Learning from Green Roofs
Standing on the rooftop at St. Simon Stock Catholic School on East 182nd Street in the Bronx, New York, Daniel Simon, marketing director of the Gaia Institute, points to a neighboring high-rise apartment building. A couple is looking down from a high window over the rooftop garden. “Those people have been living here a long time,” says Simon, “and one day they had a green roof to look at instead of city construction.”
Green roofs are usually flat, and partially or completely covered by soil and plants. Germany and Japan have incorporated them into building projects for many years, and they are now becoming popular in eco-minded cities in the U.S. According to the Green Roofs Institute, total coverage in North America climbed from 1.3 to 2.5 million square feet between 2004 and 2005. Green roofs lower cooling bills, provide research opportunity and bring the natural world to inner-city children. “The green roof movement is growing rapidly here,” says Leslie Hoffman of Earth Pledge’s Green Roofs Initiative.
Simon, with Gaia Institute founder Paul Mankiewicz, pushed the Bronx project to completion in June 2005. The roof is a lone patch of green in the quilt of gray, beige and black that stretches across the southeast Bronx. Six inches of a patented, lightweight growing medium called Gaia Soil covers 3,500 square feet, divided into plots for both elementary and graduate school research. The roof hosts 20 native species: delicate columbine flowers, milkweed that attracts migrating Monarch butterflies, tomato and cucumber plants, and black-eyed susans, favored by bumblebees. “You can see some pollination here,” says Simon, “which is rare because there aren’t a lot of plants in the Bronx to pollinate.”
Parish rector Nelson Belizario was struck by the roof idea when he arrived at the school two years ago. “I looked out at the rooftop and knew we needed gardens here,” he says. Belizario learned about green roofs in Chicago, where Mayor Richard Daley’s administration requires any city-funded project to meet Leadership in Energy and Environmental Design (LEED) Certification.
Belizario got in touch with Mankiewicz, who played matchmaker between the school and the borough. The result was an award totaling more than $130,000 from the Bronx Overall Economic Development Corporation (BOEDC). “We’re quite excited about St. Simons as a model for what’s possible with a green roof,” says Kate Shackford, director of the BOEDC’s Initiative for Energy and the Environment. “We’d like to see more green roofs in the Bronx.”
Mankiewicz estimates that the Bronx has 10 green roofs, including the municipal courthouse across the street from Yankee Stadium. Weight, apart from cost, is the largest obstacle to building more green roofs. “A lower weight soil is important because of the load-bearing limits on rooftops. For existing structures, lighter-weight soil means less restructuring,” Simon explains. The roof on St. Simon can only hold 40 pounds per cubic foot. Most soils weigh 50 pounds per cubic foot.
Mankiewicz and the Gaia Institute solved St. Simon’s problem with Gaia Soil, a growing medium that is 85 percent Styrofoam. The remaining 15 percent is a mixture of agricultural waste, clay slurry and compost that includes ground pumpkin seeds, evergreen trees and “Bronx Zoo Doo” (poop from exotic animals at the Wildlife Conservation Society’s establishment down the road). A cubic foot weighs only 10 pounds. “I started to develop it 25 years ago,” Mankiewicz says. “I’ve grown everything from red pines to vegetables with it.”
Despite its light weight, Gaia Soil can absorb twice its weight in water. Absorbency slows the rate of storm water run-off from the roof, resulting in less water in the sewer, and less overflow of untreated water into Long Island Sound and the East River. “Right now the storm water systems are overtaxed,” says Jeanette Compton, a graduate student at Cornell University who studied evaporation rates at St. Simon. “The situation is going to get worse as we build more parking lots.”
Simon claims that Gaia Soil on the school roof can absorb up to two inches of rainfall at a time; it captured 52,367 gallons total last year. He points out that the $250 million necessary for new city sewage tanks could instead fund nearly 2,000 green roofs and achieve similar reductions in sewage overflow. One hundred acres of green roofs would collect between three and nine million gallons of water a year.
“Green roofs are interesting because they don’t just have environmental returns, they also have economic returns,” says Compton. New York City, which pays about a dollar to treat every hundred gallons of wastewater, has saved a total of about $5,000 as a result of the St. Simon rooftop garden. In addition, the school met its budget for the first time in years, and Belizario credits the green roof and the mild winter for lowering the electric bill.
New York City is a victim of the heat island effect: a five- to seven-degree temperature difference between the city and the surrounding countryside. “We know that greening is the way to mitigate the heat island effect, and we know that rooftops cover one-third of New York City’s landmass, so the benefits of green roofs are logical,” says Earth Pledge’s Hoffman.
Mankiewicz hopes that the green roof will be incorporated into the educational curriculum because it teaches lessons that can’t be learned on the city streets. He’s trying to train vines to cover the rooftop fence so the red, bell-shaped flowers will attract hummingbirds. “Can you imagine that? Hummingbirds on 182nd Street,” he adds.