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A Grass-Roofs Effort - Brief Article
Sierra, May, 2001 by Wendee Holtcamp
Secret gardens conserve energy and cool the air
Imagine grass, wildflowers, shrubs, or even trees sprouting from your rooftop. Get as wild as you want--from meadowy plots to dynamic gardens stocked with waterfalls, trails, and benches. Far from a fanciful ecotopian dream, these modern versions of ancient sod roofs are now popular throughout Europe. Inspired by early Icelandic grass-topped homes, German entrepreneurs have been experimenting with different designs and weight loads for 35 years. Today, Germany installs over 10 million square feet of green roofs a year, helped in part by economic incentives and a strong national environmental conscience. The trend has spread across the continent, and green roofs now adorn homes, schools, factories, malls, and underground parking garages.
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An elevated refuge in a sea of concrete, the vegetation creates habitat for birds and butterflies, reduces storm-water runoff, and creates a weather buffer that helps the roof last twice as long as standard ones. But it is the green roof's capacity to cool ambient air temperatures and reduce energy demand that caught the attention of the EPA. In 1997, the agency initiated the Urban Heat Island Mitigation Initiative, with pilot green-roof projects in cities around the United States. A "heat island" results when a city's asphalt, buildings, and rooftops absorb the sun's rays and then release the energy at night, making the air 6 to 8 degrees hotter than in the surrounding countryside. Since higher temperatures hasten smog formation, the EPA initiative was created to reverse the urban warming trend. The idea is that reducing temperatures will help lower energy use, which in turn will mean less power-plant emissions like carbon dioxide, nitrogen oxides, and volatile organic compounds.
Relatively minor greenifications can have a significant impact on urban air pollution. EPA computer simulations for Los Angeles suggest that increasing green space by 5 percent and replacing dark roofs and asphalt with lighter surfaces could lower summer temperatures 4 degrees--resulting in 10 percent less smog and $175 million in energy savings per year.
By providing insulation, green roofs can help heat as well as air condition. In winter, green roofs can freeze, so they carry a slight heating penalty, but they still yield a net energy savings. When temperatures plummet below freezing, the roof surface remains at 32 degrees--an advantage in very cold climates.
A handful of U.S. companies install green roofs commercially, but so far high expense prevents them from greening individual residential roofs. However, do-it-yourselfers can put up their own at a sliver of commercial costs. Linda Velazquez, who recently designed a green roof for a nature center as part of her landscape-architecture degree at the University of Georgia, suggests starting with a test plot on a shed, playhouse, garage, or even a birdhouse.
That's what Tom Liptan did. A storm-water specialist in Oregon, Liptan wondered if green roofs would reduce runoff. In 1996, he installed one atop his 10-by-18-foot flat-roof garage. After adding 2-by-4s to the walls, plus cross-bracing the roof for additional structural support, Liptan waterproofed the roof by rolling out a sheet of pond-liner plastic (available at home-improvement stores). He laid newspaper above and below the plastic for added protection against punctures, then piled on two to three inches of soil from his yard and covered it with mulch. He placed boards around the roof edges to hold the plastic and soil in place, then planted native vegetation from his yard, including drought-tolerant, shallow-rooted sedums. Total cost: $80. Today, Liptan's green roof holds nearly an inch of rainfall, and the garage measures up to 15 degrees cooler than outside.
Because retained water greatly increases the weight load after a rain, it's important to have a structural engineer determine how much weight your roof can bear, or how much additional structural support is needed, before greening on your own. A meadow-like roof veneer requires significantly less structural support than a heavily landscaped garden like the 20,300-square-foot green roof atop Chicago's city hall. Last year the building was retrofitted and its roof stocked With trees, shrubs, plants, and vines. Flat or low-slope roofs work best, but green roofs have been successfully planted on steep housetops, too.
Whether a green roof requires watering beyond the rain it absorbs will depend on the vegetation used. Shallow-rooted, self-seeding native plants that can survive both drought and drenching will need little if any watering. Grasses, succulents, roadside wildflowers, and shallow-rooted sedums that grow on rocks are all recommended by landscape-architect Velazquez.
Many people wonder whether a vegetated rooftop will cause water, and possibly dangling roots, to drop through their ceiling. Not to worry: Commercial installers use specialized layers for drainage, root barriers, insulation, and waterproofing, and do-it-yourself projects can get by using shallow-rooted plants, a few inches of soil, and a layer of newspaper and waterproofing plastic, as Liptan did.
