Is that a daisy growing on your roof?

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.

"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 Dachbegrünung. 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.

In the first stage of its Green Rooftops initiative, the city of Chicago is constructing a 33,000-square-foot ecoroof on City Hall. It will hold more than 150,000 plants of many species, including wild onion, butterfly weed, aster, and buffalo grass, all hardy enough for the Illinois climate.

The city's environment department is closely monitoring the progress of this project. It has installed rooftop weather stations to keep track of temperature, wind speed, and rainfall, as well as bird and butterfly populations in order to fully understand the impact of green roofs.

Constructing ecoroofs can entail more work than conventional roofs. The roofs must be able to hold 10 to 25 pounds per square foot more than conventional roofs. The extra weight might require additional roof trusses, joists, columns, or other structures.

Underneath the soil is a waterproof membrane - a layer of modified asphalt, synthetic rubber, or reinforced PVC piping - that blocks water and keeps roots from affecting the roof's underlying surface.

A drain system must be provided because there will be some water runoff, although it's much less than with a conventional roof. Ecoroofs can handle snow and ice conditions, provided the buildings are structurally sound enough to handle both the weight of the plants and the snow load.

Besides snow, there are several other drawbacks to ecoroofs. In climates less rainy than Portland's there may concerns about the vegetation drying during the summer and becoming a fire hazard. If that's a possibility, owners may want to water the roof during dry spells.

Another obstacle is the potential for increased maintenance, depending on the plant material chosen. The roof drains should be inspected occasionally throughout the year to ensure that they are not plugged up.

The extra materials and construction increase the cost of a green roof to $10 to $15 a square foot, compared with $3 to $9 a square foot for a conventional roof. Although the cost of green roofs is higher, they last twice as long as regular ones: 40 years compared to 20 years, according to Liptan.

They can also eliminate the need for expensive retaining ponds, which many local laws require developers to create to handle storm-water runoff.

It is the cost factor, however, that keeps US construction of ecoroofs lagging behind what's happening in Europe.

The additional cost is the reason that some US cities like Portland offer financial incentives such as grants and tax breaks for ecoroof construction. "The city also gives developers credit for storm-water management if they have an ecoroof," says Liptan.

But in other American cities some consumers may be willing to pay the extra costs of green construction themselves. According to Cahners Residential Group, an industry trade publisher, eight out of 10 homeowners and builders that the company surveyed in 2002 said that new homes do not meet their demands in sustaining the environment, up from six in 10 in 2001.

The survey also found that compared with the previous year, consumers don't mind the extra cost for green features and that they want to be part of a total community environmental solution. Ecoroofs are beginning to gain a reputation as being part of that total community environmental solution.

ButLiptan believes that the best part is that ecoroofs are alive. "There are billions of living organisms [on the roofs]," he says, "all working to protect the building and provide nourishment to many birds and insects. They look great, too."