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Excerpted from articles by U.S. Environmental Protection Agency and National Wildlife Federation
 A green roof consists of vegetation and soil, or a growing medium, planted over a waterproofing membrane. Additional layers, such as a root barrier and drainage and irrigation systems may also be included.
Green roofs can be used in industrial/commercial/office facilities, and in residences. Throughout Europe, they are used for stormwater management and energy savings potential, as well as aesthetic benefits. Creating wildlife habitat on roofs is a new idea in this country, but research in Europe suggests that it can indeed provide living space for plants and animals, especially mobile species such as insects and birds. One of the pioneers of the movement to create living roofs as a strategy to preserve biodiversity is Stephan Brenneisen of the University of Applied Sciences of Wadenswil in Switzerland. As a result of his work in Basel over the past 12 years, green roofs have become an important component of the city’s plan to protect the plants and animals that live there.
Benefits
This attractive roofing option can reduce heat by providing shade and by releasing water from plants to the surrounding air. Green roofs can also:
- Reduce sewage system loads by assimilating large amounts of rainwater.
- Absorb air pollution, collect airborne particulates, and store carbon.
- Protect underlying roof material by eliminating exposure to the sun's ultraviolet (UV) radiation and extreme daily temperature fluctuations.
- Reduce noise transfer from the outdoors.
- Insulate a building from extreme temperatures, mainly by keeping the building interior cool in the summer.
Aesthetics and Habitat Restoration
Paul Kephart, executive director of Rana Creek, a Carmel, California-based company specializing in habitat restoration and living architecture, is among a growing number of biologists who see "green roofs as a way to restore biodiversity to the urban environment." His company seeks native plant communities that survive in similar conditions and can serve as models. This type of resoration offers refuge for beleagured small insects and animals in our ever-increasing cement landscapes.
 In the San Francisco Bay Area, for example, one of the world's most imperiled butterflies may soon be flourishing on rooftops. A striking butterfly with black bands and bright red, yellow and white spots, the Bay Checkerspot is found only in equally rare and threatened serpentine grasslands scattered around the area. Recreating its habitat on green roofs may help save this butterfly, as well as other wildlife species marooned in fragmented and disappearing green spaces.
Storm Water Runoff
As impermeable surfaces like buildings and pavement replace open space and vegetation, green roofs can play an increasingly important role in storm water management. During rainstorms, green roofs act as a sponge, absorbing much of the water that would otherwise run off.
Green roofs also filter pollution from rainwater through the root systems' bacteria and fungi, which utilize natural filtering processes. As a result, nnitrogen and phosphorus, are broken down and detoxified. This beneficial process increases over time as rooftop plants and root systems mature.
Green Roofs
There are two types of green roofs: intensive and extensive. Here are some characteristics of each:
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Intensive and Extensive Green Roofs |
Characteristic |
Intensive
Green Roof |
Extensive
Green Roof |
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Soil | Requires minimum of one foot of soil depth | Requires only 1 to 5 inches of soil depth |
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Vegetation | Accommodates large trees, shrubs, and well-maintained gardens | Capable of including many kinds of vegetative ground cover and grasses |
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Load | Adds 80-150 pounds per square foot of load to building structure | Adds only 12-50 pounds per square foot depending on soil characteristics and the type of substrate |
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Access | Regular access accomodated and encouraged | Usually not designed for public accessibility |
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Maintenance | Significant maintenance required | Annual maintenance walks should be performed until plants fill in |
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Drainage | Includes complex irrigation and drainage systems | Irrigation and drainage systems are simple
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Source: Schloz-Barth, Katrin. 2001. "Green Roofs: Stormwater Management From the Top Down." Environmental Design & Construction. January 15. |
Cost of Green Roofs vs Traditional Roofing Options
 Currently, the up-front cost of an extensive green roof in the U.S. starts at about $8 per square foot, which includes materials, preparation work, and installation. In comparison, the cost of a traditional built-up roof starts at about $1.25 while cool roof membranes start at approximately $1.50 per square foot.
On a summer day the temperature of a gravel roof can skyrocket to between 140 and 176 degrees F, but when covered with grass or other vegetation it is less than half as hot. The temperature inside the building stays cooler, saving energy on air-conditioning. In winter the building stays warmer, conserving energy on heating. Both of these factors will reduce energy costs.
Extensive green roofs cost more than traditional roofs because they require more material and labor for installation. Another factor affecting price is that green roof contractors are limited in number. As the demand for rooftop gardens increases in the U.S., and as additional contractors come into business, up-front costs will likely decrease.
Another factor reducing the cost of a green roof is that vegetation can extend the life of a roof. This is because less solar energy reaches the roof substrate, limiting damage from UV radiation as well as daily temperature fluctuations, which cause repeated contraction and expansion.
Green Roof Examples
- The Gap Headquarters in San Bruno, CA installed a 69,000 square foot extensive green roof in 1997.
- Ford Motor Company has installed green roofs on its corporate headquarters.
- The Church of Jesus Christ of Latter-Day Saints Conference Center in Salt Lake City, Utah installed a 348,480 square foot extensive and intensive green roof in 2000.
- Contractors recently completed a 30,000 square foot extensive green roof project on the Montgomery Park Business Center in Baltimore, Maryland.
- Ducks Unlimited, a nonprofit organization dedicated to the conservation of waterfowl, included two green roofs totaling 28,190 square feet on its national headquarters.
- Private and public interests in the City of Chicago and the City of Portland have installed or are planning to install over 43 and 42 green roof projects, respectively.
Greening Your Roof
Building one isn't simply a matter of hauling potting soil and plants to your rooftop. It's important to consult an expert, say industry groups such as Green Roofs for Healthy Cities (www.greenroofs.org), which provides resources for professionals and consumers. Look for a landscape architect, engineer or roofing contractor who has experience in green roof installation.
And, of course, check with your local planning commissions. |
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