Audubon Living
Raising the Roof
Today city skylines are getting greener. Wildflowers and grasses are carpeting rooftops, soaking up storm water, cooling buildings, and providing habitat in the clouds.
By Susan J. Tweit
From the rooftop terrace atop Portland, Oregon’s Metro Building, the Convention Center’s asymmetric glass spire juts over nearby roofs, the wide Willamette River flows beyond, and, in the background, downtown Portland’s towers rise against forested hills.
It’s what’s in the foreground though, that catches the eye: The roof of the former Sears Roebuck store is carpeted with green and burgundy sedums, ground-hugging, succulent-leaved plants whose starry white flowers dance in the breeze as cumulus clouds scud by overhead.
“Originally this was just gravel,” says Tom Liptan, a landscape architect with the City of Portland’s Bureau of Environmental Services, gesturing to the colorful turf. “When it was planted in August 2005 with little sprigs, it looked like a wasteland.” He looks around and smiles. “Now it’s alive—it’s an eco-roof.”
Liptan, tall, lean, and dressed for Portland’s outdoor-oriented culture in hiking pants and river sandals, is showing off one example of the hottest trend in green building: eco-roofs, also called green or living roofs. Blanketed with an insulating layer of soil medium and plants, these roofs are springing up in cities to fight climate change, save energy, prevent flooding, and provide habitat for birds, butterflies, and other airborne wildlife.
In Europe, meadow-style cottage roofs go back centuries, but planners and ecologists began touting industrial-size versions for city buildings in the 1980s to temper the so-called “heat island” effect and to reduce runoff. Paving and conventional tar and gravel roofs in urban areas absorb tremendous amounts of solar energy and re-radiate it, thus heating surrounding air and causing cities to be as much as 10 degrees Fahrenheit hotter than surrounding countryside.
Hotter cities cause higher demand for air-conditioning, more air pollution, increased greenhouse-gas emissions, and more heat-related illnesses and deaths. Planted roofs radically reduce heat absorption, helping to keep the heat island effect in check. They also create shade and add insulation, so that buildings are cooler in the summer and warmer in the winter. Researchers at Environment Canada, a government agency, estimate that vegetated roofs can reduce summer electricity consumption linked to cooling by up to 25 percent for one-story structures, and by about 6 percent for buildings three to eight stories high. Additionally, green roofs clean the air by removing particulates and ozone-producing compounds, and they add oxygen and sequester carbon as the living plants respire and make food.
The chief selling point for Portland was capturing storm water. Nearly 90 percent of Portland’s 37 inches of average annual precipitation falls from October through May, at rates of up to 12 inches a month. A century ago that flood of precipitation was absorbed by a spongelike cover of native forest; today it runs off impervious manmade surfaces like streets, parking lots, and roofs in volumes that cause local flooding and can overwhelm sewage-treatment facilities, sending untreated water into the Willamette River.
Portland’s green roof movement began as Liptan’s simple, though primitive, experiment. “I had this crummy old garage and the roof leaked, so I thought, why don’t I just put it up there and see what happens,” he says. So he covered his garage roof with a sheet of plastic, added a layer of soil, and planted it, finishing in the late summer of 1996, just before the wet season.
When the first rain came, Liptan found himself next to his garage, “holding a bucket under the downspout, getting soaked, and waiting and waiting for water to come out.” Finally the downspout began dripping. His bucket collected only two of the 40 gallons of rain he estimates fell on the 180-square-foot roof.
Impressed by his makeshift roof’s ability to retain water, Liptan added instrumentation: a rain gauge and a 55-gallon drum to catch the runoff. For two years he checked the gauge and barrel twice a day whenever it rained: “We get 150 days of rain a year,” he says. His data helped convince Portland to include eco-roofs in its storm-water–management manual. The city put the first one atop a downtown apartment building in 1999. Today it installs them on all new city-owned buildings where feasible, and has begun retrofitting existing roofs. Portland’s data show that green roofs can reduce runoff from intense rainstorms to roughly half of that from conventional roofs. “If we can avoid expanding sewer pipe and sewage plant capacity, we save millions of dollars,” says Liptan.
Unlike the setup on Liptan’s garage, the earliest green roofs used a continuous layer of engineered soil medium, which made them heavy. Further improvements to those soil replacements, some of which resemble potting soils, have since trimmed the weight. These new soils have also helped avoid compaction, and can be planted at depths as shallow as one inch. Some newer types use mats of plants and soil laid like sod, while modular systems employ movable trays containing soil medium and plants. No matter what kind they are, green roofs add weight to buildings, so it’s important to consult an expert such as an architect or roofing contractor experienced in these roofs before incorporating one on your home.
Once they are installed, most green roofs need to be irrigated at least until the plants are established. Some feature native species, others are planted with horticultural varieties, and some mix the two. All require plants that can withstand the extreme environments of rooftops, where temperatures may fluctuate 50 degrees between day and night and where high-velocity winds whip through urban canyons. For these reasons, varieties of sedums are often a popular choice.
In Austin, Texas, where intense summer storms can drop as much as four inches of rain in an hour, causing catastrophic flooding, property damage, and occasional deaths, Mark Simmons and his colleagues at the Lady Bird Johnson Wildflower Center, part of the University of Texas at Austin, helped install a native Texas prairie atop an area Starbucks. About the same time they started to test the performance of different roof systems and native plant species to help identify which work best at retaining water.
