Shrinkage of Logs
How much do house logs shrink as they dry, and how long does it take? This is another of those reader's questions that should be easy to answer but isn't. It seems that house logs lack the uniformity of other construction materials such as metals, plastics and concrete. Perhaps that is one reason people like log homes.
I found a useful source of information on building with logs and on other types of residential construction to be Axel Carlson of the Cooperative Extension Service of the University of Alaska. Several of his informative flyers are available free from the Extension Service. Another valuable source for me was an old book published in the 1890's that, from the looks of it, was carried by its original owner, my grandfather, on his trip into the Klondike in 1897. Smaller than a typical paperback, this manual of engineering knowledge has nearly a thousand ultra-thin, gilt-edged pages filled with tiny print. No author's or publisher's name shows in the book, but its cover is adorned with the name Haswell--in gilt print--and a loop by which the book can be attached to a belt for ease of carrying. This book tells how to build and operate windmills, water wheels, steam engines and other devices that energy-conscious Alaskans and Canadians are interested in again, now that petroleum is no longer nearly free.
Meanwhile, back at the cabin, the logs are shrinking. According to Carlson and Haswell, green logs contain water in amounts up to fifty percent: Birch 30%, Spruce 35% and poplar 50%. After two to eight years, depending upon air humidity and how the logs are treated with oil or other finishes, the logs will dry to about 12 percent water content. While drying, the radial shrinkage will be 1.5% (spruce) to perhaps 5%.
Consequently, one should expect a 96-inch high log wall to shrink at least three inches, and probably more, as it dries. Also expect the wall to seasonally vary in height by an inch or so owing to humidity changes from summer to winter. These changes can be minimized by use of overhangs to keep the rain off and by installing good foundations that keep ground moisture away from the logs.
