About Green Roofs

Intensive green roof on the Chicago City Hall building

A green roof is a roof of a building that it partially or completely covered by vegetation.  They are divided into two categories: intensive or extensive, based on their depth of planting medium.  An intensive green roof is your typical roof garden which requires a generous depth of soil in order to grow large plants.  They are considered ‘intensive’ because they are labour-intensive, requiring irrigation, feeding and other maintenance.  On the contrary, an extensive green roof can be established on a relatively thin layer of soil, and is typically self-sustaining.

Green Roof Diagram

Green Roof Diagram

The benefits of using a green roof system include but are not limited to reducing the heating and cooling loads for the building, reducing the heat island effect, filtering pollutants out of the air and rain water, providing better insulation for sound, and increasing the roof’s lifespan.  The primary disadvantages are the increased roof load, which increases the structural demands of the building; and the increased cost in comparason to a standard shingled roof.

In addition to deciding between an intensive or extensive green roof, there is also the choice between using a typical system, where the planting media is installed directly over a membrane and various other layers; or a modular system where the plants and soil sit in trays or pots, which are then placed on the roof.  It is argued that a modular system is not a true green roof system, since it is not applied directly to the roof like that of a standard system. 

Your standard extensive green roof can cost as low as $8/sf, whereas your standard modular system typically costs atleast $15/sf or more if you want fancy plants. 

Sources:  http://www.greenroofs.com/  http://en.wikipedia.org/wiki/Green_roof

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