Gardeners are always looking for new planting sites, but a college research team 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.

“Europeans have been cultivating vegetation on rooftops for decades,” says ornamental horticulturist David Beattie. “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.”

Horticulturist David Beattie is designing a roofing system that uses natural vegetation to reduce scorching roof temperatures. Inset: Turfgrass or sedum is planted in holes filled with growth medium.

Beattie and colleague Robert Berghage 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 porous expanded polypropylene (PEPP), a compressed plastic mat that can be used as a growth matrix or surface insulator for the vegetation layer.

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 last summer, the researchers measured temperatures on a gravel roof, a plastic-sheet roof, and a planted roof prototype. The temperatures on a sunny day with ambient temperatures around 88º Fahrenheit measured 140º F on the plastic sheeting, 118º F on the gravel roof, and 82º 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 can reduce the amount of energy 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 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 or extensive re-roofing projects. “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 tested different plant materials and explored how Pennsylvania’s winters affect green roofs.
In addition, they are tracking 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 a 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.”

—John Wall