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What is a Green Roof?


What Does a Green Roof Do?
Green roofs like those on the Faroe Islands can last twice as long as conventional rooftops.
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.