 "Photo: Matty Greene")
The grass is sometimes greener on the other side of the roof.
Green roofs are comprised of plants and soil rather than asphalt, gravel or shingles. But the environmentally friendly roofs vary in effectiveness, according to a study from the Lady Bird Johnson Wildflower Center.
Green roofs use plants and either soil or a man-made growing medium to cover and insulate homes, according to the center's Web site. The roofs include waterproofing and drainage layers.
Roofs from different manufacturers differed in their effectiveness at retaining rain water and conserving energy when the Wildflower Center tested them, said Mark Simmons, the UT ecologist who led the study. The type of planting medium, which ranges from soil to man-made substances, used on the roofs caused much of the difference.
Even less-effective green roofs make a difference, though. Compared to regular roofing material, green roofs do a better job of insulating buildings and lowering interior temperatures and energy costs, Simmons said. Green roofs also reduce rain runoff onto streets and in storm drains.
"A lot of cities are very interested in [controlling storm water], especially those prone to flash flooding," said Simmons, the Wildflower Center's curator. "It's one of the biggest draws of green roofs."
The Wildflower Center Web site includes a page in which visitors can view the current temperatures inside experimental boxes with different roofs, which receive heat from the sun. As of 5 p.m. Monday, the box with a green roof had a surface temperature of 93 degrees and an internal temperature of 108 degrees; the box with a traditional black roof, meanwhile, recorded temperatures of 156 and 133 degrees, respectively.
The rooftop temperatures of regular roofing material contribute to an urban "heat island effect," Simmons said, which causes urban centers to be 2 to 10 degrees hotter than outlying areas.
In addition to practical benefits, green roofs can also be aesthetically pleasing, said Fred Evins, Austin's redevelopment project manager. Evins was heavily involved in the design of the Austin City Hall building, which features a green roof.
"We were looking for sustainable strategies that would provide public spaces ... as well as landscaping that would help with the urban heat island," Evins said. "It also reflects Austin's love of nature and embraces the Hill Country."
The building's roof is irrigated by an underground stream the building team discovered while working on the parking garage, Evins said. Green roofs include native plants, trees and shrubs, which can provide animal habitats.
Austin City Hall earned a gold certification for Leadership in Energy and Environmental Design, which is distributed by the U.S. Green Building Council. The building was also recognized this summer with a 2008 award of excellence in intensive institutional design from Green Roofs for Healthy Cities.
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