GREEN ROOFS

Young birch trees waft gently in a spring breeze. A finch alights on a branch, then takes off again. In the spring sunshine, rosebuds are on the very verge of bloom. If this seems like some suburban idyll, it’s not. It’s the rooftop of the 19 th floor of a residential tower, called the Solaire, in Battery Park City in New York. Standing at the building’s edge, looking down toward the north and east, one can see green, or vegetated, roofs on other residential buildings, the Tribeca and the Rhodesian. From this vantage point, they’re like sky gardens.

Courtesy of Conservation Design Forum

Urban rooftops are turning green, and the benefits are myriad. Green roofs can serve as sponges, absorbing storm water runoff and mitigating the effects of heavy rains that might otherwise cause sewers to overflow and pollute local waterways. They also cool buildings and the surrounding areas through “evapotranspiration,” in which plants absorb water through their roots and evaporate it through their leaves. This is why parks are noticeably cooler than city streets in summer. Green roofs also filter pollutants from the air, provide habitat for urban or migrating animals, and offer opportunities for food production.

It’s not exactly a new idea. The hanging gardens of Babylon and the sod roofs of wayfaring Vikings are historical examples. But the modern green roof movement began in West Berlin, according to Steven Peck, president of the Toronto-based nonprofit trade organization Green Roofs for Healthy Cities. Its political and geographic isolation influenced the city’s urban ecology movement in the late ’70s and ’80s, and the clear environmental benefits of green roofs were incorporated into public policy. The green roof movement spread through Europe, where it remains much more advanced today than in North America.^[1] The Austrian city of Linz, for instance, mandates that new buildings in high-density areas have 80 percent green-roof coverage.^[2]

Courtesy of Conservation Design Forum

But the greening of American rooftops is well under way in such cities as Toronto, Portland, New York, and Chicago.^[3] Mayor Richard Daley, who took office in Chicago in 1989, vowed to make the Windy City “the greenest city” in the United States. According to Sadhu Johnston, Chicago’s commissioner of the environment, a million square feet of green rooftops have been completed and some 3.5 million square feet are planned or under way.^[4]

Chicago ’s 22,000-square-foot, 100-year-old City Hall building was one of the city’s first to go green, in 2001. The greenery has greatly reduced the surface temperature of the roof. In summer, says Johnston, measurements have shown that the temperature of the ambient air above the City Hall green roof was 80F degrees cooler than the air above the neighboring Administration’s black-tar roof. Not only is City Hall’s roof helping to mitigate “the urban heat island effect”—the tendency of metropolitan areas to be warmer than surrounding areas—but it also encourages biodiversity. The roof is covered with 150 kinds of trees, shrubs, and grasses, many of them native to the region. “I’ve seen peregrine falcons swooping down,” Johnston said. “There are butterflies, crickets, and even beehives.” Chicago remains at the forefront of the green-roof movement.

Leslie Hoffman, president of the New York-based environmental organization Earth Pledge, is also on a mission to spread the green. In 2002, she planted one of the first green roofs in Manhattan, on top of a renovated Georgian townhouse that serves as the Earth Pledge headquarters. It is covered with sedums, the drought-resistant succulents frequently used for green roofs, but for five years the rooftop also included a kitchen garden where employees composted lunch leftovers and grew an organic herb and vegetable garden.^[5]

“I had an epiphany,” said Hoffman. “I am a longtime gardener, and I was up on the roof in the garden. The place was buzzing; the pollinators were going crazy. And I thought, What an elegant idea this is. There’s all this wasted space—city rooftops—that can be like oases. They can help with storm water management and the urban heat island effect or serve as places to garden. Such a big idea, complex in its variety of benefits.”

So Earth Pledge has expanded its Green Roof Initiative to 16 cities, including Los Angeles, Minneapolis and Atlanta. The dramatic increase in geographic scope has resulted in the neglect of the townhouse’s rooftop—it hasn’t been watered in two years. But a visitor recently found the sedums still thriving, doing their jobs of keeping the building cooler and managing storm runoff.

Those are the basic benefits driving the American green roof movement. Also gaining momentum are newer ideas such as promoting food production. For many years, the Fairmount Waterfront Hotel in Vancouver grew herbs, flowers and vegetables on its roof, saving its kitchen an estimated $30,000 a year in food costs.^[6]

Courtesy of Earth Pledge

Paul Kephart, executive director of Rana Creek in California, is noted for emphasizing biodiversity in his living roof designs.^[7] Kephart used indigenous plants for the 69,000-square-foot roof of the Gap headquarters in San Bruno, recreating the coastal savannah ecosystem with native grasses and wildflowers. He also serves as a project design consultant on the two-acre living roof of the California Academy of Sciences museum in Golden Gate Park, designed by architect Renzo Piano and scheduled to be finished in 2008. The building’s roof mimics California’s rolling hills, with native vegetation providing habitat for bees, hummingbirds and butterflies.

As green roofs spread, they may help mitigate global warming. By reducing the urban heat island effect, the carbon footprint of energy use for air conditioning will be lessened. And they also keep us connected with nature.

Architecture critic and author Jane Jacobs put the movement in perspective: “In its need for variety and acceptance of randomness, a flourishing natural ecosystem is more like a city than a plantation. Perhaps it will be the city that reawakens our understanding and appreciation of nature, in all its teeming, unpredictable complexity.”^[8]

Footnotes

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