Green roofs -- plots of plants atop homes or commercial buildings -- are more than gardens in the air. They act as air-conditioners, solar collectors and rainwater diverters. So why don't more people do them?

The short answer is that they can be expensive, leaky if not installed correctly, and just-plain strange-looking in a traditional neighborhood of Colonial Revival houses.

But that didn't stop Eric and Mary Fisher from making one part of the second-story deck at the house Mr. Fisher designed for them in Shadyside. Though the house is surrounded on three sides by traditional homes, the living roof looks to its neighbors more like a patio garden than an overgrown roof. A rectangle measuring 41 by 14 feet, it is level with the decking and accessible from the couple's bedroom.

Gary Lichtenfels of Lichtenfels Nursery in Johnstown (www. lichtenfelsnursery.com) installed the nearly 575-square-foot roof using components made by Roofscapes Inc. of Philadelphia (www.roofmeadow.com). It cost about $10 per square foot or $5,750. Atop layers of waterproof membrane, protection/drainage, triangular French drains and 5 inches of mineral-based growth medium (not soil) grow seven types of sedum, chives (Allium schoenoprasum) and round-leaf rock flower (Talinum calycinum), all from Emory Knoll Farms and Green Roof Plants near Street, Md.

Sedums dominate because they are drought-tolerant, winter-hardy succulents that require almost nothing to survive.

"The best quality of the [succulent] plant is that it lives on existing rainfall. If you get into a lot of irrigation or maintenance, it just drives the cost of the roof up," said Edmund Snodgrass, Emory's owner and co-author of "Green Roof Plants: A Resource and Planting Guide" (Timber Press, $29.95). "Look for plants that can withstand drought, wind and all the things that make surviving on roofs more harsh. That points toward succulents."

Shop around for plants that are attractive and practical, Mr. Snodgrass said. Certain cactus varieties are rugged enough to make it in a thin, inorganic medium. And they don't go dormant, either, meaning they provide color all year.

If your plant material is 4 inches deep or more, then consider adding some herbaceous perennials -- Phlox, Dianthus, Campanula, Salvia and Potentillas, among others. Archibio, an architecture firm in Montreal, Canada, lists more than a dozen colorful flowering plants well suited to green roofs in its pamphlet "The Living Roof" ($3.50 at www.archibio.qc.ca/pages/book.html) They include 'Albys' Dianthus (white), 'Little Saphir' iris (pale blue) and 'Baby S' coreopsis (yellow).

Kitchen herbs can be a smart addition if you have a half-foot or more of soil depth. Grasses are good, too, although some may need to be mowed and irrigated, and they can become a fire hazard when allowed to dry, Mr. Snodgrass said.

He suggests using plugs, two per square foot. They'll grow into a vegetative roof in about a year and a half. The more juvenile the plant, the better, he said.

"They become more adapted to the roof environment."