In addition to being hygroscopic (gaining or losing moisture from the surrounding air), wood is also anisotropic. What this means is that wood has different properties depending on the direction or orientation of the grain—it’s not the same in all directions—and one of the areas where this property is most clearly seen is in dimensional shrinkage.
As opposed to a simple sponge or other isotropic material, wood (anisotropic) does not shrink in a perfectly uniform manner, and understanding this will help to avoid some pitfalls in preventing many shrinkage-related defects which may not crop up until months (or even years) after the wood product is finished.
A basic measurement of shrinkage—expressed as a percentage—is the amount that the wood shrinks when going from its green to ovendry state. In other words, since wood in its green state is at its largest dimension, and ovendry represents its driest (and therefore smallest) volume, green to ovendry is a measurement of the maximum possible percentage of shrinkage; this is referred to as the wood’s volumetric shrinkage.
Volumetric shrinkage tells how much a wood species will shrink, but it doesn’t indicate the direction of the shrinkage. The two primary planes or surfaces of wood where shrinkage takes place are across the radial plane, and across the tangential plane, corresponding to radial shrinkage, and tangential shrinkage; these two values, when combined, should roughly add up to the volumetric shrinkage.
The amount a piece of wood will shrink lengthwise, called longitudinal shrinkage, is so small—typically about 0.1% to 0.2%—that it is usually inconsequential to the volumetric shrinkage. However, plywood greatly benefits from the low longitudinal shrinkage of wood—layers of wood veneer are glued together with the grain direction of each ply oriented perpendicular to the adjacent ply, which has the effect of restraining most radial or tangential shrinkage within the veneer plies. As a result, the rates of shrinkage for the width and length of a plywood panel are typically less than 1%, (though changes in thickness still remain about the same as solid wood).
Radial shrinkage in solid wood can vary from less than 2% for some of the stablest wood species, upwards to around 8% for the least stable species; most woods fall in the range of about 3% to 5% radial shrinkage. Tangential shrinkage can vary from about 3% up to around 12%; most woods fall in the range of about 6% to 10% tangential shrinkage. (Accordingly, volumetric shrinkage is typically within the range of 9% to 15% for most wood species.)
The relationship between these two shrinkage values is expressed as the tangential to radial shrinkage ratio, or simply the T/R ratio. In addition to the volumetric shrinkage, (which measures the magnitude of the shrinkage), the T/R ratio serves to measure the uniformity of the shrinkage, and is another good indicator of a wood’s stability. Ideally, a wood species with good stability would have both low volumetric shrinkage and a low T/R ratio.
|A hypothetical shrinkage curve: Although shrinkage rates can vary considerably between species, (and even within the same species), this graph helps illustrate the shrinkage rates and their average proportions to one another; data was charted from values for Hard Maple (Acer saccharum), which has a T/R ratio of 2.1. Note that the values are stacked, so volumetric shrinkage is essentially equal to the sum of the three shrinkage percentages shown above. Tangential shrinkage accounts for the lion’s share of the overall shrinkage—about two thirds—with radial shrinkage making up most of the remaining third, and longitudinal shrinkage accounting for virtually nil.|
(It should be noted that just because a particular wood species experiences a high initial shrinkage during drying, doesn’t always correlate to an equal swelling after it has been dried. For instance, Basswood has fairly high initial shrinkage percentages—6.6% radial, 9.3% tangential, and 15.8% volumetric—yet its movement in service is relatively low. Using shrinkage and T/R ratio data simply offers woodworkers the best means of making an educated guess.)
In various wood species, the T/R ratio can range from just over 1, to nearly 3. At a T/R ratio of 1, shrinkage would occur in a perfectly uniform manner across the width and thickness of the board. At a T/R ratio of 3, the flatsawn surface would shrink or swell at triple the rate of the quartersawn surface.
As a general rule of thumb for most species, the tangential shrinkage is roughly double that of the radial shrinkage, which translates to an average T/R ratio of about 2. This helps explain why quartersawn boards are considered more stable than flatsawn boards: with quartersawn lumber, the thickness of the board is doing the majority of the shrinking or swelling, with the face of the board exhibiting minimal change in width—a useful characteristic for applications such as flooring planks or workbench tops.