The square footage of green roofs in the United States grew by 28.5 percent in 2010, according to the 2011 Annual Industry Survey by Green Roofs for Healthy Cities (GRHC). For the seventh year in a row, Chicago—where more than 600 green roofs have been or are being built—had the most green roofs, with more than 500,000 square feet installed. Washington, D.C., was a close second.
“Government investment in green roofs for their stormwater, air quality, green space and city cooling benefits largely fuels the growth of our industry,” GRHC president Steven Peck said. “Cities such as Chicago, Washington, New York, Portland, Seattle and Philadelphia continue to lead the way with incentives and regulations that recognize the many benefits from green roofs.”
Green, or living, roofs are planted with vegetation to absorb rainwater and help regulate temperatures. The vegetative coverings also help with energy and resource conservation, storm water management and smog. The plants’ leaves catch dust, the roots filter water and the roofs provide habitat for birds and insects. In Europe, green roofs have been successfully used on homes, office buildings, and parking garages for more than 30 years.
GRHC chair Jeffrey Bruce said, “As the green roof and wall industry develops further we will see costs come down and benefits to building owners rise, through the application of integrated design practices that turn wasted roof and wall spaces into value-added urban farms, habitat, recreational spaces, horticultural therapy centers, energy conservation, green energy production and stormwater management infrastructure.”
For urban homeowners, green roofs also open up garden space. “We’re seeing a lot of people moving toward rooftop agriculture, especially in areas where land space is difficult to come by,” Ty Voyles, green roof professional for DC Greenworks told Natural Home & Garden.
Green roofs can be divided into two categories: intensive and extensive. Intensive gardens, such as the one built on the roof of Chicago’s City Hall, include trees, shrubs, and manicured landscapes, and they require a minimum soil depth of 1 foot. Extensive green roofs, built primarily for their environmental benefits and less as human habitats, require 2 to 5 inches of soil and are more suitable for retrofitting onto existing roofs. Extensive roofs require very little maintenance.
Built-up green roof systems contain a series of layers: root-repellant, drainage, a filter membrane, the growing medium and vegetation. Modular systems come with the drainage barrier, filter layer, growing medium and plants pre-assembled in molded plastic trays. The growing medium must be lightweight and erosion-resistant; expanded shale, mushroom compost and mineral components are typical.
Natural Home & Garden tells you everything you need to know about green roofs in “Home Grown: A Green Roofs Primer” in the May/June 2011 issue, which is on sale now. Also check out Green Roofs for Healthy Cities and Greenroofs.com for more information.
Green roofs, such as these in Norway, have been common in Scandinavia for centuries.
Chicago City Hall’s living roof is part of the city’s Urban Heat Island Initiative project. The landscape design is based on a formal garden and includes a drip irrigation system fed partially by water collected from the adjacent penthouse roof.
Perennial sedum varieties make up most of the maintenance-free vegetative cover on the roof of the Fencing Academy of Philadelphia. Fescue grass, sedge, allium, burnet, and dianthus provide accents.
A rooftop garden typically includes six layers: the roof (make sure it’s in good condition), a waterproof barrier, insulation, drainage/root barrier, substrate (3 to 4 inches of growing medium, consisting of compost, shale and other organic elements) and vegetation.