Theoretically, you could dry large timbers and use them in a way that prevents any cracks from occurring. You’d have to dry them in a vacuum kiln and use them indoors and away from direct sunlight. And avoid any jarring sounds or movements near them. And . . . you get the idea. Practically, checking and cracking is often a fact of life, when you’re working with lumber, in general, and with large timbers, in particular.
Why Lumber Cracks
When you’re working with lumber sizes ranging from 4/4 to 10/4, even drying is easier to achieve; however, larger timbers, upwards of 12/4 make up a sizable percentage of the original log. Due to the significant insulation surrounding the heart wood, moisture retention at the center will outlast that of the outer layers.
As those outer layers dry, they will shrink; however, their shrinkage will be curbed by the inner layer, which retains moisture; the outer layer then responds by cracking. Also referred to as “checking,” this cracking is actually a good sign; if it is not apparent, there may be a more serious issue at hand, such as a rotten core.
While cracking and checking of the outer layer does not translate into decreased structural integrity, a rotten core most certainly does.
How Lumber Cracks
Differences among species mean that lumber cracks differently. For instance, heavier species such as Ipe tend to check more easily than, for instance, Douglas Fir. The reason is that higher density translates into less empty space to allow wood fibers to compress. Since Ipe is such a stable species, the checks may be numerous but are typically very small and shallow. Less dense species, such as Douglas Fir, often exhibit deep, wide checks because of shrinkage and compression into an empty space; the build-up of pressure causes the fibers to give way.
Either way, the cracks do not indicate lack of structural integrity. In fact, because these checks have allowed internal pressure to be released, wood with cracks and checks is often stronger than the wood that appears whole.
How To Reduce Cracks
Even though you can’t really prevent cracks and checks in large timbers — and you really don’t want to do so — you can reduce checking. The most time-tested way to do so is to cut a saw kerf into the timber. Used by Japanese carpenters as early as 600 B.C., this method is worth considering. Working as an artificial release point, such a cutting can greatly minimize cracking. Of course, the down side is that there will be a saw kerf in one face, but depending on your application, you may be able to position that part in order to hide it from view. One reason some prefer not to use this kerfing approach is that it requires cutting before the timber begins to dry.
Here at J. Gibson McIlvain Lumber, we seal the ends of timbers and store them in shady areas, helping to prevent widespread checking. Still, lumber moves and expands and contracts as it is exposed to sunlight and shifts in moisture levels.