A Green Roof on a Small Scale

A Green Roof on a Small Scale

Here is a utility shed we built to house our gardening tools and trash containers. We designed the shed to break up the deck and patio area from a service alley accessed through the garage. There is a sunny eddy on the deck backed by the shed, the key was placing it perpendicular to the house. Since its roof is so visible and accessible, we wanted to look at plants and decided to make a green roof.

First, we had to build it.

Then clad it. Note the siding is on top of vertical furring strips, creating a rain screen assembly. To learn more about this, check out: http://en.wikipedia.org/wiki/Rainscreen

Then we built and attached a barn door, the only feasible way to get a wide opening in this narrow service alley.

For the roof, we sloped the plywood then had a custom galvanized sheet metal pan fabricated, to serve as the waterproof membrane. It follows the slope of the plywood to a drain outlet. On a larger roof, it would not be feasible to have a flashing pan, and a more traditional membrane would be applied.

We researched various materials that would serve plants growing on a roof. One factor of green roofs is that the soil is not very deep, since deeper soil gets heavy. In order to have adequate moisture in shallow soil and also curb run off, one needs a mat layer. This “drainage mat” is lightweight, stores water and provides proper drainage. After hearing a talk by Certified Green Roof Professional and landscape designer Cynthia Tanyan, (http://mozaiclandscapes.com) we knew which product to use. From a company called JDR Enterprises. The product, J-Drain (http://www.j-drain.com/sub-app-green.php) is a pre-fabricated assembly. It consists of a top layer of geosynthetic filter fabric, keeping soil out but allowing water to pass through. Below this layer is a molded plastic honeycomb core where the water is retained to a degree but also drains out.

The soil is then placed directly on top of the mat. When the soil is in, the plants have a home.

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Hart Wright Architects
340 Potrero Ave
San Francisco, CA 94103
tel: (415) 503-7071