Step 1: Prep the Roof

covering the roof with a waterproofing membrane
Photo: Alison Rosa
«»
Even a flat roof should have some pitch so that it sheds water toward gutters; for a green-roof installation, that pitch should be at least ¼ inch per foot of run. Before the plants go up, a structural engineer should check to ensure that the roof can support the weight of the engineered growing medium and plants. When saturated, each tray in this system adds up to 30 pounds of load per square foot, which can stress a traditional roof built to support about 25 pounds. For this project, the roof was built to withstand the added load and then covered with a waterproofing membrane. Here, green-roof installer Rob Gaffney adds a second layer of rubber roofing membrane as a root barrier to prevent damage to the roof.

Pro advice
"Get a structural engineer to tell you what your existing roof needs to support the weight of a green roof. You might have to add more joists, strengthen existing ones, or add bracing."
Tom Silva, This Old House general contractor
Ask TOH users about Green

Contribute to This Story Below

    Tools List

    • 16-foot tape measure
      Tape measure
    • drill
      Drill/driver
    • concrete saw
      Concrete saw with masonry blade

    Shopping List

    1. EPDM

    2. Modules

    3. Engineered soil

    4. Sedums

    5. Aluminum edging