Green roofs are neither simple nor 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 both simple and cheap:
But less investment buys less return. White roofs don’t catch rainwater, help biodiversity or clean the air. Gaffin’s group found that the white portion of the Con Ed roof averaged 43 degrees cooler than black at noon on summer days. That’s something, but it’s a smaller cooling effect than green roofs offer. Green roofs improve each year as vegetation becomes denser and taller. But after a few months, a white roof tends to look like city snow — covered with soot. As a white roof dirties, it loses a lot of its cooling ability.