The Concept of Green Roofs Is 50 Years Old

Now, Columbia University puts the benefits to the test, in comprehensive green roof research.

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This podcast is part of the Sustainable Design radio program, a collaboration between IEEE Spectrum and the National Science Foundation.

Susan Hassler: One area where there’s great potential for sustainable design is above us: our rooftops. They’re called green roofs because they’re covered with vegetation. First developed in Germany in the 1960s, green roofs come with a long list of potential benefits, like saving energy, cutting pollution, and even lowering temperatures. Now, Columbia University is putting that list of benefits to the test, in one of the most comprehensive green-roof research studies in the nation. Glenn Zorpette reports:

Glenn Zorpette: Our tour of Columbia University’s green roof research actually begins below, in a basementlab.

Patricia Culligan: Um, okay, well, you’re looking at two setups here.

Glenn Zorpette: Geo-environmental engineer Patricia Culligan leads a team of scientists researching how green roofs affect things like temperature, storm water runoff, and air quality. Right now she’s peeling back the layers of a green roof sample called a “precultivated mat.”

Patricia Culligan: You can think of that as a green roof on a carpet. So you buy a carpet of green roof and you just roll it out over your waterproof membrane. Then you have a drainage layer, so that allows water that makes its way through the different layers of the green roof system to actually be directed towards a drain, and we have a little drain in the box here, so that would go to your existing building drainage system.

Glenn Zorpette: Culligan experiments with every layer of green roof design. Buildings vary as much as climates, and she’s looking for winning combinations.

Patricia Culligan: And then we have the growing medium. This is not soil. This is an engineered soil. Soil would be too heavy to put on top of an existing rooftop. If you want to put soil on a rooftop, and there are roofs like that—we call them intensive green roofs—you generally have to design a building to hold that weight.

Glenn Zorpette: Culligan is researching how various green roof designs slow down and even eliminate storm water runoff, which is a major environmental issue for many cities.

Patricia Culligan: I think people are beginning to realize that this might be, in cities like New York that have combined sewer systems, one of their primary advantages, creating permeable surfaces on areas that we normally thought were not usable, i.e., the rooftops of the city.

Glenn Zorpette: Culligan’s research partner, climate scientist Stuart Gaffin, is checking his sensors on one of their green roofs at Columbia University.

Stuart Gaffin: You can actually see some water running off from yesterday’s rain.

Glenn Zorpette: This roof is covered with what looks like a thin layer of small gravel and sedum, drought-resistant flowering plants only two or three inches high.

Stuart Gaffin: And it’s, ah, you know it’s functioning very nicely. It attracts biology. We hear crickets up here, butterflies…

Glenn Zorpette: But more importantly…

Stuart Gaffin: They are keeping the temperatures nice and comfortable in the summer and the winter. We’re measuring a lot of that with our equipment that’s embedded in the roof.

Glenn Zorpette: Gaffin is quantifying how New York temperatures would be affected if the buildings were retrofitted with green roofs, even the buildings more than 100 years old.

Stuart Gaffin: That’s not an impediment. I believe there’s a system for just about every roof in New York, no matter how light it has to be.

Glenn Zorpette: On this day, there’s been some rain in the Big Apple, giving Gaffin a good chance to compare this green roof to the asphalt jungle surrounding us.

Stuart Gaffin: What’s going to happen on the neighboring roofs is 98 percent of that water is just immediately going to go into the drains and be sent into the system. On this roof, as a rule of thumb, we expect about half that water to never get off the roof, to actually stay here and eventually evaporate.

Glenn Zorpette: The evaporation contributes to a green roof’s ability to cool temperatures, also an area of focus for another research partner, geochemist Wade McGillis.

Wade McGillis: An analogy of a human being—it sweats and when you sweat, you cool. So when the building is evaporating, it therefore drops its temperature because of the evaporation process. It loses energy.

Glenn Zorpette: McGillis leads research into the final part of the puzzle, measuring with remote sensors not only evaporation, but how green roofs affect air quality issues, like carbon dioxide levels, and water quality issues, like acid rain.

Wade McGillis: The roof is actually taking this acid rain and bringing it up to neutral condition so that the water that leaves the building isn’t as corrosive.

Glenn Zorpette: McGillis’s research involves meteorology and developing new sensors and ways to measure what is essentially a new ecosystem. And team lead Patricia Culligan stresses there are still many unknowns.

Patricia Culligan: Maybe one system that’s optimal for storm water management is not necessarily optimal for cooling the environment. We’re not necessarily pro– or anti–green roof. We’re just trying to understand how they behave.

Glenn Zorpette: Culligan will pull together the findings of her team over the next three years, providing new scientific guidance on how green roofs can help to green cities. I’m Glenn Zorpette.

Producer: Laurie Howell

This podcast is part of the Sustainable Design radio program, a collaboration between IEEE Spectrum and the National Science Foundation.