The Washington Nationals’ new baseball stadium opened the 2008 season with one. Vancouver’s 2010 Winter Olympics will feature many more. And earlier this year Minneapolis decreed that the city’s voluminous Target Center arena will have one too.
Suddenly, green roofs are sprouting across North America. Designed to curb air pollution, decrease energy expenses and reduce storm runoff, the environmentally friendly assemblies are adding a decidedly earthy element to urban skylines — a sign that the green roof industry is rapidly coming into its own.
Particularly in cities, the rise of roof-topping grasses, succulents and other vegetation is fueling a boom for landscape architects, growers, builders and consultants in the know. As the roofs bloom in size and number, cities are weighing new incentives to developers and owners to install the admittedly costly growing medium and plant life as a long-term investment that could benefit both businesses and surrounding communities. And with a strengthening infrastructure to support them, designers are branching out in new directions.
Steven Peck, founder and president of the Toronto-based industry association Green Roofs for Healthy Cities, said the industry’s “mother ship” hails from Europe, particularly Germany. Research there in the 1970s on lightweight, low-maintenance green roof systems dominated by hardy sedum grasses, he said, “opened up thousands of miles of roofscapes that had been unavailable to any sort of greenery.”
German policymakers quickly took notice of the advantages, including the potential to reduce both stormwater runoff and the urban heat island effect associated with asphalt, concrete and metal surfaces. In response, they created dozens of incentives and regulations encouraging more green roof construction. In the mid-'90s, a European industry mostly dominated by French and German firms began expanding into North America and introducing the basic concepts to a new generation of specialists.
Peck, himself introduced to the idea in 1997, was tasked with leading a federal study on its benefits and barriers in Canada, only to find that there was little scientific information available for North America. “There was no proof, it was all in German academic studies,” he said.
One of his committee member spent hours translating many of the studies into English. And even those reports sidestepped analysis of big-picture benefits that had been largely taken for granted.
A decade later, the industry has been buttressed by research and case studies detailing both individual benefits like savings on cooling costs and enhanced commercial values, and bigger-picture pluses like reduced air pollution and storm water overflows.
Another essential element has been building expertise across a talent pool that remains unevenly distributed. Peck’s group has been working for five years on an accreditation program modeled in part on LEED certification (Leadership in Environmental Energy and Design). The new Green Roof Professional, or GRP system, should roll out sometime next year, he said. In the meantime, Green Roofs for Healthy Cities has grown to include more than 80 corporate members and has trained more than 4,500 individuals. “You can’t have an industry unless you can have people who can design and deliver,” he said.
Most green roofs still feature sedum and ice plant succulents, which can tolerate harsh growing conditions and are ideally suited for low-maintenance rooftops. These “extensive” roofs, as they’re known, require only a few inches of growing medium, reducing overall weight and cost.
John Shepley, co-owner of Maryland’s Emory Knoll Farms with industry leader Ed Snodgrass, said business is booming at their green roof plant nursery, based on a former dairy farm. “We’re probably growing 50 percent annually without trying,” Shepley said. Although Washington, D.C., and New York City remain big markets, he said, the federal government has been coming on strong with new mandates for green buildings.
In mid-March, Shepley delivered pregrown sedum plugs to cover a concessions area at the new Washington Nationals baseball stadium. Two years ago, the business helped install thousands more grass plugs on a massive green roof for a Library of Congress facility in Culpepper, Va.
As green roof technology matures, new projects have begun unveiling increasingly varied designs, including “intensive” roofs that require deeper growing depths and considerably more investment but can deliver more aesthetic, conservation and other benefits. Chicago’s $480 million Millennium Park, a 24.5-acre cap over rail yards and a parking garage, is one of the world’s largest intensive green roofs to date. An award winner from Green Roofs for Healthy Cities, the project has all the advantages of a major urban park, Peck maintains.
Considered the industry frontrunner among North American cities, Chicago used a mix of intensive and extensive vegetation to cover 20,000 square feet atop its City Hall in 2001. In August of that year, researchers recorded a rooftop temperature of 119 degrees in the planted area, compared with a blistering 169 degrees on an adjoining black tar roof. Since then, the green roof has saved the city an estimated $3,600 in annual cooling and heating costs. If all Chicago roofs were similarly clad, city officials believe peak energy demand could be cut by 720 megawatts, or enough electricity for 750,000 consumers. Similarly, the load on the city’s storm sewer system could be slashed by roughly 70 percent.
Chicago is now adding green roofs to everything from office buildings to fire stations, and city governments in Toronto, Minneapolis and Seattle are following suit.
Other city-based incentives, popular in Chicago and Portland, give developers extra floor space if they add green roofs, and fast-track programs are rewarding environmentally conscious projects with a front-of-the-line approval process.
Washington, D.C., which has long struggled to control storm-water overflows into the Potomac and Anacostia Rivers, is moving toward a system that will account for impervious building surfaces that increase runoff when assessing water consumption fees. Adding more water-retaining surfaces, including green roofs, will effectively lower a building owner's city fees. Beyond relieving the impact of development, the measure is designed to encourage both public and private-sector investment into runoff-reduction technology.
Nevertheless, Mark Thomann, design director at the New York landscape design firm Balmori Associates, said high construction costs, a lack of government support, and limited expert availability are still combining to thwart many ambitious green roof plans. “We imagined doing 50 of these a year, and the reality is we do one to two a year,” he said. In cities like New York, zoning laws haven’t yet accounted for green roofs, relegating them to awkward spaces between traditional buildings and landscaping. Plant specialists, Thomann said, often don’t have the equipment for roof access, whereas contractors often don’t know enough about plant-friendly construction practices.
Even so, his firm has been trying to push the envelope on design. “Most of the green roofs have been flat green surfaces, which are great, but we’ve been looking at different ways of making those roofs three-dimensional,” Thomann said. One of his company’s completed projects, the roof of New York’s Earth Pledge Foundation, features a vegetable garden.
Other design firms are teaming up with ecologists to build roofs that incorporate hollows, small cliffs, scattered rocks, dead wood and varied vegetation to mimic everything from riverbanks to high mountain meadows. The result has been a surprising burst in habitat for urban wildlife —and more aesthetically interesting roofscapes that building owners can use to attract human tenants as well.
Endangered beetles and spiders have found refuges in Basel, Switzerland, where a city mandate requires green roofs to accompany all new flat-roofed buildings. Endangered rooftop orchids thrive in nearby Zurich, while London has created new habitats for its small population of black redstart birds with crushed-brick “brown roofs” that mimic the derelict urban sites they favor. And in San Francisco, several acres' worth of dramatically sloping rooftop on the California Academy of Sciences have been designed as native habitat for the threatened bay checkerspot butterfly.
In New York, a planned visitors’ center at the Brooklyn Botanic Garden is allowing staff to likewise experiment with a wide range of vegetation that might work as “living roof” plantings to fit the building’s sinuous profile. Patrick Cullina, the garden’s vice president of horticulture and science research, pointed out that green roofs need not be monolithic, especially ones that are meant to be seen.
The rooftop garden on Bill Clinton’s Presidential Library in Little Rock, Ark., includes yellow roses, a sentimental favorite. Cullina hopes the more publicly accessible Brooklyn rooftop will include visually interesting colors all year long. He also hopes to challenge the paradigm of which plants can be used in a living roof system, using nearby seashore and pine barren environments as inspiration.
Moving beyond simple cost-benefit calculations can pay particular dividends for an institution hoping to marry aesthetics, research and community leadership. It helps when a chartreuse rooftop grass turns a particularly vivid shade of reddish-orange during an otherwise dreary New York winter.