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.

Radial, Tangential, and 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.

Radial, Tangential, and Volumetric Shrinkage
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.

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13 Comments

  1. Thresa August 10, 2018 at 10:52 am - Reply

    It is okay to have shrinkage percentage as high as 64%

  2. Anon July 19, 2018 at 1:33 pm - Reply
  3. Luke June 26, 2018 at 4:21 am - Reply

    I have a simple question, I have made some white pine table tops that in the middle of winter split up the middle , ussaly when the table had opposite direction grain end boards at each end . I am assuming this was a result of my lack of ensuring the wood was 6-8% mc. I think if I keep the wood low in numbers like 6% or even 5.5 % mc and seal it with extra lacquer that it will handle the dry winter inside a home ? Or should I not use end boards , or switch to another species?

  4. Jacy June 17, 2018 at 7:41 pm - Reply

    Will freshly milled dimensional eastern red cedar shrink and expand greatly in the first couple of months it is laid down. Having a slow start and wondering about tight fitting on later boards.

  5. Eric March 11, 2018 at 6:15 pm - Reply

    I’m wondering how a wood species’ T/R ratio relates, if at all, to its tendency to check, especially in large boxed heart beams.

    Here are my hypotheses. I noticed that eastern white pine, even though it has very low shrinkage numbers, has a very high T/R ratio, and I’ve read elsewhere that it checks more than other species, so that makes me think higher T/R ratios would be associated with more checking. With a boxed heart beam it would seem like checking would normally be the result of wood shrinking faster tangentially than it shrinks radially, but since the circumference of a circle is 3.14 times the diameter, that makes me wonder if a T/R ratio of 3.14 would actually be ideal for minimizing checking. Then again since wood doesn’t dry evenly but rather dries from the outside in, maybe that theory goes out the window, especially with large beams.

    • Efram Golddberg May 17, 2018 at 5:27 pm - Reply

      I can offer some insight from a scientific perspective.
      In order to help understand, lets consider what causes checks. Checking occurs when the forces acting upon the wood exceed the tensile strength causing the wood fibers to pull apart and resulting in cracks. Also notice that these happen running parallel with the grain, which suggests that the forces are acting perpendicular to the grain.

      As to your first question, the higher the T/R ratio the more the relative amount of tangential shrinkage occurs. Both these shrinkages happen at right angles to the grain. As the T/R ratio gets closer to 1, the shrinking is more uniform and stresses are minimized. Yes, higher T/R ratios mean the forces are more non-uniform across the grain, and high forces across the grain cause checks.

      The fact that circumference=pi*diameter is true, but has no connection to the value of T/R, or the reason why heart box beams are more likely to check. Consider the grown rings in normal pieces of wood that dont contain a full heart box, then consider a full heart box beam. In a normal round beam, the rings will run from one end to the other. In a heart box, they run in rings around the center. So while in a normal piece think XY, T is moving in two directions, and R is moving in 2 directions. In a beam with a heart, both T and R are moving in 360 degrees effectively pushing the wood out from the center, and then tearing it apart on the outside… a wood with a T/R of 1 would still be more stable, although the orientation of the growth rings means any wood with a V number would probably be prone to checking.

  6. Bill T. March 8, 2018 at 11:07 am - Reply

    WE ARE HAVING ISSUES IN SHRINKAGE. WE USE CHIP CORE AND WE GLUE A SOLID WOOD PIECE AROUND THE PERIMETER. HOW MUCH MOISTURE VARIANCE SHOULD THERE BE BETWEEN THE TWO SUBSTRATES ?

  7. Mn7#GoFuCarthago Kid January 22, 2017 at 2:40 am - Reply

    We have problems of swelling in Oak Glulam furniture doors made by Venjakob, the door is only 688mm high and has swelled by 8mm, the relative humidity range of the room is between 45-55% and the temperature range is between 18 – 22 deg. The glue laminated timber runs horizontally and the door thickness is 20mm. Can you please advise what your assessment of the problem is.

    • L'Ola October 12, 2017 at 3:44 am - Reply

      what was the moisute content of the wood during production of the furniture? It should be between 6-8% otherwise it may cause problems as described

  8. Walt Corey April 7, 2015 at 1:02 pm - Reply

    We had hardwoods installed throughout (except bath and kitchen). In the hall the wood is laid perpendicular to the hall and terminates in the foyer. This is where the directionality changes as the foyer is laid parallel to the hall so the wood meets sort of T shaped.Very easily a quarter could be dropped in the winter gap. I’d guess it is easily 1/8″. However, at the other side of the foyer is the dining room and the direction of the planks is identical to the foyer. At the end of the dining room is the kitchen. The gap at the hall/foyer transition closes tight in the summer. Where the hardwood in the dining room terminates at the kitchen there is a single, much wider piece perpendicular to the wood in the dining room and parallel to the kichen. This transition and the identical transition between the breakfast nook side of the kitchen and family room are 100% snug winter and summer. Oddly, in walking across the gap between the hall and foyer it feels like there is in addition to the gap, buckling. We’ve had a total of 4 new homes with hardwood and never seen anything like that. Is there anything that can be done to resolve this?

    • mark November 23, 2015 at 12:46 am - Reply

      you need to find a way to control the humidity in your house. the change in gaps between boards are from a change in the humidity.

  9. Student March 1, 2015 at 11:18 pm - Reply

    Wood this be mostly for quartersawn lumber? Or do plainsawn and rift sawn shrink in the same manner?

    • ejmeier March 2, 2015 at 12:52 pm - Reply

      Quartersawn lumber tends to shrink/expand more in it’s thickness, and not as much in its width (dependent upon the wood’s T/R ratio). Plainsawn is just the opposite, and is the least stable, shrinking most along the width of the board, and least in thickness. Adding to its instability is the fact that most plainsawn boards do not have perfectly horizontal grain, but tend to have a varying pitch to the grain, edging more towards rift or even quartersawn (vertical grain) towards the edges of the board.

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