Green is good, too...
How Do Green Roofs Work?
By Mark Fischetti
Cities worldwide are promoting environmentally “green” roofs to mitigate several urban problems. Ground cover, shrubs and other flora planted across a building’s roof can reduce storm water runoff, easing the burden on local sewers and water treatment systems. And the vegetation can keep the roof cooler in summer, lowering interior air-conditioning costs and therefore peak demand on area power plants.
Green roofs have been blossoming in Europe for more than a decade, and Tokyo now requires that at least 20 percent of any new roof on medium and large buildings be cultivated. Chicago is the U.S. leader. Most installations are made on newly constructed buildings, but retrofits are rising.
In either case, the formations are built up in a series of layers that span all or part of a roof. So-called extensive roofs have fairly thin cross sections, including perhaps three inches of soil-like growing material; they weigh from 15 to 25 pounds per square foot when saturated and support low-lying plants. Intensive roofs are thicker, heavier and more costly to erect and maintain but are capable of supporting flowerbeds, shrubs, even trees. “As the plants get more demanding, the layers must become more robust, with better drainage and aeration,” says Jeff Stillman, executive vice president of ZinCo USA in Newton, Mass., a division of ZinCo, Inc., the world’s largest supplier of green roof components.
Prefabricated modules of a few feet square that contain similar layers can also be assembled like puzzle pieces; this approach can be easier to install, although it can be expensive and also results in seams.
The main drawback of all the approaches is cost. Some roofs—typically older ones—may not be strong enough to handle the weight. Standard insurance policies may construe a green roof as a structure that can create “standing water” damage, which a policy probably will not cover unless it is amended. Extensive roofs typically require minimal maintenance and an occasional dose of slow-release fertilizer; intensive roofs require more ongoing attention. Both styles, however, can turn a hot, bald roof into a pleasant space for coffee breaks, lunch, sunbathing or a simple breath of fresh air.
Did You Know ...
NOT DIRT:
Installers rarely use soil for the growing medium because it is heavy and because it packs tight after repeated rains, reducing water retention and aeration for plant roots. They instead use manufactured materials. For example, granulated clay or shale may be heated until it forms air pockets; it is then cooled. Organic compost and fertilizer are added as nutrients.
COOLER CONDITIONING:
On a sunny, 80-degree-Fahrenheit day, a tar or black-painted roof can reach 180 degrees F; a white roof 120 degrees; and a plant-covered roof 85 degrees. Even if the building has ample roof insulation to retard interior heating, the intake vents for air-conditioning units are often located on the roof. Cooler incoming air lessens the system’s burden, notes Jeff Stillman of ZinCo USA.
HEAT ISLAND EFFECT:
If installed widely, green roofs could lower a city’s cooling load, especially at night when bare rooftops radiate heat absorbed during the day. Since 1900 Tokyo’s average temperature increase has been five times that of global warming, according to Tokyo Metropolitan University—one big reason the city is pushing such construction.
SEDUM PREFERED:
Plants most recommended for green roofs belong to the genus Sedum. They grow low, store plentiful water in their leaves, and are bred to withstand temperature and moisture extremes. Common varieties include cape blanco, coral carpet and dragon’s blood.
Editor's Note: This story was originally printed with the title "Living Cover"
scientificamerican.com
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Cincinnati wants to lead green roof movement in US
By TERRY KINNEY Associated Press Writer
Oct 2nd, 2008 | CINCINNATI -- Officials want to see more green roofs on building tops in Cincinnati.
The City Council on Wednesday became the first in Ohio with a plan to channel grants and loans to residents and businesses to replace tar and shingles with vegetation.
Supporters of the idea want to see Cincinnati become a leader in green roofs, a European-born movement that has spread to only a few U.S. cities, including Chicago, Milwaukee and Seattle.
They say the greenery not only is pleasing aesthetically but reduces stormwater runoff, filters pollutants and cuts heating and cooling costs.
In Ohio, pastoral roofs grace the tops of the Toledo public library, the Ohio Environmental Protection Agency building in Columbus and the Cleveland Environmental Center, home of the Greater Cleveland Green Building Coalition.
"We call it our civic plaza rooftop," Toledo library spokeswoman Rhonda Sewell.
It was installed in 2000.
"We were inspired by what we saw in Chicago during a public library conference," said Charlie Oswanski, who heads the library's facilities and operations. "It's performing very well — very low maintenance, and it does benefit us in cooling and heating costs."
Chicago has scores of green roofs, including one atop City Hall. Other buildings elsewhere in the United States that are considered green roof pioneers include the Ford Motor Co. Rouge assembly plant in Dearborn, Mich., and the Convention Center of the Church of Jesus Christ of Latter-day Saints in Salt Lake City.
The plan approved Wednesday in Cincinnati is the first of its kind in Ohio, said Bob Monsarrat, manager of the environmental planning section of the Ohio Environmental Protection Agency, which directs certain federal grant funds toward such programs.
"We have wanted to support green infrastructure initiatives in the state directed toward improving water quality, so this was a fairly easy sell to us," Monsarrat said.
He said the handful of green roofs in Ohio have generally been on government buildings, while the Cincinnati program is mainly aimed at commercial buildings.
"It's very exciting," said Vicki Ciotti, director of the nonprofit Civic Garden Center, which plans to begin a roof garden early next year. She said the council's plan should jump-start a movement that has taken hold in a few cities but has generally been slow to grow.
"It's been slow just because it's such a new idea for us," Ciotti said. "They've been doing green roofs for years in Germany."
A report by the Green Roof Research Program at Michigan State University estimates that 12 percent of all flat-roofed buildings in Germany are covered with vegetation. It noted several barriers to widespread acceptance in the United States, including lack of government incentives or tax breaks.
"What the city of Cincinnati is doing is the largest effort I have heard of," Monsarrat said. "It will be interesting to watch that and see how it works."
About $5 million a year in below-market-rate loans through the U.S. EPA Clean Water State Revolving Fund will be available starting in 2009 for green roof projects, city officials estimate, along with an undetermined amount of grant money from other EPA funds.
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On the Net:
Michigan State Green Roof Research Program: hrt.msu.edu |
