To choose the right wood for any project, you need to know how it will perform, not just how it looks. One of the key considerations in performance is wood movement. You’ll need to predict how the particular species and cut of wood that you choose will change over the seasons and years. Ignore this and even the prettiest wood could turn out to be a pretty big disappointment.
Please refer to my previous post, “Understanding and Calculating Wood Movement,” which covers the basic concepts underlying the information here. This is indispensible knowledge for all woodworkers.
Getting oriented: flatsawn
The board in the photo above is flatsawn, as evidenced by the curved pattern of the annual growth rings on the end grain. The surfaces of this board have been planed flat and it is now at equilibrium moisture content in my shop. If the humidity lowers, the board will shrink. Let’s imagine this. The growth rings will shorten like contracting rubber bands about twice as much (per unit of length) as the spaces between the rings will narrow.
It is as if the growth rings “will try to straighten out.” This will make the board cup in a slight downward U-shape across its width.
Similarly, if the humidity in the shop rises, the wood will gain moisture, expand, and it will cup in a upward U-shape as the growth rings expand differentially from the spaces between them.
Getting oriented: quarter-sawn and rift-sawn wood
The board shown just above is quarter-sawn, meaning that it has been cut from the tree along a radius from the center of the tree. The end grain growth ring lines are therefore short, almost straight, and approximately at 90-degree angles to the face of the board. This board will undergo wood movement too but it will not cup like the flatsawn board.
Rift-sawn wood, shown below, has diagonal end grain annual growth lines. Depending on their angles and curvatures, the wood will behave intermediately between flat-sawn and quartered boards.
The schematic diagram below shows the end grain of a log and how the three types of boards could be oriented within the log.
Width and thickness
Aside from the matter of cupping, consider the simple dimensional changes of two boards of the same species, one flat-sawn and one quarter-sawn, that is at equilibrium with the ambient humidity and identical in width and thickness. Now raise the humidity and what happens?
Again, due to the tangential movement being about twice that of radial movement, the flat-sawn board will become the slightly wider of the two, and it will cup. The quarter-sawn board will become the slightly thicker of the two, and it will not cup.
What does it matter?
Wood movement continues as long as there are changes in the ambient air humidity, which for practical purposes, is forever. It is only slowed, not stopped, by finishes on the wood. Furthermore, the force of this wood movement is stronger than the wood itself and fully capable of splitting the wood if the wood does not have room to move.
Therefore, every woodwork design in solid wood must account for wood movement.
It depends so much on the species
Wood movement varies a lot among species. Values for movement are conventionally listed as the percentage of shrinkage in the tangential (T) and radial (R) dimensions as wood go from fully saturated to oven dry. Also sometimes listed as the “volumetric” shrinkage value, which is a measure of the overall shrinkage of the species and is simply the approximate sum of T + R.
Of course, wood never gets oven dry in normal circumstances. So consider these values as relative among the species, paying attention to two things:
1. Higher T and R values indicate that the species shrinks and expands more with each increment of change in humidity.
2. The higher the ratio of tangential to radial change (T/R), the greater is the tendency for flat-sawn boards of that species to cup.
The best way to become familiar with using wood movement values as an aid to selecting wood is to consider some examples. (Based on published data in the US Forest Products Laboratory Tech Sheets.)
1. Eastern white pine: T = 6.1%, R = 2.1%, Volumetric = 8.2%, T/R = 2.9
These values are low, so pine boards actually don’t move very much for a given amount of humidity change. However, this popular and plentiful softwood is often subjected to large swings in moisture content that make pine boards, as a practical matter, move quite a bit. Pine is almost always flat-sawn and the high T/R ratio makes the boards cup a lot. Plan for it though, and you’ll be fine.
2. Mahogany: T = 4.1%, R = 3.0%, Volumetric = 7.8%, T/R = 1.4
Mahogany isn’t just popular for its looks and easy workability. The movement values are very low and the T/R is low. Consequently, mahogany boards do not move much and even flatsawn mahogany does not cup much.
3. White oak: T = 10.5%, R = 5.6%, Volumetric = 16.3, T/R = 1.9
These are high values overall with a fairly high T/R and a large numerical difference between T and R. This stuff will move a lot and, if flatsawn, cup a lot. Fortunately, white oak is most attractive when quarter sawn, as shown below.
A quarter-sawn white oak table top is not prone to cupping, and the thickness change doesn’t matter because it is not fit against another component. However, the thickness changes in quarter-sawn white oak drawer sides are significant enough that they must be anticipated when fitting the drawer.
4. Walnut: T = 7.8%, R = 5.5%, Volumetric = 12.8, T/R = 1.4
Now here’s an all-star wood. Modest overall values and a nice low T/R. It’s beautiful, offers amazing variety, and it’s quite well-mannered flat-sawn, rift, or quartered.
You can’t just guess
The calculation of anticipated wood movement for a particular piece of wood – a door panel, for example – is beyond the scope of this article. It is, however, a skill all woodworkers should have and it isn’t difficult. It sure beats split panels, stuck drawers, loosened joints, and other disappointments. Basically, you need to know the wood species, the grain orientation of the board, the humidity in the shop where the piece is being built, the initial dimensions and clearances, and, if possible, the approximate range of humidity where the piece will live.
You could take out a calculator and use data and formulas found in the FPL Wood Handbook (available free online) or Understanding Wood (Taunton) by R. Bruce Hoadley. But I don’t and instead use these easy, excellent tools:
- Lee Valley Wood Movement Reference Guide, $9.50. This handy wheel chart has clear instructions that walk you through the calculations.
- Woodshop Widget, available for free online, and as an iOS app, $3.99, or Android, $1.99. This makes it so easy, there’s just no excuse to ignore wood movement!
For yet more on wood movement, read my post, “5 Things every woodworker needs to know about wood movement,” here on the Craftsy woodworking blog. Then take a great Craftsy woodworking class and build something you’ll cherish!