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June 21, 2010

Green Roofs

Ingenuity sprouting from the rooftops

The rooftops of Manhattan are as varied as the city itself. A glance at the city skyline reveals a myriad of shapes and colors. Look a little closer and the shapes reveal themselves to be the things we often see rooted on Manhattan buildings, such as water towers, ventilation systems and communications equipment. But, on a few rooftops, something else is taking root--literally.

"It's a roof with a continuous layer of living plants, and it's a new form of urban vegetation," says Columbia University climate scientist Stuart Gaffin. He's talking about a green roof just like the one he is standing on at the corner of Amsterdam and 118th street in Manhattan.

A green roof is not something you can just have installed on top of your house. The features are mainly for structures with flat roofs, such as office or apartment buildings. A green roof protects the building just like a normal roof, except instead of tar or metal, it is topped off with materials for drainage and a mineral substrate that's lighter than soil and supports plant growth. It is always bottomed with a waterproof membrane to prevent leaking. The cost can run from as little as $5 per square foot to more than $50, depending on the selection of materials and vegetation.

The 14-story building that Gaffin is standing on houses Columbia University students. Its green roof is home to a number of experiments and the vegetation that covers it stands two to four inches tall. Looking down several stories from the windows of neighboring buildings, the rooftop resembles a well-manicured, suburban lawn that is simply contained within the boundaries of a flat Manhattan rooftop. A number of small vents poke through the vegetation and on the northwest corner of the building are weather gauges for measuring wind speed, air temperature and rainfall.

With help from the National Science Foundation (NSF), Gaffin and his two partners, Columbia University civil engineer Patricia Culligan and Lamot-Doherty Earth Observatory geochemist Wade McGillis, are researching what benefits green roofs might have on harsh urban environments. Their initial data suggests the answer is--quite a bit.

Culligan, the project's leader, is working in the lab to make green roofs even greener. She and her students are experimenting with new growing materials, which they test in a special terrarium that emulates weather conditions, such as sunlight and rainfall.

Culligan points out a plastic container filled with waste pulp that has been collected from paper production. She has her hands in the container of pulp, mixing it around to demonstrate its moist, rich texture and potential as a growing medium. The pulp resembles oatmeal except it's almost black.

"Most existing green-roof growing media that support plant life are not environmentally friendly, so we are looking at waste products as an alternative," Culligan explains, now holding a scoop of the pulp in her hands. "It holds a lot of moisture, it's light weight and it offers a promising alternative to the media that are currently used." The researchers are also testing how well vegetation grows in food waste that Culligan's students collect from some of the Columbia University cafeterias and then compost.

This research is funded under the American Recovery and Reinvestment Act of 2009 (ARRA). "We have a real incentive to focus on generating new products to support a green economy for the U.S.," she adds.

Back up on the roof, her colleague, Stuart Gaffin, is waving his hands across the top of the low growing vegetation. "If you ask New Yorkers, 'What do you want to look down on: a black roof, a white roof or a green roof,' everyone chooses the green roof."

Besides the view, there are important advantages.

"You can start with temperature," Gaffin explains. "Plants are geniuses at staying cool in strong sunlight. The temperature differences on a black roof versus a green roof are astonishing." Gaffin points out that he's recorded rooftop temperatures as high as 180 degrees on black top roofs, "and that doesn't have to be during a heat wave," he adds. But on a green roof, "the plants are about the same temperature as the air."

Gaffin says there could be as much as 40 square miles of roof space in Manhattan. "We are not doing anything good with that space right now. In fact, we are doing bad things with it."

One of the bad things Gaffin is referring to is the "heat island" effect. That's a phenomenon that occurs when the concrete of a city absorbs and stores enough heat to significantly raise the city's temperature above that of surrounding areas. Gaffin believes that turning the rooftops of an urban area, such as Manhattan, into green space could have a transformative effect on cities everywhere. "Obviously, you multiply all that area up and the temperature is affected. Every degree we can shave off can save us from a blackout and, by itself, just saves money," says Gaffin.

"Water is another very important issue," he continues. "We have acid rain and water quality problems with our rain and rainwater." Work by the research team indicates that green roofs act almost like filters. "We've seen acid rain reduction and we've seen pollutants removed," he says.

Cities have also been struggling with the problem of water runoff from even the most minor of rain storms. "When we get a typical rainfall, we get a wave of water from all these buildings, sidewalks and streets hitting our municipal sewer system, which can't handle it, and so it actually triggers pollution events in our rivers," explains Gaffin. But the research team's data show that after some rainfall events, the green roofs under study were absorbing most, if not all, of the water that fell on them. "Now that's an astonishing thing because the standard roof has 100 percent runoff. Some green roofs, however, have no runoff. Imagine a zero runoff roof," he says.

Water and temperature are just the first two items on the list. The green roofs in this research have also shown that they can help reduce noise pollution and contribute to biodiversity. "When I get on these roofs in the summertime, they become oases of biological activity, including honey bees, butterflies and other things," Gaffin says. "We've restored endangered grasslands that used to be common in the New York region that are all but gone… these systems, as living systems do wonderful things," he adds.

While the initial data seems to support green roofs as beneficial to urban environments, both Gaffin and Culligan point out that research needs to continue so they can more accurately quantify the benefits. If green roofs are as beneficial to urban areas as these scientists and engineers hope, they may end up literally "shouting from the rooftops!"

Miles O'Brien, Science Nation Correspondent
Jon Baime, Science Nation Producer

Any opinions, findings, conclusions or recommendations presented in this material are only those of the presenter grantee/researcher, author, or agency employee; and do not necessarily reflect the views of the National Science Foundation.