Turning the black tar roofs that cover our cities into green spaces is not cheap or easy, but its benefits to the environment would be great.
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.
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, which could be alleviated by the addition of vegetated roof systems to absorb rainwater...