"Green roofs" flourish in eco-conscious cities

NEW YORK — When asphalt-weary American city dwellers yearn for a bit of greenery these days, more and more of them head for the roof.

The concept of planted "green roofs," which gained early popularity in Chicago, is beginning to take root in New York and other cities.

Construction of green roofs grew 30 percent in North America last year, according to the third annual Green Roof Market Industry Survey by the Toronto-based nonprofit group Green Roofs for Healthy Cities. For the third time, Chicago topped the list of U.S. and Canadian cities in terms of square footage installed in one year. Although it polls only members of its group, the Green Roofs survey estimates it reflects about 60 percent of all green-roof industry activity in the U.S. and Canada.

Due to cost, green roofs appear primarily on public and commercial buildings, such as the new Bank of America tower in Manhattan or the Apple store on Chicago's Michigan Avenue. But they also are beginning to sprout on single-family homes and other private dwellings.

Essentially composed of vegetation — often sedums and grasses — planted in soil or a lightweight growing medium above a waterproof membrane and drainage system, green roofing is more than a garden but offers the same aesthetic appeal. It also can assist in reducing stormwater runoff and resultant water pollution, reducing energy costs, cooling urban air, improving air quality, extending roof longevity and even preserving habitats for plants, insects and birds — not to mention enhancing real-estate value.

"Green roofs are a wonderful technology. They have the upside of handling storm water very well but the downside of being very expensive," said Rohit Aggarwala, director of the New York City Office of Long-Term Planning and Sustainability and chief architect of PlaNYC, Mayor Michael Bloomberg's environmentally focused program for preparing the city to absorb an estimated 1 million new residents by 2030.

Higher costs for labor and transportation make the installation of green roofs in New York at least twice as expensive as the national average of $10 to $24 per square foot estimated by the Canadian Green Roofs group.

Nonetheless, for the first time, New York — specifically the borough of Brooklyn — made it into the top five cities for green roofs last year. But with just over 100,000 square feet of green roofing installed in 2007, it still lagged far behind the 517,633 square feet constructed in Chicago.

That may change due to a new law championed by Bloomberg and signed this summer by New York Gov. David Paterson to encourage the construction and maintenance of green roofs in New York City. Under a pilot program that begins Jan. 1 and expires in 2013 unless extended, the law provides a one-year property-tax credit of $4.50 per square foot of green roof to building owners who install them on more than 50 percent of their available roof space.

"Mayor Daley got it before anybody," said Steven Peck, founder and president of Green Roofs for Healthy Cities, referring to Chicago's Richard Daley, who instituted a grant program for green roofs and turned the City Hall roof green in 2001. "New York, I hope, will surpass Chicago in a couple of years because of this new tax-incentive measure."

New York's tax-credit program, believed to be the first such state-approved green roof initiative, is designed to defray about 35 percent of the cost of installing a green roof on a standard roof, according to the text of the bill.

In general, green roofs cost about twice as much as standard ones but offer at least double the life span because they are less vulnerable to the wear and tear of temperature extremes, according to Stuart Gaffin, a research scientist at the Center for Climate Systems Research at New York's Columbia University, who has done extensive work on green roofs.

There is no question that green roofs, which soak up and slowly release large amounts of rainwater, help prevent the storm runoff that causes water pollution in many cities with combined sewer-overflow systems like New York and Chicago. When the volume of storm water in the sewer system overtakes the capacity of water-treatment systems, the excess mixture of rain and raw wastewater from toilets and other sources is diverted into rivers and other local waterways.

It also is clear that green roofs can help reduce the higher temperatures in cities, called urban heat islands. Less certain are the actual energy savings realized by the green roofs' ability to provide passive cooling and reduce heat loss in buildings.

To try to quantify those and other potential benefits, such as improvement in air quality, Columbia's Gaffin has embarked on an intensive monitoring program at a new green roof, featuring 21,000 plants covering more than a quarter-acre, just completed at a New York City facility owned by the power utility Con Edison.