From National Geographic News, by Brian Handwerk
When cities run out of valuable real estate, planners look up. The search for green space is no exception. Europe's green roofs have long provided environmental, aesthetic, and economic benefits. Is the idea growing in the United States?
Mayor Richard Daley began a green roof initiative in Chicago, Illinois, after he saw the gardenlike roofs in Europe.
"I thought, with all the flat roofs in Chicago, you could reclaim thousands of acres for the environment and also help buildings with heating and cooling and controlling rainwater going into the sewer system," he said during a recent telephone interview.
"When you look out over the city, instead of steel and concrete, you see something for the environment," he continued. "So I just thought that was the way for us to go."
Green roofs are generally composed of a low-maintenance, drought-resistant plants like sedum. These roofs are less than four inches (ten centimeters) thick and may be planted directly or laid down as pre-vegetated mats. Even sloping roofs are candidates for the green treatment.
More intensive green-roof systems may contain varieties of plants and garden elements such as trees but only on flat roofs. These plantings require deeper planting material, or soil substitute, and are heavier, are more expensive, and require more maintenance.
One of green roofs' biggest benefits is water management. They can absorb some 50 to 60 percent of the rainwater that falls on them.
Some of that water is lost when the vegetation transpires, or "exhales" the water back into the atmosphere. Some water is retained in the soil or other growing medium. The rest enters the urban rainwater drainage system in a slow, controlled flow, a process that helps dampen high-volume rainwater surges in urban water systems, which are expensive to expand.
"In a lot of cities even a thunderstorm might overflow [the rainwater system] and mix with sewage," said Brad Rowe, a plant and soil scientist at Michigan State University in East Lansing. "Who knows where that goes?"
Green roof proponents tout other benefits, like energy savings.
How much a green roof lowers energy costs depends on the type of roof and the climate in which it's installed. Warmer climates offer the greatest saving opportunities, since green roofs are more efficient at reducing air conditioning costs than they are at lowering heating bills.
Another benefit: Green roofs last longer than conventional ones. Where typical roofs might last 20 years, green roofs will survive 40 to 50 years. They protect the roofs waterproof layer from damaging ultraviolet rays and extreme day/night temperature fluctuations, which may cause cracks.
While the costs of green roofs are one-and-a-half to twice that of conventional roofs, advocates say green roofs make long-term economic sense, even without factoring in energy savings.
The rooftop green space also helps mitigate a phenomenon known as the urban heat island effect. The city microclimates occur when acres of densely-packed human-made building materials absorb heat energy from the sun.
"In Atlanta on a hot summer day, it might be 10 degrees [Fahrenheit/5.6 degrees Celsius] warmer in the city than out in the countryside," Rowe said. Green roofs can help mitigate the unnatural temperature rise, Rowe said.
Green-Roof Zoning
In Germany, where green roof are most common, the city of Stuttgart has promoted green roof and facade planting through subsidies for nearly 20 years.
Martina Laun and Klaus-Juergen Evert serve in the city's park and cemetery department. They noted that the number of green roofs in the city is permanently increasing, thanks to a combination of subsidies and zoning laws, which require green roofs in certain areas.
Under a grant-aid program established by the Stuttgart City Council, private citizens are awarded grants to help cover construction costs.
Between 1986 and 2003, the plan subsidized some 50,018 square meters (540,000 square feet) of private green roofs at a total cost of U.S. $1,076,000. During the same period, the city also invested U.S. $2,523,000 for roof plantings on public buildings. The total area: 102,423 square meters (1.1 million square feet).
Laun and Evert noted that the program has fostered aesthetic benefits as well as environmental ones. "Nature's rhythm is made visible by seasonal changes in the vegetation, even in an urban roof environment," the pair wrote in an e-mail to National Geographic News. "In terms of design, roof planting represents a considerable aesthetic improvement and altogether a significant ecological benefit."
Cultivating Converts
The green roof concept is winning more converts in the United States.
Ford Motor Company created a 10.4-acre (4.2-hectare) living roof, the world's largest, atop its Dearborn, Michigan, truck plant final-assembly building.
In Chicago, Mayor Daley is leading by example. City Hall sports a green roof that was the city's first. Over a hundred building projects, incorporating a million square feet (93,000 square meters) of green roof, are now underway. Officials are even experimenting with green roofs at O'Hare International Airport as a means to reduce noise.
At the Chicago Center for Green Technology, demos showcase green-roof technology to everyone from developers and builders to schoolchildren. The center also serves as a research center, where leading suppliers are invited to install different green-roof plots that are monitored and compared.
And the city government is providing at least a nudge to those involved in new building ventures. Projects that receive tax-increment financing support from the city are required to include some level of green roof.
"I hate to say that we're mandating, but we're really talking to big box [stores], architects, contractors, developers, and others about how important [green roofs] are to the environment and to business," Daley said.
The mayor believes that the city's cleaner, greener roofs are an important part of long-term environmental planning close to home.
"The environmental movement often seems like it's happening somewhere else and people forget about our own community," he said. "We need to be sure that we're planning well."
