By John DeFore

The green roof concept — in which some form of plant is grown atop a building — is spreading in multiple directions in the States. Not just the realm of futurists (though we love this idea) or extravagant fashionistas (see some lovely examples here), the field is drawing interest from homeowners and corporations with a range of motivations.

Now a study by the University of Texas at Austin’s Lady Bird Johnson Wildflower Center has quantified some of the issues motivating folks to put plants on the roof and found that the benefits are substantial, although results can vary widely depending on how the roof is composed and installed.

At the Wildflower Center, a team led by ecologist Dr. Mark Simmons studied roofs made by six different manufacturers with an eye toward helping the fledgling industry make better performing products. “Just having a green roof may not mean anything in terms of preventing water from reaching the street level, for instance,” Simmons has said. “Green roofs have to be done right, and our hope is to help manufacturers understand how to improve their designs.”

Rain runoff is one of the key arguments for putting plants on roofs. It’s an argument, though, that is most persuasive to government and conservation organizations, not the individuals who have to pay to mount the structures. Runoff that reaches gutters during a heavy storm can overload drainage systems and carry pollution into rivers; with certain green roof installations claiming to absorb a whopping 70% of the water that hits them, it’s not surprising that leaders like New York Governor David Paterson would include tax abatements for green roofs in packages of environmental measures.

But individuals may be more attracted, especially in August, by the prospect of lower temperatures in buildings with plants atop them. “For Energy folks, water retention isn’t a priority,” Simmons told GreenRightNow, and the Center’s study has an ongoing component that speaks directly to those people: A web page that provides real-time data comparing three different kinds of roofing as they perform throughout the day.

The researchers have installed metal insulated boxes at their field study site with three configurations: one is covered with plants and soil, one has a reflective white material, and one is topped with the kind of black tar substance found on many roofs. Not surprisingly, black works worst when it comes to keeping the box from warming up; around midday on the day this story was written, the temperature inside that box was 133.1°. But one might wonder how reflective white and a roof of plants would compare:

“Early morning is an exception,” Simmons acknowledges, during which the white-topped box sometimes has the lowest temperature, but “generally green roofs are more effective than white roofs at cooling internal temperatures when it matters most.” On this midday, for instance, green was 13.6 degrees cooler.

The variation is far more impressive when it comes to the roof surface itself, another piece of data that may not seem to affect the people inside the building, but does: Lower roof temperatures on a substantial percentage of structures in a city would diminish the “heat island” effect, which the Wildflower Center describes as “the difference in temperature between urban areas and the surrounding countryside caused by a lack of vegetation and a large number of reflective surfaces that absorb heat.” With green roofs as much as 80 degrees cooler than conventional ones, that could add up to make “summer in the city” a lot more pleasant.

Obviously, any plants added to an environment will mean some increase in air quality: more CO2 and pollutants sucked out of the air, more oxygen released. That aspect of the roofs was beyond the scope of this study. “We do intend to look at gas exchange,” Simmons tells us, but so far they’ve focused on quantifying runoff and temperature effects.

As with everything at the Wildflower Center, this study emphasized plants that are native to Central Texas. When asked what he thought about a decision made by Con Ed in New York to use non-indigenous plants because they were “unlikely to attract potential pests,” Simmons said it was “interesting — but probably wrong. Having non-natives does not mean you don’t attract animals (they’re not always that fussy about where the plants comes from when it comes to food and nesting).

In fact it may be worse because non-native plants may not support a potential predator of any pest that takes up residence, increasing the problem.”

Speaking of pests and predators, homeowners may be concerned about putting tiles of dirt on their roofs — even with the waterproof barriers included in green roof set-ups, mightn’t all that soil encourage critters that would burrow through to the attic? While Simmons concedes that “in biology anything is possible,” he’s not worried about that: “the Europeans have looked at green roofs over the long term (decades) and I am not aware of any major negative issues with animals.”

He adds what could be a clincher for homeowners considering the investment: Though he cautions that installation on a sloped residential roof will raise costs, he says “remember a green roof can actually preserve the integrity of the roof” — protecting it from heat damage, hail storms, and wear from sunlight — “doubling its life.”

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