Last Wednesday, we had the opportunity to attend the Green Roof workshop sponsored by the Urban Ecology Institute and CityRoots.  At the workshop, Filomena from CityRoots installed an extensive green roof on a tool shed at the Claybourne Street Community Garden.  The tool shed had been built last year with the help of YouthBuild with the intention of installing a green roof in the future.

Rooftop gardens can be built on any horizontal or slightly tilted roof, and they have many benefits.  They provide insulation, cooling in hot weather and keeping warmth in during the winter; they reduce pollution caused by excessive storm water runoff, and when installed properly, they prevent roof leaks.

Here’s a quick guide to installing your own extensive (shallow) green roof:

• Choose a roof that can support between 15 to 30 lbs. per square foot
• Install a root barrier – a heavy-duty waterproofing layer.  A good choice is a double layer of 20 mil. pond liner.
• Add a drainage layer – Urban Ecology Institute recommends a lightweight granular medium, such as pebbles, or a synthetic layer that helps retain moisture.  An absorbent old carpet or a recycled blanket can work.
• Install a filter fabric on top of the drainage layer, with a honeycomb texture that can prevent fine soil particles from passing into the drainage layer.
• Add a lightweight, easily draining soil medium.  A good guideline is approximately 75% inorganic (such as crushed slate, clay or vermiculite) and 25% organic (compost or clean topsoil).  The picture to the left shows a mixture containing lightweight expanded clay aggregate (LECA) round stones.
• Choose a hardy and drought-tolerant plant such as sedum, hens and chicks, ice plants, chives, or columbine for the final layer.  Some species of grass are an option if you are planting in a soil mixture deeper than 6 inches.
• Weed once or twice per year – roots of invading woody plants can damage the waterproof roof lining.

For the full guide to building a simple green roof, written by CityRoots and the Urban Ecology Institute, click here.