Architects raise a roof

By Wade Rawlins
Raleigh News & Observer
© Copyright 2003

Feb. 22, 2003

When the weather warms, the new roof that architect Charles Brown and two dozen helpers planted Friday on his office in downtown Raleigh should burst to life with succulent blooms of red, yellow and white.

The more pressing question is whether the spongy new roof also will help reduce pollution in the Neuse River and ease flooding. Such expectations are a heavy load for a roof to bear. In a back-to-the-future approach to building design, researchers at N.C. State University -- in conjunction with Brown & Jones architects -- laid a roof consisting of 4 inches of lightweight soil and sprigs of hardy, drought-resistant ground cover plants atop the architects' office at 701 N. Person St.

"I don't know if the roof will be blooming this year, but next year it will be blooming," Brown said. Brown & Jones focuses on environmentally friendly designs. Turning half its roof into a desert garden drainage project might be pushing the design envelope.

"We wanted to experiment on this more advanced technology on ourselves, rather than making our clients the guinea pigs," said Brown, as he planted sprigs of ground cover. "It was a matter of putting our money where our mouth is. We are making our own building into a research tool.

"When we told a landscape gardener we were doing this, she thought we were crazy," he said. So-called green roofs are common in Europe, where dense, cobblestone cities have been struggling for decades to control stormwater that overloads their ancient sewer systems. The concept is just gaining attention in the United States as an environmentally friendly alternative to hard surface roofs that contribute to runoff pollution.

That is where the researchers enter the story.

"What can you do in a place like downtown Raleigh or downtown Durham to treat stormwater before it leaves your site, before it enters the Neuse River?" said Bill Hunt, an extension specialist in the biological and agricultural engineering department at NCSU, who is overseeing the research. "What is the impact on flooding? Do green roofs hold water? We are going to try to see how all these things work in an ultra-urban setting."

Green roofs are a layer of vegetation over multiple layers of drainage and roof membrane. The bottom layer is the waterproof membrane. On top of that is a drainage layer, which resembles a mini egg carton and retains moisture. Above that is a root barrier fabric, then the soil and the plants.

The soil is a mixture of sand, compost and pebbles that have been burned to remove nutrients and are airy as pumice. Like the soil in little plastic-potted cacti, this soil is very lightweight. Even so, several inches of dirt is a much heavier load than traditional roofing material, so the architects' office, a former gas station, had to be reinforced.

"A green roof load is equivalent to a 6- or 8-inch snowstorm," Hunt said.

Green roofs, while they may last longer, cost $6 per square foot more than traditional roofs, so they may be beyond the budget of all but the most environmentally conscious homeowners. But they are more practical for flat-roof office buildings in urban areas, supporters say.

Workers used rakes to spread the pebbly soil evenly over a black egg-carton drainage mat that was laid on the gently sloping roof up to the center ridge. On the other side of the ridge, there is no soil or vegetation, just the traditional waterproof membrane.

For two years, Hunt and fellow researchers will collect rainwater that runs off each side of the roof into gutters and into collection basins. They will compare how much water is coming off each side to test the absorbency of the green roof, and they'll check the levels of contaminants.

"It might sound simple, but there are no studies on it," Hunt said. "There is a chance these things could reduce nitrogen and phosphorous runoff. A rooftop or a parking lot will contribute nitrogen. Fertilizer is not the only source of nitrogen."

An $80,000 grant from the Department of Environmental and Natural Resources is helping finance the rooftop research as a potential additional way to reduce runoff pollution in the state's waterways.

In North Carolina, regulators have established pollution thresholds for waterways. That includes a 30 percent reduction in nitrogen by 2003 in the Neuse River, which is overloaded with waste from sewage plants and septic tanks. Nitrogen and phosphorous are nutrient pollutants that state officials and environmentalists blame for massive fish kills in the Neuse over the past few years.

Environmental administrators are considering counting green roofs as a tool to remove pollutants. At this point, they have no basis to calculate the efficiency of green roofs in removing nutrients. The research, along with two green roof projects begun last spring in Kinston and Goldsboro, should provide more information about their benefits, Hunt said.

"The amount of knowledge is limited but growing," Hunt said. "It will be an important part of the national picture."

[Reprinted with permission of The News & Observer of Raleigh, North Carolina]

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