Down to Earth Homes

Living Roofs

Most of us appreciate a roof over our heads, but don't give a second thought to the roof itself. It's no wonder: We're used to austere metal panels or lifeless asphalt shingles. The roofs of earth-sheltered homes, however, can be peppered with a profusion of Indian Paintbrush, Black-Eyed Susans, Sky-Blue Asters or other floral delights, In the summertime, elk come to nibble on the luxuriant grasses that grow on my living roof.

A living roof consists of soil and plants on top of a wooden or steel-reinforced concrete roof. The three most important considerations when designing a living roof are providing adequate support, waterproofing and proper drainage.

Living roofs can be nearly flat, domed or sloping, up to 6 to 8 inches per running foot. To ensure that the substructure, the decking and the roof framing are kept dry, a waterproof membrane must be applied over the decking. Some builders use polyethylene because it is inexpensive and can be doubled over for additional protection. Most builders use more sturdy waterproofing material.

After the roof is waterproofed, soil is carefully placed over the roof in layers from 6 inches to 9 feet thick, depending on the strength of the underlying roof system. The soil can then be covered with sod that was removed with the excavation of your foundation or grading of your driveway, which greatly accelerates the establishment of a healthy living cover. You can plant the soil with native seeds, which are well-adapted to local soil and weather conditions, and more likely to survive. During the first year, the roof may need to be watered to ensure good seed germination and growth.

Although living roofs are beautiful and functional, you may want to leave a portion of your roof uncovered to house solar hot water or photovoltaic panels. The clerestory design of my house leaves space for both, and also allows me to harvest rainwater.