for National Geographic News
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.
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