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Is that a daisy growing on your roof?

More people are finding that 'ecoroofs' help both their property and the environment blossom

By Liz NakazawaSpecial to The Christian Science Monitor / April 9, 2003


Most people in the rainy Pacific Northwest want to remove moss from their roofs, but Portlander Tom Liptan is happy to see vegetation growing on his rooftop. He turned his garage green on purpose by buttressing his roof and covering it with two inches of soil. Then he planted ground covers in the soil, making what's called an ecoroof or a green roof.

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"Ecoroofs replace conventional roofing with a lightweight, living system of growing medium [soil, compost, and perlite] and vegetation such as sedums, sword ferns, and woodland strawberry," says Mr. Liptan. He's an environmental specialist with the city of Portland, who has spearheaded the city's ecoroof movement.

"They protect buildings from the natural elements with an ecological system that accepts and uses nature to grow and cover the roof area," he explains.

Reducing storm runoff is one of the most important features of ecoroofs, which act as sponges. "Rainfall is captured in the vegetation," says Liptan. "A roof with four to five inches of soil will hold about a one-inch storm event before any water runs off."

During larger storms the excess water is detained or slowed to about one-tenth of what conventional roof runoff would be. This prevents flooding and sewage problems that occur when a city's storm-water system overflows. This is an important benefit in Portland, which is under a court order to reduce the release of raw sewage into its rivers 94 percent by 2011.

Liptan says the conventional way to deal with excess storm water is to build bigger pipes for it to flow through and storage ponds to hold it. "These roofs, however, catch rainwater at the source, before it becomes a problem."

Improved air quality is another benefit of ecoroofs. Through the process of photosynthesis, plants convert carbon dioxide, water, and sunlight into oxygen.

Plants on the roof can also help to cool cities during hot summer months. A US Department of Energy climate model in New York City showed that urban roof vegetation could reduce the temperature of the ambient air by 3.6 degrees F.

A typical ecoroof is flat, but one with a slant of as much as 45 degrees is possible. The steeper the slope, the more materials and labor are required.

Europe has been a leader in the green-roof movement. Ecoroofs make economic sense in many European countries, where it is common to assess an annual fee on the building's owner based on the amount of storm water the building generates, according to Charlie Miller, a Philadelphia contractor who specializes in ecoroofs.

Since 1989, Germans have constructed 34 million square meters of ecoroofs - called Dachbegr├╝nung. Swiss cities mandate that new buildings have green roofs to make up for the vegetation they destroy.

The ecoroof growth in the United States, though slower than in Europe, is moving forward. In Chicago, a redevelopment firm installed an ecoroof on a bungalow garage as part of an effort to increase the amount of environmentally friendly housing being developedon the city's Southwest Side.

In New York, the Earth Pledge Viridian Project is constructing ecoroofs on the top of low- and moderate-income housing in the East Village.

Large civic and commercial buildings in a handful of cities are also turning green.

Ford Motor Co. will soon complete a living roof on its Dearborn Truck Plant in Michigan.