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Green Roofs Bring Relief From Above,
by Tina Roseberg, The New York Times

It’s spring — time to plant your roof. Roofs, like coffee, used to be black tar. Now both have gone gourmet: for roofs, the choices are white, green, blue and solar-panel black.

All are green in one sense. In different ways, each helps to solve serious environmental problems. One issue is air pollution, which needs no introduction.The second is the urban heat island. Because cities have lots of dark surfaces that absorb heat and relatively little green cover, they tend to be hotter than surrounding areas — the average summer temperature in New York City is more than 7 degrees hotter than in the Westchester suburbs. This leads to heavy air-conditioning use — not good — and makes city dwellers miserable. For a few people every year, the heat is more than a discomfort — it’s fatal.

The other problem is storm water runoff. In New York, as in about a fifth of American cities, there is only one sewer system to conduct both rainwater and wastewater. About every other rainfall in New York, sewers flood and back up, discharging their mix of rainwater and wastewater into the city’s waterways. It doesn’t take much to overload New York’s sewers — it can take only 20 minutes of rainfall to start water from toilets flowing into Brooklyn’s waterways. The water does more than flood streets. It makes us sick — cases of diarrhea spike when sewers overflow. When sewers back up, polluted water runs into our lakes and oceans, closing beaches.

How can a new roof help?

At 1:45 in the afternoon on August 9, 2001, the temperature in Chicago was in the 90s. Eleven stories up, on the roof of City Hall, the surface temperature of the black tar measured 169 degrees. But Mayor Daley, environmental innovator — yes, that Mayor Daley — had done something interesting. The year before, a section of the City Hall roof had been painted white. The surface temperature there was between 126 and 130 degrees. And much of the roof of the building had become a garden — 20,000 plants in 150 varieties, chosen for their abilities to thrive without irrigation and stand up to Chicago’s notorious wind. The surface temperature of the green roof varied between 91 and 119 degrees.

So the difference between a black tar roof and a green roof was at minimum 50 degrees. And the green roof was able to retain 75 percent of a one-inch rainfall. The two tasks go hand in hand — green roofs cool by capturing moisture and evaporating it.
Putting living vegetation on the roof is not a new idea. For thousands of years people have made sod roofs to protect and insulate their houses, keeping them cooler in summer and warmer in winter. The modern movement for green roofs began in the last 50 years in Europe. Germany, where about 10 percent of roofs are green, is the leader; some parts of Germany require green roofs on all new buildings.

Greening a roof is not simple or cheap. Over a black roof — flat is easiest but sloped can work — goes insulation, then a waterproof membrane, then a barrier to keep roots from poking holes in the membrane. On top of that there is a drainage layer, such as gravel or clay, then a mat to prevent erosion. Next is a lightweight soil (Chicago City Hall uses a blend of mulch, compost and spongy stuff) and finally, plants.

An extensive roof — less than 6 inches of soil planted with hardy cover such as sedum — can cost $15 per square foot. An intensive roof — essentially a garden, with deeper soil and plants that require watering and weeding — can double that. But because the vegetation is thicker, it will do a better job of cooling a building and collecting rainwater. Plants reduce sewer discharge in two ways. They retain rainfall, and what does run off is delayed until after the waters have peaked.

A study conducted by Columbia University and City University of New York of three test roofs built by Con Edison in Queens found that the green roof — an extensive roof, planted with sedum — cut the rate of heat gained through the roof in summer by 84 percent, and the rate of heat lost through the roof in winter by 34 percent.

Another study (same researchers, same Con Ed test sites) found that green roofs are a very cost-effective way to reduce storm water runoff. If New York has one billion square feet of possibly greenable roof, planting it all could retain 10 to 15 billion gallons of annual rainfall — which would cut a substantial amount of sewage overflow. “If you add in all the other green infrastructure, such as street trees, permeable pavement and ground collection pits, it might be possible to eliminate the combined sewage overflow without building specialized water detention tanks, which are hugely expensive,” said Stuart Gaffin, a research scientist at Columbia’s Center for Climate Systems Research, who co-authored both studies with colleagues from City College.

Green roofs have other advantages.They scrub the air: one square meter can absorb all the emissions from a car being driven 12,000 miles a year, said Amy Norquist, chief executive of Greensulate, which installs green roofs.And green roofs can provide the plants that animals, birds and bees need where parks are far apart.

White roofs are cheap and don’t require any engineering — just a layer of special paint. New York City is trying to coat a million square feet of roof a year. Building owners can do the work themselves, or they can engage CoolRoofs, a city initiative that promotes white roofs and organizes hundreds of volunteer painters. Since 2010, about 3,000 volunteers have coated 288 buildings.

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