Bringing a Lofty Garden Down to Earth

Advocates of a growing movement for green roofs on city skyscrapers anticipate a day when people will look down from airplanes and see a quilt of vegetated building tops. Katrin Scholz-Barth doesn't have to imagine it, or take a plane ride for that matter. She sees her own green patch from the dining room of her Capitol Hill row house.

Last month she constructed and planted a flat roof above a bicycle storage area in her back yard. Just over 3 feet by 6 feet -- barely big enough to lie on -- it consists of a rubber lined tray filled with a thin layer of gritty soil and a mat planted with hundreds of little sedum plants.

"It's poco, poco , but it demonstrates that the smallest of applications makes a difference," she said. "And it makes me happy."

Scholz-Barth is an environmental engineer and consultant who took part last week in a green roof conference in Washington organized by the Toronto-based nonprofit Green Roofs for Healthy Cities.

Toronto is among a number of North American communities where green roofs are making a splash. Long popular in Scholz-Barth's native Germany and other European nations, the roofs are designed to hold, cleanse and then slowly release storm water that would otherwise erode and pollute waterways. The mini rooftop gardens also keep buildings cooler in summer and help reduce the heat island effect of large cities.

The offices of the Casey Trees Endowment Fund, at 1425 K St. NW, became one of the first downtown buildings last year to have a green roof. Other examples are found on local government buildings and apartments in suburban Maryland and Northern Virginia. These gardens in the sky are not merely plants in pots, but the final layer in an elaborate and highly engineered roof system that must drain excessive moisture and keep buildings leakproof. The plants must grow in thin, poor soil and take care of themselves in highly exposed sites, narrowing the choices to a few hardy succulents.

Such roofs are more expensive than conventional rubber-and-tar versions, but advocates say that in addition to their environmental benefits, they have a longer life span. Plants, it seems, can take damaging ultraviolet light for years in a way that rubber and caulk cannot.

Despite their many advantages, they are not widely used yet in townhouses or detached homes. Scholz-Barth thought it time to bring green roofs down to earth and adapt one to a home setting.

For flat-roofed commercial buildings, various green roof systems were on display at the conference -- some with thick, rubber barriers against root penetration, others also employing a blanket of copper to halt unwanted root growth. Pitched roofs, up to about 45 degrees, can be vegetated, too, but they are more expensive to install because each plant grows in its own soil-retaining cup.

Scholz-Barth sees a time when green roofs on single-family homes will become accepted in the United States, as they have in Germany, but only if grants or tax incentives to install them are available because of their inherent increased cost over conventional roofs. And just as most people wouldn't dream of installing their own roofs on a house, green roof installation is for professionals.

But that's not to stop you from having some fun and getting into the spirit of it by planting one on a small roof somewhere about the home -- on a doghouse or shed, even a birdhouse, she said, where "it doesn't matter how perfect it is."

For her little shed, sandwiched between a fence and the side of the house, she formed a traylike roof from surplus cedar fence boards, lined it with a sheet of roofing rubber, and tilted the structure slightly to allow the excess moisture to empty through a sink drain set in one corner. In the tray, she placed a two-inch layer of gritty soil and covered it with a pre-grown mat of sedum on top. A green roof should absorb the first 1 1/2 inches of a deluge; the rest is drained. The retained moisture is released later or evaporates "so the peak flow is reduced," she said.