The plants on green roofs also work like a giant sponge, soaking up water and therefore reducing the amount of rainwater that reaches street level, lowering the risk of flooding, minimizing sewer system overflow and filtering dirty runoff.
“Cities are built to be impervious,” Mr. Peck said. “The water runs off buildings and streets, and when it hits streams and estuaries, it can cause erosion. It’s also incredibly contaminated. If you want to have drinkable, fishable, swimmable water in and around our cities, we need to capture storm water and use it as a resource, instead of just getting it away from the buildings as soon as possible.”
Beyond the long list of financial, health and environmental benefits, green roofs have become valued amenities in residential buildings.
“We were really keen on finding a place with outdoor space,” said Carolyn Kushner, who chose the apartment building 525W52 in New York in part because of the roof, which features lounge chairs overlooking the Hudson River, a large lawn, grilling stations, bocce ball and Ping-Pong and shuffle board tables, all surrounded by shrubs and greenery. Dr. Kushner and her partner — who proposed on the roof — go up there several times a week. “We have a glass of wine and watch the sunset. The city can be really overwhelming if you don’t get that kind of greenery. It’s nice to have someplace to escape to.”
One early cause of hesitation in adopting living roofs was the upfront cost. Green roofs cost two to three times as much as a traditional roof.
“With so many environmental initiatives, it often boils down to money,” Ms. Domanska said. “One of the ways we’re able to sell this is that a green roof significantly extends the life span of a roof. Over time it saves money. That was very attractive to the people who live here. Now people see all the benefits, but initially it was about the savings.”
Studies bear this out. A 2006 study from the University of Michigan compared the expected costs of conventional roofs with the cost of a 21,000-square-foot green roof and found that over its lifetime, the green roof would save about $200,000. Almost two-thirds of that would be in reduced energy needs for the building below.
“We’ve established best practices for creating green roofs,” Mr. Peck said. “We have political leaders looking for ways to improve the health and well-being of the people in their cities and also address climate change. We have research that shows the benefits of green roofs. It seems clear that the cities that invest in green infrastructure will be the cities that thrive.”