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Minimizing Drying Cracks in Large Timbers
Question
Forum Responses
From contributor M: I started out working for a log home builder. We peeled and fitted the logs green and factored the shrink into the building. Never did see a log that did not split or check. It could be controlled with the grove cut in the bottom of the log before it was stacked on the building but not eliminated. From contributor M: Hard to tell but it looks to me like the beams are all cut free of heart. You will need a very nice piece of wood to start making FOHC beams. Start with logs 30" and bigger, nice and straight with no twist and very little taper. You won’t be able to make good beams from bad logs. If you have hardwood you might be able to trade it for FOHC Douglas fir. From the original questioner: Ok a bit of clarification. First of all fachwerk, it can mean one of two things. It can either be the classic German/Swiss half timbered structure, or in older usage it is a style of log cabin. The German style is squared timbers laid horizontally with the walls mortised into each other. Many of the squared log cabins are in this style. This is the fachwerk I am talking about, although I very much love the half timbered houses (being of Swiss background myself) and would love to do a structure like that some time. I used beams in my post because the "logs" used to build cabins in this style are squared and more accurately called beams. For splitting, I know it is unavoidable but I also know there are certain ways to lessen its impact, and appearance. Keep also in mind that these are squared and not round logs. The beams I would be using would need to be all heart wood, as they would be exposed to the elements. The cabins I am talking about would not be the normal round notched log style. These logs would require milling (or rough hewing with a broadaxe in the old days) but only minimal carving after that.
From Gene Wengert, forum technical advisor: First, you want a species that has the radial and tangential shrinkages close to each other. That will result in less shrinkage stress. Also, you want both r and t values to be rather small, compared to most other species. Once you have the wood, if the logs or beams you are making can avoid the pith (large log diameter), you will have better success. Next, if you can put a fairly large hole down the length, then this will relieve the shrinkage stress as the wood can shrink into the hole. Finally, the last alternative is to put a deep kerf on a face that will not be seen and then let this kerf crack absorb the shrinkage stress. Once dry, it is advisable to coat the exterior with a water repellant. From contributor J: Found some radial and tangential shrinkage values in USDA handbook #72 (1955 p.315). Looking only at the ratio of tangential to radial shrinkage, from green to 6% MC, some values would be: black walnut (1.4), white ash (1.6), pignut hickory (1.6 a low ratio but highest shrinkage at 9.2T), white oak (1.7), black cherry (1.9 lowest shrinkage 5.7T), hackberry (1.9), sugar maple (1.9), n, red oak (2.1), red maple (2.1), and American elm (2.2).
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