Zone Zero: An Introduction To Cordwood Building
By Rob Roy
Since 1976, my wife Jaki and I have worked with cordwood masonry. In 1980, we began teaching it at Earthwood Building School in northern New York and around the world. There is a pattern of common questions we have observed over the years that anyone interested in learning more about this creative building technique might find of interest. Here are some of the key questions:
How did you get started in cordwood?
In 1975, we needed affordable shelter and liked the look of cordwood, which we’d seen in Eric Sloan’s book An Age of Barns and in the April, 1974 issue of National Geographic. Before we began to build, however, we traveled over the border to Ontario to see some old cordwood homes and barns. On the way home, we found a congenial cordwood builder actually laying up the walls of a cordwood barn. Jaki took notes.
What are cordwood’s pros and cons?
Cordwood buildings can be beautiful, combining the warmth of wood with the pleasing texture of fine stone masonry and special artistic features that can be incorporated. Cordwood masonry is inexpensive, particularly if you have a ready source of wood on hand. The walls are easy to build, although fairly labor intensive. When you lay the key component, a log-end, you attend to interior finish, exterior finish, structure, insulation, and thermal mass all at once. A conventional stick-frame might have 10 to 12 operations or layers to achieve the same functions. Through tons of mortar, a cordwood wall’s corners are insulated at the mortar joint, placing valuable thermal mass on each side of the insulation, helping to keep the buildings cool in the summer, while maintaining a comfortable interior temperature in the winter
Are there places or climates that are unfavorable?
Cordwood homes have been built successfully throughout Europe and North America. We’ve also seen them in Central and South America, as well as Australia and New Zealand. A very successful two-story cordwood hexadecagon – 16-sided building – was built of Captain Cook Pine near Mountain View, on the Big Island of Hawaii, which gets 200″ of rain a year. The only unsuitable places, in my view, would be those lacking a ready source of local wood.
How do you build a cordwood wall?
Slowly and carefully. The work is not strenuous or difficult – I have often said, “Children, grandparents, and beavers can all do it, and do it well.” However, basic time-tested methodologies should be adhered to. This is the main subject of my new book, Essential Cordwood Building. What follows here is a very short description of the building process. Yet the old adage, “a little bit of knowledge can be a dangerous thing,” is particularly appropriate with cordwood.
There have been a few bad examples built by people who did not pay attention to detail or the experience (including the mistakes) of others. The best cordwood work by novices is done by those who have taken a 3 to 5 day long course. The next best cordwood work is by people who have studied a good video and books on the subject. Careful study of a book alone is better than nothing, while “none of the above” – trying to re-invent the wheel – invites failure, or, at the least, poor quality. So … please … take the following as an introduction, not a definitive treatise in the “how-to” of cordwood masonry.
Roofing support: Our home, called “Earthwood,” is a round, load-bearing cordwood building with a heavy living-roof. Many other successful “stackwall cornered” homes have been built in North America. However, as a caveat, it is a great advantage to build under the protection of an already established roof that is supported by a strong timber frame. The main change that we would make if building our house again would be to do the cordwood masonry under the umbrella of a timber-framed hexadecagon, like our friend in Hawaii did. In seismic areas, the timber frame is a structural necessity.
The Logs: The building units for cordwood masonry are called “log-ends.” Cut logs to the same length as the width of wall to be built. Sizes that work well: 8″ for a sauna, shed, small guest house, etc; up to 16″ to 24″ for a home, depending on size, climate, and the thermal characteristics of the log-ends. In North America, the R-value of wood on end grain varies from R-1 per inch of thickness for northern white cedar, to about half of that for hardwoods, like maple and oak.
Log-ends can be rounds, splits, or a combination of the two. With the more favorable species (the lighter, airier softwoods such as white pine, white cedar, and spruce), it is best to cut them to log-end length for use the following year. Otherwise, wood shrinkage can be a visual and energy problem due to heat loss through the resulting gaps.
Mortar: a few options. Portland cement-based, lime-based, or if you have a source of favorable clay nearby, cob can all be used as mortar. Some builders have used paper-crete and hempcrete as cordwood masonry mortar. Recipes and instructions on how to make the mortar, occupy several pages of Essential Cordwood Building.
