Marin Master Gardeners
University of California
Marin Master Gardeners

Marin IJ Articles


Green Roofs

May 14, 2007
Marilyn Geary

by Marilyn Geary

One of my favorite books as a child was the classic Heidi, by Johanna Spyri. Most of all I loved the vivid illustrations: the colorful alpine wildflowers, the icy mountain peaks, and Heidi’s little chalet with its roof carpeted in tall green grass. One illustration showed a goat munching on the rooftop greenery. I wondered how all that grass could grow up there. How deep was the soil? Did the roots grow through the roof?
Recently my questions were answered at a seminar at the San Francisco Botanical Garden given by Paul Kephart of Rana Creek Habitat Restoration, an ecological design company. I learned that roofs supporting living plant material have existed for centuries. They have become a status symbol in Germany and are also making a comeback in the United States due to their many environmental benefits.
Green roofs, or living roofs, as they are also called, can dramatically reduce energy use. Alec Hoffman, Green Building Coordinator on Marin County’s Sustainability Team, cites many benefits. “Green roofs are great,” he says, “for absorbing first flush water runoffs, modulating temperature, and providing habitat for invertebrates.” They also modulate noise, and filter pollutants and carbon dioxide from the air.
Green roofs improve water quality by slowing or reducing storm water runoff that carries pesticides, fertilizers, oil, and other pollutants into nearby ecosystems. “Pollution in the form of particulate matter settles on the roof,” Kephart says, “and storm water runoff carries these toxins into the groundwater. Green roofs can absorb 70 percent of this water.”
Black tar covers many urban rooftops and traps heat and sunlight, raising temperature in the surroundings. This phenomenon is called the “urban heat island,” a main cause of ozone production. The heat trapped by dark, flat roofs can raise city temperatures by as much as ten degrees Fahrenheit. According to Kephart, “Green roofs have huge energy conservation benefits since their surfaces can be 35-80 degrees cooler than traditional roofs.”
A particularly challenging living roof is currently under development by Kephart’s firm for the new California Academy of Sciences in Golden Gate Park. Italian architect Renzo Piano has designed a 2.5-acre living roof with undulating slopes to mimic the surrounding topography. Eighty per cent of the roof surface will be planted with over 1.7 million native California plants belonging to nine species. These plants will not require artificial irrigation.
Kephart’s crew and the Academy’s in-house botanists worked for over two years to subject about thirty native plants to San Francisco’s weather extremes, including intense fog. A few plants survived the intense two-year testing phase: beach strawberries (Fragaria chiloensis) produce berries that attract native birds; stonecrop (Sedum spathulifolium) produces nectar for the Hairstreak butterfly and food for endangered San Bruno Elfin butterfly larvae; sea thrift or sea pink (Armeria maritime) attracts moths and butterflies with flowers like pom-poms; self-heal (Prunella vulgaris) bears large tubular flowers that attract hummingbirds and bumble bees; miniature lupine (Lupinus bicolor) and California poppies (Eschscholzia californica) provide nectar for bees and butterflies;goldfield plants (Lasthenia californica) attract a wide variety of beneficial native insects; tidy tips (Layia platyglossa) attract parasitic wasps and pirate bugs that feed on pest insects; and California plantain (Plantago erecta)hosts a variety of butterfly larvae.
Green roof initiatives are underway in Los Angeles and San Francisco, but Chicago has led the way, with more green roofs that any other U.S. city. The City of Chicago requires any developer receiving city financial aid to create a green roof. Large green roofs in the San Francisco Bay Area include The Gap’s offices in San Bruno, where a 69,000-square-foot green roof is planted in native California grasses and wildflowers. Air traffic noise from nearby San Francisco International Airport has been significantly reduced by the roof’s insulating properties.
Here in Marin, we have an early example of green roof construction. In the 1960s, Frank Lloyd Wright incorporated a green roof for the cafeteria terrace in his design of the Marin Civic Center. The outdoor garden with lawn and fountain is a favorite rest spot for county employees and visitors alike, including birds and invertebrates. Weddings and receptions are frequently held on the living roof terrace. Marin architect Sim Van der Ryn recently designed a green roof for a residence in Sonoma County, and Rana Creek is working on a project to create a living roof for a private residence in Ross.
Green roofs consist of several layers: a structural frame, the growing substrate, and vegetation layered over a waterproof membrane. Additional layers might include a root barrier, a filter sheet to hold the substrate and allow water passage, and a moisture blanket to absorb water.
Kephart emphasizes the importance of consulting an engineer to make sure the structure will hold. Once structural concerns are addressed, select the appropriate soils and aggregates, and plant species most likely to meet your project goals and flourish in your particular setting.
Green Building Coordinator Hoffman supports the installation of green roofs for their many environmental benefits. Marin County requires no building codes for green roofs other than they meet the standard structural requirements. Hoffman advises, “Green roofs are longer-lasting because they are not subject to damage from ultraviolet rays which break down common roofing materials.”
Green roofs are easiest to install and maintain on flat surfaces. Although your green roof will last longer than standard roofs, it will not be maintenance-free. In the first year or so, you must weed and water to get the plants established. Later, if you have planted drought-resistant plants, watering should not be necessary and periodic weeding may be all that’s required.
If the faded browns of winter don’t suit your aesthetic tastes, you can plant evergreen perennials for a greener look year-round. To experiment with living roofs, you might start out with a do-it-yourself project on a garden shed. For more information, see www.greenroofs.com.
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