As I mentioned before, as part of our cob-building course I learned how to install a living roof (also known as a green roof or a sod roof). Virtually all the houses at the Cob Cottage Company had living roofs, so perfect for the temperate coastal rainforest climate, and their soft mossy contours only added to the feeling of a hobbit village. The roof I helped build was a small one, of course, but the same principles apply to the installation of a larger living roof.Living roofs have many benefits. When installed and maintained correctly, they can last longer than conventional roofs. They help divert waste from landfill, as DIY builders can use many recycled materials. They also reduce the amount of storm water run-off, moderate the temperature of the room below (thus reducing utility costs), absorb air pollutants, and reduce the so-called Urban Heat Island effect. Plus, did I mention they help create beautiful spaces?
Here are the basic steps for an inexpensive, attractive, and easy-to-install green roof.
1. Create a cushioning layer. Cover the roof boards with a layer of cardboard. Use longish staples to attach the cardboard, and make sure they are secure. The idea is to protect the liner from being punctured by any sharp nails etc. in the roof.
2. Create supporting structure. Attach a raised fascia or edge board along the edge of the roof to help anchor the sod. We used 2×4’s. At this stage, it’s important to consider where rainwater will flow off your roof.
3. Lay down the liner. You can use almost any material, as long as it’s sturdy, waterproof, and either comes in large sheets or can be safely sealed at the seams. The most durable, but also more expensive, option is an EPDM (pond liner) membrane. But for a garden sitting area roof such as ours, two layers of 4″ thick black polyethylene was sufficient. Run the membrane over the edge boards and fold underneath so that no water can permeate the roof structure. The soil above will protect the membrane from the deteriorating effect of UV rays and other damage.
6. Plant! Plant whatever grows well on the forest floor or a meadow in your region. Moss, low-growing succulents such as sedum, ferns and grasses are ideal. Or you can throw on some wildflower or cover crop seeds, experiment with ornamental herbs, or even edibles such as strawberries. Over time, your roof will develop its own ecology, and the roots will tie the whole thing together so that it stays securely in place.
Sources consulted: The Hand-Sculpted House; http://www.greenroofs.org.