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Power Plants

Putting soil and plants on roofs—an idea that began as a way to help manage stormwater runoff—is turning out to have important energy-saving benefits as well.

You’ve probably noticed the difference plants make at ground level. Walking across a paved parking lot in the middle of July you feel like you’re roasting—but step off the pavement into a grassy field and you immediately feel cooler. That’s because plants keep the ground temperature much lower than a big expanse of asphalt.

Traditional flat roofs on commercial buildings are a lot like parking lots. Heat builds up to extreme levels on such roofs on sunny days, and that heat quickly transfers to the interior of the building. Adding a layer of green plants to roofs can make their surfaces much cooler.

Several university research projects under way in the United States and Canada are providing statistical proof of the difference. In these controlled experiments, groups of simple test buildings, constructed rather like children’s playhouses, are identical except for the roofs. Some have conventional roofs, while others have “green” roofs with a special layer of soil and plants. Sensors monitor every aspect—the ambient weather, the temperature on the roof and at various levels within each building throughout the day and night, and energy used to cool the structure throughout each 24-hour period.

Dr. Robert Berghage, director of the Center for Green Roof Research at Penn State University, says, “In our test buildings here in central Pennsylvania, we’re finding that with a green roof we can save about 10 percent of our energy costs in terms of air conditioning.

“Our test buildings are not typical of real-world buildings,” Berghage points out. “We have to build small structures, which means our structures have a low roof-to-wall area ratio. A typical commercial building would have a much larger roof area relative to the walls, so we predict a greater savings in energy costs in that circumstance.”

Ed Snodgrass, owner of Emory Knoll Farms in Maryland, a leading supplier of plants for green roofs, says, “The temperature difference between a plain black roof and a green plant-covered roof can vary by 50 to 100 degrees. When it’s that hot, solar panels don’t operate as efficiently. So green roofs help improve the performance of photovoltaic panels installed on roofs to collect solar energy.”

The most useful applications for green roofs could be large low buildings, things like one- or two-story buildings that sprawl over several acres. Warehouses, shopping malls, freight terminals, and schools with large surface areas are likely candidates for green roofs. Building owners in at least a dozen states have already installed green roofs as signature projects.

Berghage points out that saving energy costs on a daily basis isn’t the only advantage to a green roof. “From a utility company’s perspective, the important thing about these energy savings is when they occur,” Berghage says. “A green roof saves energy during peak usage times, during the sunniest, hottest part of the day. If enough people install green roofs in an area, together they could save enough energy so that the local utility company does not have to build another power plant to meet peak energy demands.”

To find out more about green roofs, visit these Web sites:,, or

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