Rob Roy's Earthwood Home

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Haven't got old fence rails? Use what you've got. I don't know of any wood that will rot out when used as log ends, provided it has been debarked and it has no deterioration to begin with. The secret of cordwood masonry's longevity—100 years or forever, whichever comes first—is that the wood "breathes" along the end grain, something that does not occur by other construction methods. Also, one log end never touches another, so moisture is not trapped. A cordwood wall might get drenched during a driving rain, but it soon dries out again, and the bacteria that cause rot never get a foothold.

I have never treated either side of any cordwood wall I've built—five houses and numerous out-buildings—and the walls have never experienced deterioration. As added precautions, I always make sure that I keep the first course of wood well clear (at least 6") off the ground, and I employ a substantial (16" minimum) overhang around the building.

A word about the mortar, or "mud," which holds the whole thing together. After years of research and testing, we have arrived at the following recipe (proportions are equal parts by volume, not weight): 9 sand, 3 sawdust, 2 Portland cement, 3 type S (builder's) lime. The sawdust should be softwood from a sawmill, passed through a lf" screen to remove bark and the like, and soaked overnight in an open-topped drum. The sawdust in the mix is the secret spice that prevents the mortar from shrinking. It works by storing moisture, giving it off as needed to replace that absorbed by the dry cordwood. This mortar takes four or five days to fully cure, instead of one or two without it. Retarding the set in this way protects against shrinkage and cracking. Thoroughly mix the ingredients with enough water to give a "wet snowball" consistency. Not crumbly. Not soupy.

The cordwood construction procedure is simple: (1) Lay down a double bed of mortar, an inch thick and about 4" wide for a 16"-thick wall. (2) Fill the space between the mortar beds with loose fill insulation, such as vermiculite or perlite. I actually use sawmill sawdust, treated with lime at the rate of about 1/12 part lime, well mixed in with a hoe. Sawdust has been found to be equivalent to fiberglass in insulation value, a little over R3 per inch. Incidentally, fiberglass is not recommended as insulation between the mortar joints of a cordwood wall: if it gets wet, it may mat down and not fluff up again. If the lime-treated sawdust gets wet, it sets up like rigid foam when it dries out, still excellent insulation that completely fills the voids. (3) Lay the log end in the wall with a gentle pressure. The 16" log end establishes the width of the wall. (4) Subsequent courses follow the same pattern: mud, insulation, cordwood, except now you are following the hills and valleys established by the first course. It's really quite therapeutic. (5) Before the mortar gets too stiff to work, we "point" (or grout) the wall with a stainless steel knife made from an old kitchen butter knife which has had its last inch of blade bent to about a 30° angle. A firm pressure smooths the mortar nicely. Pointing makes the wall stronger, more beautiful, and much less porous. I think that pointing makes the difference between a good wall and a great wall. Jaki and I like to recess the pointing 1/4" to 1/2" back from the wood, for the sake of a pleasing masonry texture.

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