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Changing city rooftops into green landscapes

Copenhagen’s sewage overflow facility has been covered with prefabricated mats of highly absorbent sedum plants.

Living roofs are becoming an aerial line of defence against flooding caused by the heavier rainfalls associated with climate change.

Fly in over cities like New York, Stuttgart or Madrid, and other cities around the world and you see they are literally going green, square meter by square metre.

Those square metres, however, are in the rare air of city rooftops. More than just a refreshing break from the barren, black landscape at the top of the city skyline, green roofs are an effective way for cities to adapt to climate change.

The tiny, hardy plants that make up living roofs act as sponges when it rains, reducing the amount of runoff that makes its way to treatment plants.

A great green initiative
The newest city to embrace living roofs is Copenhagen, where city officials intend to incorporate it as part of their climate adaptation strategy.

“We studied green roofs around the world and were like, ‘Wow, these things are great’,” says Dorthe Rømø, of the City of Copenhagen’s Park and Nature Centre. “Adding more greenspaces also adds to quality of life for people in the city. Green roofs are the single most beneficial sustainable initiative there is.”

Green roofs can do something else that traditional roofs can’t: they can change colour with the seasons – from green to red and then to golden.

Keeping water out of the sewer
Because climate change has heavier rainfalls in the forecast for the future, one of the most important things green roofs can do is help reduce the amount of runoff reaching sewers.

Small plants such as sedum or hens and chicks plants can absorb as much as 80 percent of the rain that falls on the roofs. Such plants are often better suited for use in living roofs than grass because they are more resistant to drought.

“Water doesn’t start running off until the soil on the roof is saturated,” explains Senior Project Manager Arne Bernt Hasling. “Delaying and decreasing the amount of runoff helps prevent streams from overflowing and people from getting water in their basements.

“The rain that falls on the roof gets absorbed by the plants, and then gradually evaporates. Much of the rainwater never even reaches the sewer.”

COWI has helped the City of Copenhagen form its climate adaptation efforts by creating the overall strategy. Blue roofs that collect rainwater as well as green roofs are among COWI’s suggestions to help the city realise its goals.

By Eva Isager evis@cowi.com
Published: 21.01.2010

The green roofs of Copenhagen

Green roofs are just one of 50 initiatives making up the City of Copenhagen’s plan for adapting to tomorrow’s climate.

By 2015, the city expects to establish 325,000 square metres of green roof – roughly the same as five football pitches. According to COWI’s calculations, that translates into a CO2 emissions savings of 218 tonnes annually.