof experts discourage DIY.

The first question to ask your contractor is how much weight your roof can hold, according to Bruce Dvorak of the Conservation Design Forum, an Illinois-based, environmentally friendly design firm.

Extensive roofs have a soil depth of 1 to 6 inches (2.5 to 15.2 centimeters) and can handle a weight load of 15 to 50 pounds (6.8 to 22.7 kilograms) per square foot of living roof.

Intensive roofs have a soil depth of 6 to 24 inches (15.2 to 61 centimeters) and hold 80 to 150 pounds (36.3 to 68.1 kilograms) per square foot.

Extensive projects may cost as little as U.S. $8 per square foot, while intensive green roofs tend to start at $15 per square foot.

The roofs' energy savings and longevity—green roofs can last up to twice as long as conventional shingled roofs—will eventually make up the difference in the cost of installation, experts say.

Flat roofs are typically easier and less expensive to work with than sloped roofs. Homeowners with steep sloped roofs may spend more on a baffle system to hold the soil in place.

(See green roof pictures.)

The DIY Version

If installing a full-on living roof is not an option because of accessibility or cost, Chris Dunn of the Peggy Notebaert Nature Museum in Chicago suggests container gardening and window boxes as a low-cost first step.

Dunn, whose museum has a green roof, recommends collecting rainwater that runs off of the roof to irrigate containers and boxes as a small storm-water prevention measure.

Another level up from container gardening is setting up trays filled with sedum plants on a flat roof. Sedums are hardy leaf succulents that usually grow close to the ground and are often used in green roofing.

Some nurseries, such as MotherPlants, specialize in drought-tolerant, shallow-rooted species suited to extensive and semi-extensive living roofs.

If nurseries are too pricey, you could follow in the footsteps of Green-roof blogger Matt Schley, who found a cheaper source of materials on eBay.

Schley said he spent $675 on trays, soil, plants, and pond liner as a root barrier to cover 200 feet of his New Jersey row house roof—less than he would have buying trays and plants from a green-roof nursery.

However deep you dig into green roofs, the environment—and your wallet—will thank you.

For more on gardening, check out Green Guide's Gardening page.