Swelling and shrinkage of wood. Consequences of moisture variation

I'm sure you've all heard that wood "works" permanently. What does this mean? Solid wood changes its dimensions according to the humidity outside and does so even if it has been properly dried and varnished. The swelling and shrinking of wood occurs because of the natural exchange of moisture with the environment. This is a property that must be taken into account because it has consequences that can sometimes be very unpleasant.

Wood movement. The relationship between wood moisture and environmental humidity

About equilibrium humidity of wood we have already talked about when we touched on the subject of drying solid wood, but I will remind you of a few facts because they are useful in the following explanations. Equilibrium humidity of wood is closely related to the humidity of the environment in which it is used. For this reason, wood, whose moisture content is very different from that of the environment, tends to lose or absorb moisture in order to return to equilibrium. In addition, wood is a hygroscopic material, which makes it naturally absorb moisture.

The following relationship has been established between the equilibrium moisture content of the wood and that of the environment:

• outdoor humidity 25% - wood 5%
• outdoor humidity 50% - wood 9%
• outdoor humidity 75% - wood 14%
• Outdoor humidity 99% - wood 28-30%

When wood that has reached equilibrium is moved to an environment with a different moisture content, it will absorb or release moisture until equilibrium is restored. This leads to swelling and shrinkage of the wood which can result in dimensional variation, deformation or appearance of cracks. This is, for example, why solid wood flooring should be allowed to acclimatise for a few days in the space where it is to be laid before being installed. This avoids gaps between the planks or swelling.

Swelling and shrinkage of wood

It's all about wood's property of absorbing moisture, hygroscopy (or hygroscopicity, according to some sources). The absorbed water binds in the cell walls, changing the dimensions of the wood. Only this type of water - bound water - causes dimensional change in the wood. There is another type of water - free water - which enters the wood without being bound. It fills existing voids and so, when it is lost, neither swelling nor shrinkage of the wood occurs.

The changes due to the bound water are different because the inner structure of the wood to which it is bound is also different. Thus the dimensional variation will depend on:

• species
• how to cut the piece
• size of parts
• anatomy of wood

When choosing wood to be used in environments with high humidity variations (bathroom, kitchen, pools, sauna) or outside, we need to take these factors into account.

In terms of shrinkage when bound water is released, there are 3 categories of wood:

• with small shrinkage: nuc, plop, chestnut, tea
• with medium shrinkage: resinous, cherry, birch, maple
• with high contraction: oak, frasin

That's why cracks are more common in beams oak compared to other species. High shrinkage leads to high stresses resulting in cracking. Species in the first two categories are recommended in construction because their dimensional changes have low consequences.

Dimensional variation within the same trunk

The dimensional variation is also different within a tree trunk. In the axial (longitudinal) direction, the variation is very small, so it is not really taken into account by carpenters. On the other hand, in the tangential direction (tangential cut at the annual rings), the variation is very important, reaching 8-9% in softwoods and 11-12% in hardwoods. In the radial direction (cutting perpendicular to the annual rings) the variation is half that of the tangential direction.

When cutting lumber, the dimensional variation in relation to the swelling and shrinkage of the wood must be taken into account.

The lowest shrinkage is in radially cut lumber - when the annual rings are perpendicular on both sides of the plank. Tangential cutting leads to warping when moisture variations occur and a plank cut in this way will curve like a tree. Deformation is greater the closer the cutting is made to the outside of the trunk. The cutting method is clearly visible on the end of the plank. If the annual rings are seen as straight lines, the cut is radial and the change is small. When cutting tangentially the annual rings are seen as curved lines, which are closer and closer together the closer the cut is to the outside of the trunk.

In terms of piece size, a small piece absorbs more moisture than a large one. The result is that it will warp more compared to a large one.

Fibre saturation point

We said that there are two types of water in wood, but only the one bound at the cellular level leads to swelling and shrinkage. The cell wall cannot hold water indefinitely and there is a maximum amount of water that gets into the wood because of its hygroscopicity. This maximum is called saturation point being lower in resinous (25-28%) and higher in foioase (35-38%).

Different value leads to different behaviour. Softwoods can be cut all year round without the risk of large cracks when drying, whereas hardwoods are only cut outside the growing season (November-January). Deciduous wood should be sliced lengthwise as soon as it is cut because there is a risk of deep cracks, whereas softwood can reach equilibrium moisture as a trunk (see houses in bușteniThe trunk of the resinous trees can be peeled immediately after cutting without great consequences, while the hardwoods should be left with the bark as long as possible to slow down the water outflow.

Don't confuse between the total water content of the freshly cut wood (35-40% for hardwoods and 50-55% for softwoods) and the saturation point. Only the percentage of water represented by the saturation point produces dimensional changes when removed because it is bound to the wood fibre. The escape of this water can lead to tension and cracking. The quicker it comes out, the more sudden, the more unpleasant the consequences.

Why is it important to know about wood swelling and shrinkage?

Why did I insist so much on the swelling and shrinking of wood and the consequences of water removal? Because, as I said at the beginning, wood is constantly moving and there is nothing we can do about it. But we can dry it, cut it and assemble it without its movements having any consequences. Problems with dimensional variation can also occur with wood used indoors, not just exposed outdoors. Wherever two pieces of wood cut differently meet, a problem can arise if the wood moves. This is best known to those who make doors and windows. Problems are not only related to growth or shrinkage but also to bending or warping. A wrong choice of elements can lead to forcing and warping of joints and even splitting. The difference between a work that keeps its shape and one that warps over time is the correct choice of wood. You can have good and very good results no matter what wood you use.

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