UNIVERSITY PARK, Pa. -- Gardeners are always looking for new planting sites, but a research team in Penn State's College of Agricultural Sciences is aiming higher by developing a system that would replace the tar-and-gravel materials used on flat roofs with a surface covered by living plants -- a green roof.

"Europeans have been cultivating vegetation on rooftops for decades," says David Beattie, associate professor of ornamental horticulture. "The roofs of large buildings -- flat roofs in particular -- are subjected to large swings in temperatures that cause cracking and eventually leakage into the floors below."

Beattie and Robert Berghage, associate professor of ornamental horticulture, are working with JSP International Inc., a Malvern, Pa., manufacturer of plastics used in automotive bumpers, to create a plastic-based roofing system that can be used as a planting site for vegetation.

The company uses PEPP, porous expanded polypropylene, which is a compressed plastic mat that can be used as a growth matrix or surface insulator for the vegetation layer.

For the past six months, Beattie and Berghage have tested prototypes planted with sedum, a drought-tolerant succulent plant variety that is related to cactus, and hard fescue, a grass. "We think sedum will be effective because some varieties stay short, don't shed a lot of dead plant material as winter sets in, and are easy to propagate," Berghage says. "The temperatures and winds on top of a building can be brutal, so the best plant material to use should be low-lying and need minimal maintenance."

In a series of tests this summer, the researchers measured temperatures on a gravel roof, on a plastic-sheet roof and on a planted roof prototype. The temperatures on a sunny day with ambient temperatures around 88 degrees Fahrenheit measured 140 degrees F on the plastic sheeting, 118 degrees F on the gravel roof, and 82 degrees F on the planted roof prototype.

Beattie sees several advantages to installing a green roof.

--Aesthetics: "A green expanse of lush vegetation looks great," Beattie says. "Even in winter, brown plants and soils will look a lot better than a grimy gravel roof."

--Durability: Using a vegetated roof eliminates the large temperature extremes and fluctuations that cause cracking and failure in flat roofs. "Europeans estimate that roof life can be doubled by using green roofs," Berghage says.

--Energy Savings: The insulation provided by an expanse of plants across a roof can reduce the energy load required to heat or cool the building. "Vegetation is a natural temperature moderator," Beattie says. "That's why it feels cooler in the middle of your lawn than it does in the middle of your driveway."

According to Berghage, green roofs also can alleviate the temperature rise associated with large metropolitan areas. "In big cities such as Washington, D.C., and Philadelphia, where asphalt, concrete and building materials absorb and radiate heat, the temperature can be 10 to 15 degrees higher than nearby suburban areas," he says. "The loss of green space in cities can be reduced by using green roofs."

Beattie says green roofs require more structural support than a conventional flat roof, but enough support for a vegetative roofing system can easily be built into new construction projects or an extensive re-roofing project. "You have to provide an irrigation and drainage system, which requires additional support," Beattie explains. "Also rain and snow would add weight to the soil."

This winter, Beattie and Berghage will test different plant materials and explore how Pennsylvania's winter conditions affect green roofs. In addition, they will track how, and if, heat is retained within a building outfitted with a green roof. Eventually, the research team hopes to get permission to install a green roof system on an existing Penn State building.

"Using these systems doesn't mean you won't have to pull a few weeds or irrigate occasionally, but that can be handled on a contract basis with the maintenance organization doing the building landscaping," Beattie says. "This isn't zero maintenance, but it is minimum maintenance."

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EDITORS: To contact Robert Berghage, please call 814-863-2190. To contact David Beattie, please call 814-863-2251.

Contacts: John Wall jtw3@psu.edu 814-863-2719 814-865-1068 fax