Green Roofs

Looking out his hotel window on a gray and rainy day, all David Beattie could see was a flat gravel roof. A heating duct stuck out of it. Otherwise it was just gray, a grimy-looking gray.

white house with green roof on green grass

It might be green, he thought. Green and growing, like the roofs he'd seen in Germany. Perhaps with a mosaic effect, a pattern made of plants and a red-gravel path for maintenance. Bettering the view would make the hotel rooms more attractive. Being green was good for the roof too—the plants would insulate it, protect the roof from freezing and thawing, cracking and leaking, and make it last longer. And last, a green roof could lessen the heating and cooling costs of the hotel lobby down below.

Back at Penn State, Beattie, an associate professor of ornamental horticulture, began to look into the idea. It's not a new idea. Says Beattie, "In Scandinavia and the British Isles, it's rooted in history. My grandfather was born in a sod-roofed house in Ireland. He remembered seeing goats grazing on the roof. Today, it's promoted by the Germans more than anyone else." Since 1989, Beattie learned, Germans have constructed on the order of 34 million square meters of green roofs. "They believe it's part of their duty, just as recycling is, to use as little energy as possible." It's also less expensive. "There are real economic benefits to green roofs," he says. "In this country, we just haven't gotten to the point where energy costs matter."

In collaboration with Robert Berghage, an associate professor of ornamental horticulture, Dan Sterns, an associate professor of landscape contracting, both at Penn State, and Robert Herman, a horticultural consultant in Connecticut, Beattie is attempting to "recreate" the German success in Pennsylvania. He envisions a roof built in three layers: First, seal the building's top with a vapor barrier, as usual. Then, instead of gravel, scatter a layer of drainage material to hold water for the roots (his current prototype uses a type of clay chips imported from Germany). Cover that with plant-growing modules that lock into place like ceiling tiles.

To develop these modules, the Penn State team is working with JSP International, a company based in Malvern, Pa. JSP International manufactures a molded plastic material called PEPP that is used mainly for automobile bumpers. "I can grow plants on this in the greenhouse, turn it on its side, ship it to Pittsburgh—or anywhere—and lay it on a roof," Beattie says. He envisions molded joints that will lock together and a frame to weight the whole structure down.

So far, Beattie has grown hard fescue, a slow-growing grass, and sedum, a succulent related to cactuses, in ground-level plots using this technique. Both are hardy and need minimum maintenance. "At one point, the plots were all covered with flowers," he says. "The bees from the entomology department were all here, hundreds and hundreds of them."

Temperature monitors showed that on a 90-degree summer day, the uncovered PEPP registered 145 degrees F, roof gravel was 119 degrees, and the roof plants were 82 degrees. To find out if that cooling is felt in the building below, Beattie needs to grow an actual green roof. With the help of a local company, R. H. Markon Inc., he is in the process of reroofing a 5,000-square-foot storage building on campus, turning a gravel-topped eyesore into a garden.

David J. Beattie, Ph.D., is associate professor of ornamental horticulture in the College of Agricultural Sciences, 102 Tyson Bldg., University Park, PA 16802; 814-863-2263;

Last Updated May 01, 2001