At their research site, in the prairie a short walk from the center’s main offices, 24 miniature flat roofs sit in the hot Texas sun, testing eight different roof types—black, white, and six versions of green roofs supplied by manufacturers. (Each system is repeated on three test roofs in order to replicate results for statistical purposes, and each sprouts the same native Texas grasses, wildflowers, and shrubs.) Instruments record temperature automatically at and under the soil medium; gauges measure runoff.
In every case, says Simmons, how well the plants grow combined with the success of the materials is key to performance. On some of the test roofs the plants are large and healthy—and thus provide shade and absorb water—while on others they are small and clearly struggling. “These are living systems,” he says, “not machines.”
Sixteen stories above downtown Portland, Liptan opens a locked steel door at the City Building and steps outside onto the newest municipal eco-roof. It’s designed with rows of colorful sedums, sprawling iceplant with magenta flowers, and the taller forms of blue fescue, a bunchgrass.
He kneels down, pulls two stainless steel temperature probes out of his pocket, and sticks one into the middle of a clump of sedum and the other into bare soil. “On a hot, sunny day,” he says, “you’ll get a temperature difference of 20 to 30 degrees,” showing that green roofs not only collect water but that they can literally cool cities, especially when they blanket large swaths of the urban landscape. As places such as Portland continue to grow, the potential for incorporating green roofs is also increasing. “There are about 12,400 acres of rooftops in Portland right now,” says Liptan, “with the potential for 10,000 more in years to come.”
In Chicago, where summer temperatures sizzle and winters bring frigid winds off Lake Michigan, the city spent $1.5 million to retrofit the 1911-vintage City Hall with a green roof that mimics a hilly native prairie. With a layer of soil ranging from 3 to 18 inches deep and some 20,000 plants, the roof succeeded beyond expectations. When summer air temperatures hit 90 degrees Fahrenheit, so do surface temperatures of the vegetated roof—compared with nearly 170 degrees above the black bituminous roof of the County Building next door.
That kind of dramatic benefit convinced the Friends Committee on National Legislation, Washington’s largest peace lobby, to install a 1,227-square-foot living roof when it remodeled two Civil War–era brownstone offices on Capitol Hill. Nancy Pelosi, Speaker of the U.S. House of Representatives, lauded the project as a model for greening the Capitol Hill area.
Such payoffs also swayed Majora Carter, founder of Sustainable South Bronx and winner of an Audubon Rachel Carson conservation award in 2007. After she saw how easy installation was above the former industrial complex that houses Sustainable South Bronx’s offices—a crane simply lifted trays of plants and soil up onto the roof—the organization added green roof installation and maintenance to its “green collar” training program, which prepares inner-city residents for such environmental jobs as water-quality testing and urban tree maintenance.
The roofs don’t have to be big, or high-tech. Mace Vaughan, conservation director for the Portland-based Xerces Society, built a gatehouse with a 40-square-foot living roof using ordinary materials from a hardware store, a pond liner for the waterproof layer, a nylon soil erosion net to hold the soil on the sloping roof, and flats of plants he grew himself. “I like the idea of adding a piece of habitat over my gate,” he says—habitat for native bees, that is, just the kind of pollinators the Xerces Society is involved in conserving. He’s seen loads up there: “sweat bees, bumble bees, mason bees.”
At the Portland City Building’s new eco-roof, Liptan has photographed so many species of bees, butterflies, spiders, and damselflies that he hopes to enlist the Xerces Society to document the burgeoning rooftop community.
In Chicago, Jerry Garden, stewardship chair for Chicago Audubon, counts birds on the City Hall roof at least once a week during spring and fall migration. The tally so far: 57 species. Garden’s favorites include the sora rail flushed from high above downtown, and the ovenbird, a woods-dwelling warbler that used the wall of the elevator shafts as a tree trunk from which to sally forth and snatch insects. “My personal thrill,” says Chicago Department of Transportation assistant project director Kevin Carroll, who nurtures the prairielike roof and its 160-plus plant species, “was seeing a ruby-throated hummingbird sip nectar from a rooftop columbine.”
Designing wildlife habitat in the sky is certainly a growing business. In Carmel Valley, California, Rana Creek Nursery, which specializes in restoring native ecosystems, tested and grew 1.7 million native wildflowers that will carpet a 2.5-acre undulating meadow of roof sheltering the rebuilt California Academy of Sciences complex. The roof will provide not only the city’s largest swath of native vegetation but critical habitat for the endangered bay checkerspot butterfly, a dusky brown species with dazzling cream and orange spots.
In Portland, Liptan recently hosted a group of wildlife biologists who were surveying the city’s eco-roofs. There he showed them the potential habitat for species that are declining in urban areas. “We took them up to the Portland City Building eco-roof,” he says, “and they were excited about it as bat habitat.” That’s the magic of green roofs. Sure, they counter climate change, save energy, prevent runoff, and clean the air. They also live and breathe, sustaining flourishing plant worlds that can give wildlife new homes in cities, where, often, they’re needed the most.
Susan J. Tweit, a frequent contributor to Audubon, is the author of 10 books about animals, plants, and landscapes.
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Let It Rain
One of the major advantages of green roofs is their ability to capture storm water, but if you’re not ready to install your own, there are a variety of steps you can take to curb runoff.
• Dig swales, low spots to collect runoff water in your yard. (Your soil must be permeable to avoid standing water and mosquito problems.) Drainage ditches leading away from the building and lined with rocks will also work.
• Plant low spots with “rain gardens”: water-loving native plants that quickly absorb water and transpire it to the air (see “Good to the Last Drop,” Audubon, September-October 2003).
• Use rain barrels, available at garden-supply stores, to capture roof runoff for landscape watering and nonpotable household uses (check local codes and water laws). |
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