In several cities, the impetus for green roofs has come from federal mandates for improving storm-water management. When rain falls in the woods, most of it is absorbed by the soil. But in densely built cities, about three-quarters of the rainfall fails to sink into the ground or evaporate. Instead, it runs into city sewer systems, which often overflow during intense storms. "One out of two times that it rains in New York City, raw sewage flows into city waterways like the East River," says Colin Cheney, director of the Green Roofs Initiative for Earth Pledge, a nonprofit group that facilitates the development of green roofs in New York.
The soil on green roofs can slow the runoff from buildings. The water evaporates into the atmosphere or is taken up by the vegetation. "In summer, depending on the plants, growing medium and climate, green roofs retain 70 percent to 80 percent of the precipitation that falls on them; in winter, they retain between 25 percent to 40 percent," says Peck.
To encourage these benefits, incentives are sometimes offered. The developer of Manhattan's Chelsea Residence will receive a check for $55,000 from the New York State Energy Research and Development Authority for using green roofs, thermally efficient insulation and energy-efficient windows, according to Richard Vitto, the project's architect.
Incentives are necessary since, in the short run, green roofs are more expensive to construct than standard-issue roofing. "You have to convince policymakers that green roofs deliver tangible benefits to the public that justify the investment," says Peck.
Those who experience green roofs are easier to convince--any new patch of greenery gets raves from urban dwellers enveloped by concrete and steel. The two green roofs on the Chelsea Residence, including a vegetable plot maintained by the tenants, are intended to make up for the complete lack of open space in the area. "Part of the reason you create green roofs is that they're good for the environment," says Vitto, "but it's the human aspect people get excited about." Other projects on the Earth Pledge roster include low-income apartment houses in the green-deficient South Bronx and Harlem.
Concludes Peck, "It's astounding how many benefits you can generate with green roofs. You can create a sense of community, keep people off the streets, provide social assistance and help the ecology."
greening your roof
If you want to install a green roof on your home, odds are you'll be the first on your block to do so. Unless, that is, you live in the ecologically savvy Northwest U.S., where around 50 residential roofs already exist, according to Patrick Carey, a Seattle architect and director of the green-roofs project of the Northwest Eco-Building Guild.
Pioneers reap many benefits: Not only does green-roof technology on your home make your roof last longer, it reduces air-conditioning bills, cuts down on water runoff and improves your view. "You can look out the window and instead of seeing a hot and dry roof, you see something green," says Tom Liptan, an environmental specialist for the city of Portland, Ore. "You're improving livability."
So far, green roofs cost more than the standard sort. Suppliers are oriented toward commercial clients and haven't yet scaled down their products or their prices for homeowners. And you can count on hiring an architect or structural engineer to make sure your roof will bear the weight of plants and the soil medium.
Because of the expense, and because green roofs are simplest on flat surfaces, Liptan recommends that first-timers start off by having work done on their garages or on outdoor structures like garden sheds. If you plan on redoing the roof on your home, he suggests waiting until your current one needs replacing or at least serious repair.
Meanwhile, you can help reduce storm-water runoff by putting "storm-water planters" under the downspouts of your house's gutters. "Built above the surface, and filled with sandy soil and plants that can tolerate temporary flooding, they slow down rainwater, filtering it and allowing some of it to return to the atmosphere by transpiration," says Liptan. "The more storm water you remove from your sewage system, the less pollution enters your rivers and lakes."
You can also improve your environment by putting plants in containers on your roof or terrace. "To notice a difference in terms of reducing air pollution and lowering summer heat, cover at least one-fifth of the surface with plants," says Liptan.
Another low-tech way to boost air quality and reduce energy bills involves planting vines on your home's walls. "You are especially likely to conserve energy if you plant on the south side," Liptan says.
If you want tall vines that need support, such as clematis, large lightweight trellises are now available. "It's time for people to get used to the idea of mixing plants and buildings," asserts Noël Kingsbury, co-author of Planting Green Roofs and Living Walls.