The most forgiving mix is the Portland and lime mix: 9 parts sand, 3 parts builder’s lime, 2 parts Portland, and 3 parts softwood sawdust (that has been passed through a quarter-inch screen and totally soaked overnight). The sawdust acts as a cement retarder. Without it, the dry log-ends can rapidly rob the mortar of its moisture, causing mortar shrinkage cracks. If you don’t have a favorable sawdust, a commercial cement retarder can be used.
Insulation: Finally, you will need loose-fill insulation to place between the inner and outer mortar. Our favorite is sawdust (passed this time through a half-inch screen) mixed with builder’s lime at the ratio of 12 parts sawdust to 1 part lime. Other people have used cellulose, Perlite, vermiculite, and the like. The insulation is key to the thermal performance of the wall. In fact, the insulated mortar joint – about 40% of the wall by area – actually has a superior R-value and thermal performance than the wooden portion.
Wheelbarrow for mixing, two shovels, a hoe, mortar pans, a trowel, pointing knives, ¼” and ½” screens, small spouted buckets and tin cans (for placing the sawdust insulation – very important!), amd cloth-lined rubber gloves to protect your hands from caustic cement and lime.
Steps to Building:
First, the foundation (concrete footings or floating slab, or stone or block) needs a brushed application of a bonding agent, such as DAP Bonding Agent or Acryl-60 from Thoro Corporation. Do this at the beginning of each day of masonry work.
Mortar is mixed in a wheelbarrow, also a convenient vessel for transporting and storing the “mud” during building. However, a plastic mortar pan is able to get you closer to where you are working at different heights and in tight corners. Once you get the process down, making a batch of mud takes only about ten minutes.
The mantra of cordwood masonry is: “Mortar, insulation, wood.” This is the order of building. Using a M-I-M stick (seen in the image on p. 65), lay down the inner and outer mortar joints, about an inch thick and of a width appropriate for the wall thickness: 2.5″ for 8-inch walls, 4″ for 16-inch walls, 5″ for 24-inch walls. Go 2″ or 3″ up the posts of the timber frame with mortar as well. Beginners should only tackle 24″- 30″ sections at a time. As you get faster, you will be able to handle 36″- 48″ long sections of wall without the mortar stiffening.
Next, with the spouted bucket, pour in the sawdust insulation between the mortar joints. Now you are ready for placing the log-end, usually beginning at a post. Set it down on the mortar with a gentle vibrating motion. You can put a little more mortar up against the first log-end prior to placing the second. It is easier at this stage than trying to jam it in later. But, the insulation in this cavity can wait until you do the double-mortar joint in the second course of masonry. It will find its way in, without you spilling it on the mortar.
Using a variety of sizes of log-ends during the first course will send the wall into what we call the “random rubble” style – pleasing to the eye. This uses up your various log sizes at a consistent rate. If you use all 6″ diameter logs on the first course, for example, you will soon run out of that size.
The second course proceeds as the first. Follow the mantra: mortar, insulation, wood. Get the mortar (and insulation!) down first and the wall will almost build itself. Using your gloved hands, keep a consistent 1″ thickness of mortar. You will have formed various “cradles” of different sizes and shapes. You simply take a mental snapshot of the space you need to fill, turn to look at your log-ends (stacked vertically within arm’s reach) and select the right one. If it rolls around in the cradle, select a larger diameter. If it won’t fit into the cradle, find a smaller one. After a day or two, you’ll consistently reach for the right size.
Pointing: The wall is finished by “smoothening,” or pointing, the mortar with a pointing knife. We make ours from old, non-serrated butter knives. Pointing is your chance to cut away excess mortar and to push it off your gloved hand into recessed areas. The log-ends will stick out a little past the mortar, “proud” of the mortar with about a quarter-inch “reveal.” Finally, smoothen out the mortar by drawing your knife fairly stiffly along the mortar. Pointing strengthens and beautifies the wall.
Well, that’s it in a nutshell, but remember the value of “a little bit of knowledge …”
Here’s the article as a pdf, as seen in Issue 07.
Click the “pop-out” button on the top right corner of the image to get a closer look.