Tuesday, July 24, 2012

Green Roofs in Washington, DC

In 2011, the Washington area led the nation, installing 800,000 square feet of green roofs (doubling the amount in 2010), and for the first time surpassed Chicago, which is known for its green roof initiatives. The District now has more than 1.2 million square feet of green roofs, according to the Green Roofs for Healthy Cities survey. DC Greenworks,, a nonprofit helping build green roofs,  believes 2012 probably will surpass 2011.

Commercial roofs form the bulk of green roofs because they generate the most return in terms of insulation and offsetting District stormwater and impervious surface area fees. Residential green roofs are increasing due to their ability to insulate against weather and noise, as well as for aesthetic benefits.

The District gives permits for three types of roofs:

1) Extensive Design, which is usually approved quickly, according to DC Greenworks. The roof has a shallow bed of growing medium (three to four inches deep), consisting mostly of expanded shale and slate and only 30 percent organic material. A higher percentage would make the roof heavier and potentially lead to rot.

2) Semi-Intensive, which contain deeper growing mediums that support plants with deeper roots.

3) Intensive systems, , which contain vegetables, bushes and even trees can be grown

In all three types, the green roof components (the plants, growing medium, drainage system, irrigation, root barrier and insulation), can either be laid down in layers across the roof, or they can come pre-installed in small modular trays that hook together.

The benefit of the former is that you can easily include or exclude different components, such as irrigation, to customize to a user’s need. A modular system, on the other hand, allows one to easily take up different sections to look for leaks or to replace dead plants. Also, modules usually come pre-planted, and thus the new roof has complete plant coverage sooner. A modular system is heavier and more expensive.
Resources on green roof design and function can be found on the District Department of the Environment, DC Greenworks and Green Roofs for Healthy Cities. (Wash Post, 7/23/2012) 

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