a green roof is a roof on a structure which contain plantings.

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Introduction

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Types of green roofs

There are two types of green roofs, intensive and extensive (sometimes called "roof gardens").

Intensive

Itensive green roofs tend to be what most people think to be a green roof. Also called "roof gardens", intensive green roofs require a greated depth of soil for larger plant material, more maintenance, irrigation, and are generally more labor intensive.

Extensive

By contrast, "extensive" green roofs are meant to be virtually self sustaining, requiring very little maintenance. Extensive green roofs usually consist of a thin layer of soil that sits atop an impervious, waterproof membrane. This think layer of soil usually supports a hardy species of planting, such as many variety of Sedum or mosses.

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Advantages

Green roofs have many advantages which include:

Reduced stormwater runoff
By capturing and using the water that falls on it, green roofs reduce the amount of stormwater runoff that a typical roof treatment would create, thus lessening the burden of municipal storm water systems and keeping runoff on site

Reduced urban heat island effect
Plant material on a green roof absorbs much of the suns heat, as opposed to reflecting it back into the atmosphere as a traditional hard surface roof would.

Maintaining more constant building temperature
The plant material and soil layers help insulate buildings from temperature extremes, helping keep a more constant core building temperature, thereby reducing the need to cool buildings in the summer and heat them in the winter.

Reduce pollutants
Plant material on roofs assist in filtering pollutants from the air as well as from stormwater.

Provide additional beneficial public space
Additional green space is created

Additional wildlife habitat
Additional much needed wildlife habitat is created in urban areas.

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Disadvantages

Added cost
Depending on the type of green roof used, additional cost may be added to a project. Some types of extensive green roofs, however are comparable to traditional roof installations.

Added maintenance
Depending on the type of green roof, maintenance cost may be added to a project.

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Additional Resources

Green Roof Association:
www.igra-world.com/intro.html
Penn State Center for Green Roof Research:
hortweb.cas.psu.edu/research/greenroofcenter/index.html
The Green Roof Research Program at Michigan State University:
www.hrt.msu.edu/greenroof/