Chicago"s Millennium Park boasts a "green roof" covering a parking garage and train terminal.© Greenroofs.org
But few writers noticed one of the greenest things about Millennium Park: it’s a living "green roof" of plants on top of a huge subterranean parking garage and commuter train terminal. With 120 living roofs built or planned citywide, including one on City Hall, Chicago is pioneering a trend that has taken off across North America. The impetus came from Mayor Richard M. Daley, a proponent of green technology who was impressed by the green roofs he saw when traveling in Europe—where they’ve been built for 30 years and have become commonplace.
Building green roofs "is a very important initiative for the mayor," says Chicago Green Projects Administrator Michael Berkshire, an urban planner. "He’s a very popular mayor, so when he gets behind something, it really means a lot."
In a city where more than 700 people died during a July 1995 heat wave, one benefit of green roofs that was clearly important to Daley is their capacity to mitigate the "urban heat island effect" that makes cities hotter than surrounding suburbs. Higher urban temperatures derive from mile after mile of concrete and other heat-absorbing materials; in contrast, the plants on green roofs cool the air and reduce ozone formation.
Green roofs also reduce flooding and sewer overflows by absorbing rain like sponges, slowing its release and filtering out pollutants. The plants oxygenate the air, attract birds and insects and soften the urban "viewscape." In addition, the living systems insulate roofs, keeping buildings cooler in summer and cutting electricity use. (An unexpected benefit of the Millennium Park green roof is that the South Shore Terminal directly below the park stays cool, despite the heat of the trains.)
Chicago has taken its cue from German cities by requiring green roofs and other "green" strategies for certain construction projects that are built with public money, that are large enough to require special review or that sit close to Lake Michigan. For instance, a new Target store on the city’s north side is getting a green roof. The Millennium Park green roof is unusual; most green roofs are not designed for everyday foot traffic. Those that are require a heavier layer of soil and stronger roofs.
Other examples in North America include a 1.5-acre living roof at the Gap corporate office in San Bruno, California; a 3/4-acre roof on a business complex adapted from a 1925 Montgomery Ward catalog warehouse in Baltimore; more than two acres of green roofs and terraces at the Solaire high-rise in Manhattan; and an eight-acre multi-level roof at Salt Lake City’s Church of Jesus Christ of Latter-day Saints Conference Center. The Salt Lake City center’s roofs resemble mountain meadows, planted with 300 types of wildflowers.
The world’s largest green roof shelters the Ford Motor Company’s Rouge River truck plant in Dearborn, Michigan. The 10.4-acre roof, completed in 2003, absorbs four million gallons of rainwater each year, minimizing storm runoff into the nearby Rouge River. The $3.6 million cost of the roof was double that of a conventional roof, but the German experience shows that green roofs last at least twice as long because of the protection they provide.
Americans should take more advantage of the German experience, says German native Peter Philippi of Green Roof Service in Street, Maryland. "In the States, it seems people want to invent the wheel anew," Philippi says, adding that North Americans can learn from reading the German standards, now available in English.
New York City is working to add green roofs to mixed-use neighborhoods where parks are scarce. In Long Island City, Queens, a nonprofit consortium called Earth Pledge is backing a living roof on a metal manufacturing plant in a noisy and drab neighborhood. The aim is to make the area more livable. "There’s been a lot of interest in trying to focus green development in areas that are at risk," says Colin Cheney of Earth Pledge.
Cheney notes that storm water control is a prime motivator. Even a 1/20th inch of rain can trigger a flood, and when any of the city’s 14 sewage treatment plants is overwhelmed, the overflow—raw sewage—dumps into the Hudson River, the East River or the New York Bay.
Another ambitious project is the green roof planned for the remodeled California Academy of Sciences in San Francisco’s Golden Gate Park. The challenge is to plant greenery on two domes that will house a planetarium and a rainforest exhibit. "It’s like a flying carpet with big bumps in the middle," says Lawrence Reed, a principal with the landscape architects SWA Group. Reed notes that in a Mediterranean climate like California"s, "green roof" is a misnomer. The roof won’t be green during the dry season. And the plants are chosen less for looks than for their hardiness. "The romantic vision is the sod roof and grasses," says Reed, "but the most durable plant is the sedum plant," a humble succulent. Reed’s firm will plant the Academy roof with wild strawberry, sea thrift, the herb self-heal and stonecrop, a sedum that attracts the endangered Mission Blue Butterfly.
The possibility of butterflies pleases Paul Kephart of Rana Creek Habitat Restoration, a project consultant. He sees living roofs as the key to restoring urban "hardscapes." His aim is "encouraging habitat where it’s otherwise not found
using living architecture and restoration ecology to restore the missing components of our environment." Although some roofs incorporate irrigation, Kephart prefers roofscapes that survive on their own once established.
Kephart also advocates green walls: the more living ecosystems in the city, the better. Fostering urban greenery has a long history, he says, citing the ancient Hanging Gardens of Babylon. "They did it to convey water through the site and cool their buildings, and to provide pleasant courtyards and plazas," says Kephart. "There’s nothing new under the sun."