A new study, published in Urban Ecosystems, comparing the performance of different types of green roofs suggests that buyers shouldn't assume these roofs are created equal.

Interest in vegetated roofs has increased as water and energy conservation becomes more important to property owners. Yet the study of six different manufacturers' products found the green roofs varied greatly in capabilities such as how much they cooled down a building's interior and how much rainwater they captured during downpours.

"Just having a green roof may not mean anything in terms of preventing water from reaching the street level, for instance," said Dr. Mark Simmons, a center ecologist and the lead investigator on the study. "Green roofs have to be done right, and our hope is to help manufacturers understand how to improve their designs."

Some of the key findings of the study were:

• The study of 24 experimental roof tops suggested a green roof could reduce a building's air conditioning bills about 21 percent compared to traditional, tar-based black-top roofs.
• A key part of the difference performance of green rooftops resulted from the native plants used on the green roofs. In comparison to sedums, a type of succulents traditionally used on most green roofs, native plants can take in more water and release more of it to the atmosphere.
• The ability of green roofs to capture water, lessening runoff onto streets and storm drains is an important benefit. Yet this feature varied the most among the six manufacturers. The better green roofs retained all of the water during a ½-inch rainfall, and just under half the water when 2 inches of rain fell. Some roofs, however, only retained about a quarter of the water in a light, ½-inch rain and as little as 8 percent during deluges.

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Source:
Barbra Rodriguez
University of Texas at Austin