THE SHORT ANSWER
Splits and cracks occur when timber shrinks as it dries. Timber shrinks roughly twice as much along the growth rings (radially) as it does across the rings (tangentially) and it is this uneven shrinkage that causes cracks to develop.
ARE CRACKS BAD?
We think of them like wrinkles in a cotton shirt. It proves the timber is real. Cracks are what make a solid timber look organic. It is what gives timber so much of its character.
WHY DOES TIMBER SHRINK?
It sometimes surprises people to learn that roughly half of a living tree’s weight is water. Wood is hydroscopic, meaning it’s like a sponge in that it can absorb, hold and release water.
HOW LONG DOES IT TAKE A TIMBER TO DRY?
It’s not exactly a fast process, and it depends on the humidity of the environment in which the timber is located, but one rule of thumb is that a timber air dries about 25mm per year. So a 150mm x 150mm timber post would take around 6 years to dry to the center.
HOW DRY WILL THE TIMBER BECOME?
Timber will eventually air dry to the Equilibrium Moisture Content (EMC) of its environment. The EMC is the point at which wood is neither losing nor absorbing water. The timber’s moisture content is determined by the atmospheric humidity of the timber’s environment. That environment varies, of course. Is the timber located inside, or outside? What part of the country is it located in? What time of year is it?
MOST TIMBER FRAMES ARE BUILT FROM GREEN (WET) WOOD
It is impractical (it takes years) to air dry timber, so for the last two thousand years, people have been building timber frames from green (wet) timber. Yes, green timber will shrink, crack, and sometimes twist as it dries. But timber framers and engineers understand and account for the movement. Cracks begin on the exterior surface of a timber and almost always stop at the heart (center) of the timber, and are almost never a structural concern. By the way, if a crack were to develop all the way through the timber (splitting it into two separate pieces), it would be called a split and might be cause for concern.