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Warning

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A green roof should not be confused with green roofing. Green roofing materials are simply conventional roofing materials re-engineered in ways that minimize their environmental impact. A green roof is a roof that mimics a lawn or garden. It has a bed of some growing medium and is planted with grasses, flowers and sometimes even shrubbery. Though they have been popular in Europe for some time, they have only recently gained popularity in the US.

In the prairie days of the US the sod roof was a common way for settlers to inexpensively waterproof the simple shelters they were able to build. Sod roofs also had the advantage of helping to cool the house in summer and retain heat in winter.

In their modern incarnation green roofs provide many of the same benefits.

Green roofs fall into two broad categories. Intensive green roofs can have up to a foot of soil and are capable of growing plants up to and including small trees. The structural requirements of supporting an intensive roof make them impractical in many applications. More common is the extensive green roof, having only two to four inches of soil or other growing medium.

Extensive roofs are normally covered by shallow rooted plant species that are drought tolerant. Low growing sedums are a common choice. Intrusion by weed species is limited by the shallow depth of the soil medium, which at times can dry out completely. The extensive roof is the only truly viable choice for residential construction.

In cities or suburbs where housing density is high, green roofs can lessen the effects of storm runoff, capturing the water that impervious surfaces like roads, walkways, and standard roofs shed. Even in heavy rainfall events, green roofs can decrease the damaging effects of runoff, since even the water they can't absorb is released over a period of time rather than all at once. Green roofs have the added benefit of increasing the life of the membrane roofing material by blocking the ultraviolet rays of the sun and protecting the membrane from the elements..

Though it is possible to apply green roofing strategies to pitched roofs, in fact most green installations have very little pitch, as the techniques needed to establish this type of roof at any significant slope are quite difficult to implement.

Green roofs are still very expensive to install, though as with many alternative strategies, cost differences are reduced when the increased lifespan of the application is considered.

Green roofs require considerable structural reinforcement when compared to conventional roofing, and the actual construction of the underlying roofing requires additional layers of material. Several membrane layers, a barrier layer to prevent root penetration and a drainage layer are required for successful performance of the roof. Currently there are modular components coming to market that reduce the difficulty of installation and so reduce costs, and as green roofs become more common, efficiencies of scale should reduce these costs further.

Green roofs are practical, attractive, and environmentally friendly. They are likely to become more prevalent in the future, in the US as well as in the rest of the world.