Large timbers present a unique set of challenges, and even those familiar with other sizes of lumber might not realize how to treat large timbers or what to expect from them. We often field questions about what many consider “cracks” in their large timbers, so today we’ll take some time to explore that issue.
The Good News
Okay, for starters, your timbers probably don’t have cracks; what they actually have are technically considered “checks.” What’s the difference? A lot, actually. A check can be defined as “a split in wood that runs parallel to the grain, across the annual ring growth” or “the separation of wood tissues along the grain of sawn lumber.” So the main way to distinguish a check from a crack is that a check runs along the length of a board, whereas a crack runs across the grain.
In summary, checking is caused either during the drying process or as a result of natural shrinkage and fluctuation in moisture levels, whereas a crack is a serious defect that requires attention. The good news is that checking is not a cause for concern.
The Bad News
It’s not really bad news, unless you really wish you could completely avoid checking or rid your large timbers of checks: They’re really not preventable. While checking can be controlled through careful drying when it comes to 4/4 through 8/4 lumber, the situation changes when you get into 12/4 lumber or timbers ranging from 6×6 to 12×12.
When you get to that large timber size, you’re dealing with a huge cross-section of the original log, causing the board to act more like a log than a board. The insulation surrounding the heart of the timber allows the central portion to retain moisture longer than the outer layers do; as the outer layers dry, they shrink. The moist inner layer won’t allow the outer layers to shrink much, though, so those outer layers have no other option but to check.
So the bad news is that while there’s really nothing you can do to prevent checking, the counterpoint is that you really don’t want to stop it. In fact, if you see a large timber with absolutely no checking, there’s actually extremely bad news in store: the core may be rotten.
More Good News
Just like wood movement, checking occurs predictably and varies in how it occurs in different species. For instance, dense, heavy species such as Ipe check more readily than others because the higher density means less empty space for wood fiber compression. While the checks occur quickly, they’re typically pretty small. In species with less density, such as Douglas Fir, checks occur only after wood fibers have been compressed into empty space, causing a buildup of pressure. When the fibers finally do give way, the result is a wide check that appears similar to an explosive crack.
What’s the good news in that? Checking is far from negatively impacting the structural integrity of a timber; in fact, checks actually strengthen the timbers because they allow the internal tension to be released.
To learn more about what not to do about checking, check out Part 2.
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J. Gibson McIlvain Company
Since 1798, when Hugh McIlvain established a lumber business near Philadelphia, the McIlvain family has been immersed in the premium import and domestic lumber industry. With its headquarters located just outside of Baltimore, the J. Gibson McIlvain Company (www.mcilvain.com) is one of the largest U.S. importers of exotic woods.
As an active supporter of sustainable lumber practices, the J. Gibson McIlvain Company has provided fine lumber for notable projects throughout the world, including the White House, Capitol building, Supreme Court, and the Smithsonian museums.
Contact a representative at J. Gibson McIlvain today by calling (800) 638-9100.