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Green Roofs

Green roofs are becoming more mainstream. Here's why.

Rooftops around the Northwest are sprouting tufts of grass, forming a greenbelt above our heads. Quite recently, it seems as if green roofs have gone from funky and alternative to upscale and mainstream. The Seattle architecture firm Olson Sundberg recently designed a living roof atop the home and garage of a lakeside estate, as part of a complex geothermal climate control system. And the Northwest Eco-Building Guild green roof tour was greeted with great enthusiasm last spring, when participants climbed up to take a look at planted roofs on homes, offices, studios, and even a chicken coop.

‘The Germans are light years ahead of us,” says Patrick Carey, an architect who heads the Guild's Green Roof Project. In German cities such as Stuttgart and Mannheim, commercial buildings are required to have green roofs. The City of Seattle is taking an active role in encouraging public and residential green roofs, and in Portland, Oregon, developers can build taller if they top out with a living roof. Last June, Portland hosted “Greening Rooftops for Sustainable Communities,” the second annual international green roof conference, which featured 55 speakers from 10 different countries.

Green roofs consist of a layered sandwich of roof deck, waterproof membrane, and soil filter fabric, topped with several inches of lightweight, pumice-rich soil mix. All you see is the top layer of drought-tolerant hardy plants, often enlivened with vegetables, annuals, and perennials. Even after a rainstorm, a fully saturated green roof weighs no more than one made of tile—about 15 pounds per square foot. Surprisingly, the soil need only be three to four inches deep to gain all the ecological benefits, although deeper soil (built up where supports are strongest) allows for a greater variety of plants.

A residential green roof can be built for $8 to $12 per square foot—about the same price as tile or slate, and about twice as much as metal. Green roofs need to be irrigated and weeded regularly for the first year, but when planted with drought-tolerant fescues, mosses, and sedums, they can get by thereafter on rainfall and a twice-yearly weeding. The dirt and plantings provide a cushion, mitigating the extremes of temperature that cause a roof to wear out, and protecting it from sunlight. A living roof often lasts two to three times longer than a standard one.

And there are manifold ecological benefits. A green roof acts like a storm window to reduce energy use, insulating the house from summer heat and winter chill, produces more oxygen than a family uses in a year, and both absorbs and reflects noise. It also collects pollution from the air, as well as capturing, filtering, and slowing water runoff—a great benefit for salmon streams and overburdened storm sewers.

‘This is the greenest thing you can do in building, other than not building at all,” says Carey. He says that the idea of capping homes with alpine meadows “struck my radical core—this is seriously sustainable and a building element that actually gives back to the environment.” 

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