The roof of a commercial building near Westmoreland Mall might seem an odd place to plant more than 6,000 young sedums.

But Greg Phillips, manager and chief executive officer of Westmoreland Conservation District, said he hopes the idea will take root and spread.

Mr. Phillips said the county's first "green roof" will help control storm-water runoff and conserve energy at the Hempfield building owned by the county Industrial Development Corp.

The 25-year-old GreenForge building, empty and gutted, will reopen in December as a home for the U.S. Department of Agriculture's Rural Development agency and conservation-related groups. It sits next door to another green building, the 125-year-old barn where Mr. Phillips has his office.

"We're trying to make this campus an alternative energy showcase," said Mr. Phillips, who recruited about two dozen conservation staffers and volunteers to plant 11 kinds of sedum yesterday on two flat sections of the GreenForge roof.

The campus also includes Donohoe Center, a home to government agencies that gets a small portion of its energy from a rooftop windmill.

Like the barn, the 23,000-square-foot GreenForge building will have a geothermal energy system, using water, two miles of tubing and 30 200-foot wells to draw heat--or coolness, depending on the season--from the earth. A sloped portion of the roof will have a white polymer coating as a sunblock.

"Traditionally, roofs have always been black or gray. The white coating, instead of absorbing heat from the sun, will reflect it back," said Jim Pillsbury, a storm-water engineer for the conservation district.

The sedums may be the building's most eye-catching feature.

The sedums, flowering perennials that grow about four inches high and eight to 12 inches wide, were planted in an absorbent soil mixture. Beneath that are a layer of rubber baffling, two layers of "filter fabric" and, to prevent leaks, a watertight membrane.

The plants and filter material trap and gradually release storm water that otherwise would tumble off the building and into the watershed, burdening the sewer system. The green roof insulates the building, helping to keep it cool in summer and warm in winter. The plants also absorb sound and purify the air by taking in carbon dioxide and emitting oxygen.

The roof costs $21 per square foot, perhaps $7 or $8 per square foot more than many other commercial roofs, said Tim Clement, area representative for the designer, W.P. Hickman Systems Inc. Besides the environmental benefits, he said, the green roof will last longer than others.

Phipps Conservatory and Botanical Gardens and Carnegie Mellon University's Hamerschlag Hall also have green roofs. Mr. Phillips said he wants developers to visit the Hempfield campus and consider incorporating green technology in their construction plans.

"I've always cared about the environment. I've always loved the outdoors," said Kristen Krempasky, a Westmoreland County Community College student who was cold and muddy from sedum planting. Volunteers also included nine members of the Botanical Society of Westmoreland County.