What is a Green Roof?

The Chicago City Hall green roof helps cool the building and minimize water run-off. See more green science pictures.
Photo courtesy of courtesy of DOE/NREL I Photographer: Katrin Scholz-Barth

An aerial view of most urban areas shows swathes of asphalt, black tar and gravel-ballasted rooftops. Heat radiates off of the dark roofs, and water rushes over the hard, hopefully impermeable surfaces. Yet, there is a new trend that breaks up the monotony of common roofs: green rooftops. Long popular in Europe, green rooftops have begun to appeal to homeowners, businesses and even cities as an attractive way to promote environmentalism while solving the problems of conventional roofs. Green roofs supplement traditional vegetation without disrupting urban infrastructure -- they take a neglected space and make it useful.

Green roofs last longer than conventional roofs, reduce energy costs with natural insulation, create peaceful retreats for people and animals, and absorb storm water, potentially lessening the need for complex and expensive drainage systems. On a wider scale, green roofs improve air quality and help reduce the Urban Heat Island Effect, a condition in which city and suburban developments absorb and trap heat. Anyone who has walked across a scalding parking lot on a hot, summer day has felt one effect of an Urban Heat Island.


The layers of a green roof must, like any roof, accommodate drainage and protect the building from the elements with a waterproof membrane. But they also must create a growing area and potentially provide support, irrigation and root protection barriers while staying as light as possible.

Two types of green roof exist: intensive and extensive. Intensive green roofs are essentially elevated parks. They can sustain shrubs, trees, walkways and benches with their complex structural support, irrigation, drainage and root protection layers. The foot or more of growing medium needed for an intensive green roof creates a load of 80-150 pounds (36-68 kilograms) per square foot. Extensive green roofs are relatively light at 15-50 pounds (7-23 kilograms) per square foot. They support hearty native ground cover that requires little maintenance. Extensive green roofs usually exist solely for their environmental benefits and don't function as accessible rooftop gardens.

One of the most famous American green rooftops, Chicago's City Hall, combines extensive, intensive, and the intermediary semi-intensive systems on one retrofitted roof. Under the Mayor's direction, the City of Chicago's Department of Environment City Hall pilot program kicked off a citywide push to support green rooftop systems with incentives and grants.

In the next section, we'll learn about the cost of green roofs and the benefits that make them worth their weight.


What Does a Green Roof Do?

Green roofs like those on the Faroe Islands can last twice as long as conventional rooftops.
Image used under the GNU Free Documenation License

The initial expense of green rooftops often turns away prospective clients. Because green roofs require professional design, careful structural analysis and multiple layers and systems, even extensive green roofs usually start at $8 per square foot, significantly more expensive than the $1.25 per square foot for built-up roofs (BURs) [Source: EPA]. But benefits and incentives, like those laid down by the City of Chicago, are prompting new green-rooftop projects. And, as the American green-roof industry grows, prices will drop.

In the meantime, long-term economic benefits already outweigh the start-up costs. Because green roofs protect the roof membrane from harsh weather and ultraviolet (UV) radiation, they can last twice as long traditional roofs. Green roofs also have a fairly stable surface temperature, remaining at air temperature or cooler while traditional rooftops can soar up to 90º F (32º C) above air temperature [Source: EPA]. The extra growing medium and vegetation insulates the building from intense temperatures and minimizes heat gain. According to a Canadian study, even a six-inch extensive green roof can reduce summer energy demands by 75 percent [Source: Professional Roofing].


These benefits are encouraging eco-minded homeowners, businesses and cities to build green rooftops. Green roofs mitigate water runoff and sewer overflows. Vegetation and soil act as a sponge, absorbing and filtering water that would normally plunge down gutters, wash through polluted streets and over-tax sewer systems. A green roof's plants remove air particulates, produce oxygen and provide shade. They use heat energy during evapotranspiration, a natural process that cools the air as water evaporates from plant leaves.

Evapotranspiration and the shading provided by plants help counter the Urban Heat Island Effect brought about by an excess of reflective and impermeable surfaces in cities and suburbs. Because Urban Heat Islands increase temperatures in urban and suburban areas, they amplify the demand for air conditioning and launch a cycle of energy consumption that contributes to global warming. If green roofs become a common building initiative, cities can reduce the uncomfortable effects of Urban Heat Islands.

Green roofs replace a hard infrastructure with one that's not only more efficient, but also beautiful and useful. Green rooftops offer office workers a rooftop retreat and apartment residents a place to plant gardens or relax. Even non-accessible green roofs create stunning aerial views for surrounding neighbors and provide wildlife with a secluded, safe space.

For more information about green roofs, global warming and other related topics, check out the links on the next page.


Lots More Information

Related Articles

More Great Links

  • Ashmawy, Alaa K. “The Hanging Gardens of Babylon.” http://ce.eng.usf.edu/pharos/wonders/gardens.html.
  • “About Green Roofs.” Green Roofs for Healthy Cities. May, 2005. http://www.greenroofs.org/index2.php?option=com_content&task=view&id=26&Itemid=40&pop=1&page=0.
  • “Chicago City Hall Green Roof.” ASLA Online. http://www.asla.org/meetings/awards/awds02/chicagocityhall.html.
  • “Chicago’s Green Rooftops.” Chicago Department of Environment. http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/GuidetoRooftopGardening_v2.pdf.
  • Eisenman, Theodore. “Raising the Bar on Green Roof Design.” Landscape Architecture. November, 2006. http://www.asla.org/land/050205/pdf/Greenroof_articleLAM11_06.pdf.
  • “Green Roof Technology Adapted to Cold Climates.” EnviroZine. February 16, 2006. http://www.ec.gc.ca/EnviroZine/english/issues/62/feature2_e.cfm.
  • “Heat Island Effect: Basic Information.” U.S. Environmental Protection Agency. http://www.epa.gov/heatisland/about/index.html.
  • “Heat Island Effect: Green Roofs.” U.S. Environmental Protection Agency. http://www.epa.gov/hiri/strategies/greenroofs.html.
  • “History of Green Roofs.” Chicago Green Roofs. http://www.artic.edu/webspaces/greeninitiatives/greenroofs/history.html.
  • Liu, Karen. “Going Green.” Professional Roofing. September, 2002. http://www.professionalroofing.net/article.aspx?A_ID=130#fig4.
  • Scholz-Barth, Katrin. “Greenroofs: Stormwater Management from the Top Down.” Environmental Design + Construction. January 15, 2001. http://www.edcmag.com/CDA/Archives/d568f635d8697010VgnVCM100000f932a8c0