Why does timber change shape over time?
Source: bunnings.com.au
If you’ve ever worked with wood in the past, lived in or around timber constructed buildings and furniture, you may have come across splits and cracks throughout pieces of timber. This can be most easily seen on old fixed table and desk tops, like the one pictured here, with a crack opening up along the whole length of one of the boards.
This crack was clearly not there when the desk was first made, so how did it open up like that? They appear due to the natural movement of timber – a very slight expansion and contraction that occurs in all wood, no matter how new or old it is.
What is causing the movement?
This can be counterintuitive at first – after all, isn’t the tree dead once it’s cut down? How can it continue to change and move after then?
There are two main ways that timber moves. The first is as it is initially dried (or seasoned) after being felled and machined, where most of the water content from the tree evaporates, and then the second over the course of its lifetime as the small amount of remaining water content changes. Even after timber is “dried,” the moisture content can still be between 4-11%. In fact, it’s this moisture that makes wood the material we know and work with. However, no matter how long it’s been since the original tree was milled, wood continues to be affected by things like temperature and humidity.
Possible timber movement as green wood is seasoned can be up to almost 10% of its tangential size.
Wood is a stable material, and an increase in local humidity doesn’t result in an equal increase in timber’s water content, but changes in the environment will affect timber and cause it to grow and shrink. This can be exacerbated with exposure to larger swings in environment, for example wood that’s outside rather than inside and protected from the worst of the elements, or timber furniture that is in a spot that gets direct sunlight or is directly under an air conditioning unit.
How much will timber move?
Seasoned timber will move different amounts along each of it’s axes. Timber is most stable along it’s longitudinal axis, or along the length of the grain, and has very little movement in that direction. However, along the tangential and radial axes (the two you see when looking at the end grain) timber will move a lot more.
This timber movement is a big reason that different cuts of timber are desired for different uses, such as plain-sawn vs quartersawn timber. By controlling the direction of the grain in every axis, we can both predict and plan for timber movement that is more stable along one or more axes, rather than happening at a diagonal angle to a face or side, or that cups or bends as it grows and shrinks.
A good rule of thumb is to always remember that when timber shrinks, the internal pressure is trying to flatten out the grain as much as possible. In a quatersawn piece where all the grain is parallel to the edge, that movement will be flat across the whole board, but in a plain-sawn board this will cause the entire piece to cup as the grain attempts to straighten itself.
What do I do about timber movement?
The first thing to do is to know that it will happen. When we design furniture or products, we always must think about the movement of timber and make room for it to expand and contract without binding or splitting. Luckily for us, we are able to plan and predict for the movement of timber and take that into account when we design and build pieces.
You can find out more about how to calculate exactly for this wood movement in the links below, but there are some good general rules to keep in mind when working with timber:
Leave room for timber to expand and contract. This can look like using a fixing for a table or desk top that has a small amount of movement in it rather than nailing or screwing the top down directly (and ending up with the desk pictured at the start of this article!)
Let timber season to your workspace before building something that is dimensionally precise. Just the act of leaving the timber stacked to acclimatise to your space for a few months will make a huge difference, and won’t result in you marking and cutting a piece and coming back the next day to find it no longer fits.
Use a non-moving timber product for non-face work. Materials like plywood, while made from timber, are produced in such a way to counteract timber’s movement and provide a great deal more dimensional stability. If you need to build something that won’t change shape or size, such as shop jigs or the internals of cabinetry, using plywood will take out the calculations for movement that solid timber requires.
Don’t worry if you make a mistake! You might build something and come back to it later and find that it’s warped slightly, but if you get to it early all those issues are fixable. Learn from the experience, and build something a bit more stable next time!
Illustrations by Liz, © Among The Trees
References
1. Wood movement 101 - Australian Wood Review
2. The nature of wood - wood movement - Workshop companion
3. Why does timber move? - Logic Bespoke
