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Green roofs start to sprout on urban homes

Low-maintenance sedum cuts energy costs as well as greenhouse gases. Roofs are costly, though.

By Caitlin CarpenterCorrespondent of The Christian Science Monitor / October 31, 2007

Rooftop garden: Karen Weber of Boston looks over sedum in a display.

Tony Azios


New York

Every time it rained, Majora Carter cringed. "I lived in mortal terror whenever I thought it was going to rain," Dr. Carter says, remembering how the rainwater seeped from the street into her Bronx brownstone.

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Then she and her husband, James Burling Chase, realized that the source of the problem wasn't on the ground, but on the roof. The stormwater system in their neighborhood backed up so quickly that the water rushed straight from their roof to the street – and into their home.

They decided to try a new strategy to fix an old problem: a green roof.

Now, after a substantial renovation, their flat roof has come alive – literally. Flowers and baby sedum are anchored in a thin bed of soil and gravel covering the roof. Golf-ball-sized stones frame this rooftop oasis.

Now their roof will retain about half the rainwater that falls on it, once the sedum matures in about two years. But besides finding a practical solution to a recurrent problem, Carter and Chase wanted a tangible way to show they were "walking the walk" when it came to their environmentalism.

Their home is the first in New York to feature such a roof. Green roofs have taken root on numerous commercial buildings across the country, but now people are exploring the possibility of planting a little shrubbery atop their own homes.

Karen Weber, founder of the green-roof promoting organization, Earth Our Only Home, says there are numerous benefits to green roofs:

•Energy savings of 10 to 60 percent, as the greenery acts as another layer of insulation from heat loss in winter and cooling loss in summer.

•Less noise (extra layers of plants and drainage materials act as insulators) and less greenhouse gas (like any green plants, those on a roof absorb carbon dioxide and give off oxygen).

•Doesn't trap heat from the sun the way conventional roofs do. Conventional roofs can overheat entire cities.

•Attracts pollinators like honeybees and bumblebees, which are often scarce in urban areas.

•Doubles or triples the life of the underlying roof.

But all the benefits must be balanced with the hefty cost. And turning a roof "green" demands more than flipping through the phone book for a contractor.

Although some may be tempted to do it themselves, Ms. Weber advises homeowners to resist the urge. First, call a structural engineer to analyze how much weight the house can hold. And while planting sedum on your roof may sound easy, it requires expert installation of layers of protection, drainage, and growing medium, says Dustin Brackney, a landscape architect with Cypress Landscapes in Cambridge, Mass.

Until recently, several factors have limited the number of green roofs on homes, says Melissa Muroff of Roofscapes in Philadelphia. Contractors like Roofscapes (one of the oldest and largest green-roof designers and installers) are not used to doing small roofing projects, while conventional residential roofers often lack green-roof expertise. That's changing, though, as more people are demanding green renovations, Ms. Muroff says.