We apologize if this title was a tad misleading: if you’re looking for a way to prevent any cracks from occurring in your large timbers, we hate to break it to you, but it’s simply not possible. However, by learning why these cracks occur and what makes them more likely, you can at least learn some ways to reduce cracking and choose species accordingly.
Why Cracks Occur in Large Timbers
Cracks (or checks, as they’re sometimes called) are typical reactions of wood to shifts in moisture content. While checking can be more easily controlled in smaller sizes of lumber, all bets are off by the time you get to lumber in excess of 10/4 sizes. The thinner stuff is simply easier to dry. Once you get to 12/4 and thicker timbers up to 6×6 and even 12×12, what you’re getting is actually a cross section of a log; as a result, the timber will end up behaving more like a log than an individual board.
Think of it this way: the more material surrounding the center of a given timber, the more insulation there is to protect the interior portion from shedding moisture. The outer layers will naturally dry and shrink, but they’ll be kept from moving as much as they are trying to, since they’re connected to the moist inner layer; that’s why the cracking occurs. While you might not like seeing these checks or cracks, they really don’t affect the integrity of the wood; however, not seeing them would actually indicate a problem, such as a rotten inner core.
Why Cracks Occur More in Some Species than Others
When you compare a timber made from Douglas Fir to a same-sized timber made from Ipe, you’ll notice there’s a difference in how those respective timbers behave. A dense, heavy species such as Ipe will have less empty space to allow the wood fibers to compress; as a result, such a timber will check more readily. At the same time, due to the high stability of Ipe, the resulting checks, though numerous, will generally remain small as well as shallow. Fir timbers, by contrast, will have fewer but wider and deeper checks, since the pressure is built up as the fibers have compressed, and then when they finally give way, the result is more extreme. Either way, though, the important thing to remember is that the checks or cracks in no way impact the integrity of the timber; by contrast, they actually improve strength due to the release of internal tension.
In the future, we’ll take a look at a couple possible solutions to this whole checking-and-cracking dilemma. But at least for now, you at least know that while you (or your customer) might not prefer how the cracks in large timbers look, those cracks will not compromise the structural integrity of large timbers.