Common Wood Defects From Moisture

Wood is a hygro-scopic material. This means it naturally absorbs and releases water (moisture) to balance its internal moisture content with the surrounding environment.

Because wood retains its hygro-scopic nature after it is put into use, it is subjected to fluctuating humidity. Shrinkage and swelling may occur in wood when there are changes in humidity and temperature. This may eventually result in cracks, gaps, and weak joints.

One major problem that occurs when drying wood is the tendency of its outer layers to dry out more rapidly than the interior ones. If these layers are allowed to dry much below the fiber saturation point while the interior is still saturated, drying stresses are set up because the shrinkage of the outer layer is restricted by the wet interior. Rupture in the wood tissues occurs, and consequently splits and cracks occur if these stresses across the grain exceed the strength across the grain.

Since wood shrinks and changes shape as it dries, the bulk of that shrinkage and change of shape should occur before a woodworker starts working with it.

The amount of shrinkage varies from species to species, but generally wood shrinks 8 to 10 percent tangentially, 4 to 5 percent radially, and close to zero percent lengthwise. In other words, the surface of the board where the grain intersects it perpendicularly, or close to perpendicularly, shrinks the most. This means woods of different shapes will shrink differently based on how they’re cut from the tree.

Some kiln-dried wood can change 1/8” to ¼” in width for every foot. They may not seem like much, but when you add it up, a 4-foot wide table can vary in width by as much as an inch from dry season to wet season.

Moisture changes and wood defects

Moisture, therefore, is certainly the most important factor affecting the performance and service life of wood and wood products. Because moisture affects the dimensional movement of wood and wood products, under certain conditions moisture change can lead to major dimensional change.

Once kiln-dried wood arrives, woodworkers should store the wood in a climate-controlled shed or shop in order to avoid wood movement. Seasonal changes in humidity can cause wood to shrink or swell.

Some woodworkers, however, prefer to buy green lumber and dry it themselves – either by air drying or using a solar kiln. This can help offset the high price of kiln-dried lumber.

Depending on where wood is stored and under what conditions, wood can easily absorb or release moisture. For that reason, all wood should be monitored regularly and certainly before use in order to avoid moisture-related problems.

This is one of the reasons why more woodworkers today are using moisture meters to prevent problems such as warping, loose joints, or cracked table tops.