Advice provided by: David M. Pratt, P.E., CEM, LEED AP, MWH

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Green roofs are simply vegetated roof covers constructed atop and across a roofdeck.

While the concept of a green roof has been used since the 10th century B.C., the modern green roof was introduced in Germany in the 1970s and the technology became more widely accepted in Europe throughout the 1980s (Velazquez, 2005).

There are two types of green roofs, extensive and intensive.

Extensive green roofs typically have lower growing plants than intensive green roofs, and also have less variety or species. Intensive green roofs are usually designed for human recreation (as in a rooftop garden), while extensive are typically non-accessible.

Extensive green roofs are also less expensive to install and maintain than intensive green roofs (Velazquez, 2005). However, both types of green roofs are built in the same fashion. The vegetation is planted in a type of growth medium with a drainage layer beneath it. Beneath the drainage layer is a root barrier and waterproof membrane, all constructed atop the insulated roof deck.

As a contrast to cool roofs, green roofs not only have lower surface temperatures than conventional roofs, they also insulate more than conventional roofs, keeping more of the heat inside the facility during the winter and lowering the energy demand not only in the summer but also in winter (FEMP, 2004).

Green roofs also have additional environmental benefits such as reduction of stormwater runoff as well as water quality and air quality improvement (Velazquez, 2005). The absorbed radiation of a typical black asphalt roof not only raises the exterior temperature of the roof; it also raises the temperature of the surrounding air in densely populated urban areas. This also occurs with other impervious surfaces such as pavements. This higher ambient temperature acts as a catalyst and adds to smog, making air pollution problems worse as well. This is known as the urban heat island effect (FEMP, 2004).

Like cool roofs, green roofs can reduce the surface temperature of the roof as well as the surrounding ambient air temperature, thus combating the urban heat island effect (Velazquez, 2005).

Other benefits of green roofs include extended lifespan of the roof membrane, sound attenuation, and reduction of green house gases (GhG). Adding a green roof to a new construction project can also add up to 7 credits in the LEED (Leadership in Energy and Environmental Design) Green Building Rating system.

Green roofs do come with some disadvantages however. Initial installation costs are typically higher than traditional roofs, and the building and roof structure will have to be reinforced to handle the additional load. Maintenance costs may likely be higher, some roof repairs may be more difficult, and green roofs aren’t appropriate for every climate.

All in all, the benefits outweigh the disadvantages, and green roofs are a wonderful energy efficient alternative to traditional roofs that come with added environmental advantages.

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