Skip to content

Green roof gambit: How plant-covered rooftops will defend cities against global warming


Gaffin's green roof in March 2008. The plants are just starting to grow out of the netting that secures them to the roof. The plants are sedum, a stocky succulent that can survive frigid winters and sweltering summers.

On a mild April afternoon, the sun baked a Bronx, N.Y., rooftop to the tune of over 104 degrees. Just yards away, another rooftop also sat beneath the sun, but only measured about 70 degrees.

The difference? The cooler roof was a green roof, 5,600 square feet of shrubby plants called sedum. And the temperature difference between the two roofs is just one type of data climatologist Stuart Gaffin is gathering in an effort to convince politicians and developers that green roofs are critical to the future of cities.

Gaffin, an atmospheric researcher at Columbia University's Center for Climate Systems Research, has been studying how green roofs affect the environment since January, when he began gathering data from his rooftop laboratory on the recently constructed buildings of the campus of the Ethical Culture Fieldston School, a private school in the Bronx. Gaffin is one of a growing number of climatologists who believe green roofs are a powerful technology to help cities confront the threats of global climate change.

“Global warming has been my issue for my whole career,” said Gaffin, and the data he’s collecting is bolstering the case for green roofs. Compared to that other, conventional roof, the green roof stays cooler in the sun, drinks up rainwater, and reflects sunlight that would otherwise heat the roof like a blowtorch on a steel girder.

Cities are hotspots of worry for climate scientists because expansive urban areas, coated in asphalt and belching the summertime exhaust of countless air conditioners, already pose problems for air quality, heat and excess rainfall, which can foul water systems. In the coming years, global warming is expected to make these problems worse. “Two of the biggest predictions for cities is more heat and more rain,” said Gaffin. “Global warming is going to actually amplify both of those things.”

Global warming will increase the intensity, duration and number of heat waves in the United States, said Radley Horton, another climatologist at Columbia. And this is in addition to an existing problem: the urban heat island, a stifling pocket of air saturated with pollutants that surrounds cities on hot summer days. While Horton won’t link global warming to the urban-heat-island effect, he believes the combination of the two poses a dramatic threat.

Heat kills more people than any weather phenomenon except for extreme cold, according to the National Weather Service. An average of 170 people die every year from excess heat, but in some years that number rises drastically. In 1980, 1,250 died from a nationwide heat wave, and in 1994, more than 400 people suffered heat-related deaths in the city of Chicago alone.

Politicians have responded to such disasters by seeking environmental “sustainability,” making the use of environmentally friendly building materials and green roof technology, a policy issue. Chicago Mayor Richard M. Daley installed 20,000 square feet of green roof on City Hall as an experiment to encourage green development in the Windy City. Seattle, too, has a green roof on top of its City Hall.

Portland, Boston and Baltimore are some of the other cities with policies to promote green roofing for new development. And New York City has proposed a tax incentive for green roof installations in Mayor Michael R. Bloomberg's PlaNYC 2030 environmental initiative.

Yet in many cities, including New York, green roof production has been slow to gain momentum. “Many mainstream decision-makers have not yet been convinced that high-performance design is good business practice,” said a report by the non-profit U.S. Green Buildings Council. Part of the reason is cost. Installing a green roof in the United States can cost anywhere from 50 to 500 percent more than a normal roof, depending on factors like the design, height and location of the building.

But Gaffin believes his research will offer convincing proof to policy makers that green roofs could avert summer power outages from strained electrical grids, as well as other problems, by lowering air conditioning needs of buildings, and cooling and filtering the local air. The plants absorb carbon dioxide, the villainous greenhouse gas. They filter pollution from the air, which helps lower air temperatures around the building. And green roofs act as insulation, lowering heating and cooling costs for buildings during the winter and summer months, experts said.

And while a single green roof benefits one building, advocates said a wide area of green-roofed buildings can affect the climate of a whole community.

“Is one roof (going to) make the entire neighborhood better air quality? A little,” said Mark Thomann, a landscape architect at Balmori Associates, a design firm in New York. “Would a whole bunch of green roofs do it? Sure.”

The sedum plants themselves are easy to maintain, experts said, requiring weeding for only the first few seasons until they fill the roof with a dense, verdant carpet. The sedum plant can survive long periods of drought, heavy rainstorms, and severe winds. “They're tough,” said Kurt Horvath, president of Intrinsic Landscaping, which planted the roofs of Chicago's transit authority and the Quaker Oats headquarters. "They can take a lot of abuse."

Horvath argues that green roofs aren't just good for the environment, but they can also be good business. “People are starting to realize the benefits,” said Horvath. “One of the biggest drivers is the architectural community has grasped this and run with it.’

In the Midwest, interest in green roofs is sprouting up, and entrepreneurs are taking notice. From 2004 to 2005, green roof square footage grew 80 percent in the United States, with Chicago and Washington, D.C. leading the way, according to Green Roofs for Healthy Cities, a green roof industry organization with more than 4,000 individual and 75 corporate members.

While green roof technology is off to a strong start in some cities, it struggles to grow in others. “We've had a lot of requests from architects,” said Thomann of the Balmori firm, which created a green roof for the Sopranos studio at Silvercup Studios in Queens, N.Y. “But,” he said, “they just want a deck terrace with a couple of planters.”

Gaffin said even in winter he’s seeing a return on the green roof investment. “These roofs are insulating in the winter,” he said. “They're keeping the building warmer in the cold weather.”

On a recent morning after a rainstorm, Gaffin noted that the roof had captured 2,000 gallons of water, “2,000 gallons of water that didn't go into the (sewer) system,” he said, referring to a problem called Combined Sewer Overflow, in which rain floods the sewers and sewage leaks into local waters.

“It’s really the number one source of pathogens to the harbor,” he said.

Gaffin will begin publishing his data later this spring, but thinks his research is only a start to understanding the potential of this technology. “One of the things about green roofs,” he said, “is there’s really lots to be discovered.”