Camouflage housing (Image: Jim Brickett)

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Green roofs are not just a load of greenwash. That’s according to a new study which has measured the amount of carbon absorbed by 13 different green roofs.

“I can’t imagine why anyone wouldn’t want a green roof,” says Kristin Getter, who carried out the research with colleagues at Michigan State University in East Lansing.


Getter’s team examined 12 existing green roofs and grew their own Sedum-covered roof. They found that the roofs absorbed up to 375 grams per square metre over the two years of their study.

That may not sound like much, but it adds up. If a city the size of Detroit, Michigan, with around a million inhabitants, were to switch to green roofs, it would remove as much carbon from the atmosphere as taking 10,000 mid-sized SUVs and trucks off the road for a year.

There’s a catch, though: starting a garden on the roof won’t immediately lower your carbon footprint. Greening conventional roofs requires special materials, which come at a carbon cost. It takes seven years for the roof to offset the carbon used for its building materials and become truly carbon negative. Developing low-carbon building materials could bring it down to two or three years, says Getter.

Jon Sadler of the University of Birmingham, UK, says green roofs have many benefits, such as increased biodiversity, but is unconvinced by their potential for absorbing carbon. “You need a big area to make it work,” he says. “It’s not a quick fix.”

Journal reference: Environmental Science & Technology, DOI: 10.1021/es901539x