Green Buckeye RN

NYT: Green Roofs in Big Cities Bring Relief From Above
June 5, 2012, 9:12 am
Filed under: News


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, and the adjacent Cook County  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.

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