Expert answers to cordwood construction questions about building a home using this energy efficient wood.
Cordwood Construction Questions? Ask the Experts!
Practiced throughout the ages, cordwood masonry construction is experiencing a renaissance as hands-on home builders learn of its simplicity, energy efficiency and unique beauty. But with any unconventional building technique, cordwood construction comes with its own set of challenges. Here, we've addressed the most frequently asked questions.
Won't the log-ends rot? If basic care is taken, log-ends will not rot. Fungi, which need constant moisture to thrive, cause wood to rot. Cordwood breathes wonderfully along its end-grain, foiling fungi's propagation. To ensure long-lived log-ends: 1) Debark the wood; 2) Don't place wood against wood (this can trap moisture); 3) Don't use wood that already shows signs of deterioration; 4) Design your roof with at least a 12- to 16-inch overhang; and 5) Set your bottom course of cordwood at least 6 inches off the ground, on a good masonry foundation of stone, block or concrete.
What kind of wood should I use? Select light and airy woods like white cedar, white pine, cottonwood, poplar, spruce or larch (tamarack). These shrink (and expand) less than dense woods such as maple, oak, elm, beech, and some of the heavy Southern pines. Denser woods can be used only if special building-design strategies are used.
How long should I dry the wood? Light, airy woods should be dried at least a year, if you can wait that long. This will greatly minimize shrinkage. Dry dense wood just a few weeks, as there is the very real danger of wood expansion with dry hardwoods that get wet from driving rain. Wood expansion is a more serious problem than shrinkage; expansion can break up the wall, whereas wood shrinkage can be attended to in several ways. Research the chosen wood's shrinkage characteristics before deciding upon a drying time.
What mortar mix should be used? For more than 20 years, we have had good success with a mix of 9 sand, 3 sawdust, 3 lime and 2 Portland cement (equal parts by volume). Use sawdust from a light softwood rather than from a dense hardwood. First, pass the sawdust through a half-inch screen. Then, overnight, completely saturate it by placing it in a soaking vessel such as an old bathtub or an open-topped metal drum. The purpose of the sawdust is to slow mortar-curing time, which reduces mortar shrinkage. If softwood sawdust is not available, use a commercially available cement retarder. Also, use hydrated or Type-S lime, also called builder's lime, which makes the mix more plastic, and, as the lime calcifies, makes the mortar stronger over time. The Portland cement (either Type I or Type II) lends strength to the mix.
In addition to writing books and articles about cordwood masonry, Rob Roy also is the director of Earthwood Building School in West Chazy, New York. He and his wife, Jaki, have taught cordwood masonry construction across North America, and in Chile and New Zealand.