By Kent Steinriede
Gannett News Service

This summer Beverly Barnett and Alton Dahl hope to have a meadow on their roof. It will be a who's who of native plants, with irises, lobelia, sedge and phlox.

Green roofs - in which plants grow in soil on a roof - are rare in the United States, but fairly common in Europe, especially in Germany.

The soil and plants act like a sponge to absorb rainwater, create insulation and protect the roof.

Homes must be adequate

Most green roofs have small succulents, such as sedum and ice plant, but for her Arden, Del., home, Barnett chose native plants because she wants to attract wildlife to her property.

"If I get a new bird, I'm doing things right," she says.

Early this spring, Buzz Ferver of Overbrook Design & Construction, based in Oxford, Pa., delivered lightweight soil, a mix of popcorn-like expanded slate, compost and sand. Ferver and three workers used a crane to get the soil on top of the home's carport and then used rakes to spread it on the roof. The next step is to recruit a few friends to plant a dozen varieties of native plants in the six inches of soil.

Because she doesn't know if the roof will be dry or moist, Barnett chose a half-dozen plants suited for a moist meadow and a half-dozen for a dry meadow. "Nobody knows what's going to grow," she says.

New construction easily can be designed with the weight demands of a green roof in mind. That's what Barnett and Dahl did when they decided to build a carport in an area that was once a screened-in porch. The roof is supported by steel beams.

Not every existing building is suited for a green roof. It must be able to support at least 13 pounds per square foot, says Charlie Miller, head of Roofscapes Inc., a Philadelphia firm that has designed and installed green roofs on commercial buildings across the country.

Because of changes in building standards in the United States, most buildings put up after 1960 are not sturdy enough to support the weight of soil, plants and the water that they absorb.

However, many older buildings can be retrofitted with a green roof. Five years ago, Miller installed one of the first green roofs in the region on top of the Fencing Academy of Philadelphia. The brick building, built in the 1920s, was perfect for a green roof, says academy owner Mark Masters.

Originally planted with sedum, grasses, onion bulbs and dianthus, the roof now has a wide variety of plants growing in different ecosystems. Tall grasses have popped up where air conditioners drip water in the summer. Wild garlic, tomatoes and a few weeds have been sown by the wind. In the spring, grasses, onion bulbs and dianthus flourish. In the summer, sedum blossoms.

The benefits of going green

The biggest environmental benefit of such a roof is its reduction of rainwater runoff. A three-inch green roof reduces runoff by 50 percent. This puts less runoff water, which is high in pollutants and nitrogen, into streams, storm drains and ultimately rivers.

A green roof is also cooler than a conventional roof. It reduces the cost of air conditioning by 10 percent because there's "no fluctuation in temperature," says Robert Berghage, an associate professor of floriculture at Penn State University.

Studies at Penn State have found that when the air temperature is 90 degrees, gravel on a flat roof can be 119 degrees. However, the sedum of a green roof is only 82 degrees.

Because the soil and plants insulate the roof itself from heat and cold, a green roof can also double the lifespan of a roof. Longer-lasting roofs means less environmentally hazardous material ending up in landfills, Berghage says.

Aside from the environmental benefits, Beverly Barnett is looking forward to the birds that will be attracted by the native grasses.

But she might have to go aloft and mow the roof. "That's not so bad," Barnett says. "I'll use a scythe."