As a rough rule of thumb, wood will shrink or swell about 1% in size across the grain, thickness and width, when the moisture changes by 4% moisture content.
As another generalization, wood does not shrink appreciatively along the grain, length-wise, even with large moisture changes.
Eugene Wengert, known to many as the wood doctor, is a professor and extension specialist at the University of Wisconsin. He explains this shrinkage and swelling in more detail.
Although 1% shrinkage which occurs with the 4% moisture content change, seems like a very small-size change, perhaps being only several thousands of an inch, such a small change can quickly lead the gluing problems, especially cracks and joints, after the furniture, cabinets or other items have been manufactured. In a piece of furniture or in a cabinet, 1% shrinkage or swelling can result in poor fitting drawers and doors, as well as poor fitting joints. In addition, 1% shrinkage can often result in a development of finished cracks and work.
Although, we have already mentioned that wood shrinks and swells in thickness and width, let's look at a few examples of the moisture effects on manufactured wood products.
Consider two pieces of wood joined by a mightier-joint, as the wood dries out, the ends of the pieces will become more pointed with the gap developing on the inside of the joint. If the wood gains moisture, the ends become more blunt and the joint opens at the tips.
Consider several pieces of wood, edge-glued together, but then lose moisture quickly; the ends will dry and try to shrink. However, the rest of the panel is losing moisture and shrinking more slowly, leading to the development of stress, which can result in a crack in the end.
If the individual pieces that will be used to glue up a panel, are cut to précised sizes, but then, are allowed to change moisture content, there will be gaps between the individual strips. Such gaps, if more than 6,000 of an inch, will lead to a weak glue joint.
Consider a panel that has a cleat on the end, because wood does not shrink appreciatively in the length-wise direction, but does shrink along the grain, there will be when the moisture content changes, considerable stress developed, where the cleat joins the panel, sufficient to break a glue joint and crack the wood or bend the nails or screws.
Incidentally, if this cleat where moved to one face, the cleat would result in a strange shrinkage and swelling on the backside, while the other side would be free to move. The net result would be warp.
A similar situation exist, when two pieces are joined with the grain at right angles, one piece is changed in size when the moisture content changes, the other is not. This creates stress and potentially some cracking.
Or consider a panel that is exposed to a very dry atmosphere after being stored in a more humid condition. The quarter-sawn piece in the center will change in thickness twice as much as the two flat-sawn pieces, leading to observable bumps in the panel when it is finished.
A similar situation exist, if solid wood components are attached to a particleboard core, but, the wood and the core have not been exposed to the same EMC prior to assembly.
Note that in these previous examples, it is the change in moisture content that causes the problem.
But, moisture content doesn't change unless the EMC and moisture content are not equal. So the key is to know the EMC in the customer's home or office. And once the lumber has been dried to the correct moisture content, keep the EMC in storage and in the plant, at the same moisture content condition.
As a rule of thumb to avoid moisture-related problems, the moisture content of wood components and wood products should change less than 2%, from initial manufacturing until the wood is in use and achieves its final moisture content, in a customer's home or office.
Several gluing problems from components that are too wet have already been mentioned.
Consider one special problem, if wood is too dry, the wood will absorb moisture or other liquid in the glue too quickly, before the joint is assembled and pressure is applied. The net result is a weak glue joint.
Although machining of wood including planing, mowing, boring and sawing is greatly affected by speed rates, blade, knife-sharpness and machine set-up variables, moisture is an important variable as well.
If wood is too wet, the wood fibers are weaker than normal and there will be an increase in fuzzing.
On the other hand, if wood is too dry, under 5% moisture content for hard woods, and under 8% moisture content for soft woods, the wood will be too brittle and will split or crack while machining. Also, overly dry wood is more prone to developing chip grain and torn grain during machining. And then raise grain later on, after the wood soaks up moisture from the air around it.
A special note, wood only changes size or shape for one reason, a change in its moisture content. There is one exception, wood that has drying stresses or case hardening in it will warp or change size immediately while machining.
Any change in size that occurs after machining is related to a change in moisture content.
With water-based coatings becoming more popular, the effect of water in the coating must be considered.
We are only now learning some of the subtle effects of wood moisture content on water-based coating quality. Certainly, overly dry or overly wet wood results in poor quality coatings.