Earth-sheltered Homes

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Surface-bonded block walls. You can build below-grade walls using poured concrete, stone masonry or conventionally mortared blocks. But my favorite is concrete blocks, stacked dry (no mortar between blocks) and coated with an eighth-inch layer of fiber-reinforced surface-bonding cement. Even inexperienced builders can construct such walls — applying the surface-bonding cement is not unlike frosting a cake. A surface-bonded wall does not have the tremendous moisture content of a poured concrete wall, which I would not advise inexperienced builders to tackle anyway. And the millions of embedded reinforcing fibers make it much stronger than a conventionally mortared wall against the lateral pressure of the earth berm.

Heavy timber roof framework. I advocate construction using heavy timbers you might get at a local sawmill at reasonable cost. For example, Earthwood is stoutly built, with a post-and-beam frame consisting of 8-by-8-inch posts and girders varying from 8 by 8 inches up to a 14-foot oak girder with a 10-by-12-inch cross-section. Our roof rafters are 5 by 10 inches, or 6 by 10 inches where spans are a little greater. Over the rafters, we use 2-by-6-inch tongue-and-groove planking.

Have your plans designed or checked by a qualified structural engineer. It is imperative that the framework be engineered to support the required load. This includes the load of the structure itself, typically 10 to 15 pounds per square foot (PSF) for this type of structure; the weight of the saturated earth (about 10 PSF for every inch); and your snow load (ask local officials; mine is 70 PSF). Earthwood is designed to carry a combined load of 185 PSF.

Waterproofing and drainage. In an earth-sheltered home, the earth is your friend, but the water in it is not. We place at least 4 inches of tamped sand below the floor for drainage, but the roof and any walls in contact with earth must be thoroughly waterproofed. Several good waterproofing membranes are available on the market, including large thick sheets of rubber, Neoprene or EPDM (two kinds of artificial rubber) and various bentonite clay products.

I prefer the membranes composed of laminated top layers of black polyethylene backed with a sticky bituminous material. Backing paper protects the membrane from sticking prematurely to unintended objects. The rolls are typically 3 feet wide and 50 to 75 feet long. I like these membranes because they provide high-quality waterproofing and they’re moderately priced (50 cents to $1 per square foot) and easy for inexperienced builders to apply. You simply pull the backing paper off the underside of the membrane as you roll it onto the surface to be sealed. Each sheet typically laps over the previous one by 2 1/2 to 4 inches. Installers press the sticky bituminous mastic down with the heels of their hands. Some of these products work better after the application of a “surface conditioner” (like a primer), which improves the adhesion of the sticky surface to the wood. Factory-finished edges are pre-caulked, but cut edges must be sealed with a compatible caulking. A membrane of this type goes on very quickly. A couple of people could cover a simple, shallow-pitched roof of, say, 1,200 square feet in a few hours.

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