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Over and over again, I tell students in my earth-sheltered building classes that “drainage is the better part of waterproofing.” If you provide an easier place for rainwater to go than into your house, it will go there. I used to make my own drainage layer out of crushed stone covered with loose hay or straw as a filtration mat to keep the fine particles of earth out of the drainage layer. It worked well, but was a lot of hard work to install and added another 20 PSF to the design load.

Now, I use one of the composite drainage materials made for this purpose by several different companies. They essentially create a quarter-inch channel of air over the roof, and the water travels swiftly along it down the pitched roof and drips off the edge. They consist of a tough nylon mesh covered on one or both sides with a filtration mat. These drainage products typically cost between 50 cents and $1 per square foot.

Building the Living Roof

I recommend a pitch for an earth roof between 1 in 12 (1 foot of rise in 12 feet of horizontal run) and 2 in 12; 1.5 in 12 is my favorite. Water drains readily in this pitch range, yet the earth does not slump off the roof.

I install the waterproofing membrane directly over the roof planking. Over the membrane, we place sheets of extruded polystyrene insulation, the thickness depending on location, local codes and the purpose of the building.

Atop the insulation, I install a layer of 6 mil black polyethylene. This inexpensive layer is not intended as a waterproofing membrane — you’ve already installed that. Rather, it acts as the base of the all-important drainage layer. Water rides this plastic quickly to the drip edge.

The composite drainage matting goes on top of the polyethylene. The earth goes over the drainage matting. There is no gain in placing more earth on the roof than needed to maintain the chosen green cover. Remember, saturated earth is heavy — don’t surpass your total engineered load.

Finally, plant the roof, choosing native plants. Sedum is a succulent plant that stores moisture in its leaves, perfect for seeing a living roof through a drought situation without the need for watering. Sedum can grow in 3 inches of soil, and many varieties are quite colorful. Grasses and wildflowers need 7 to 8 inches of soil. In arid climes, go with desert plants.

Rob Roy is director of Earthwood Building School in West Chazy, N.Y., which he has operated with his wife, Jaki, since 1981. Earthwood offers instruction in cordwood masonry, earth-sheltered housing and post-and-beam framing techniques “for the rest of us.”

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