Why Does Wood Shrink in One Direction?

Types

• Shrinkage in wood can be divided into three different types. Radial shrinkage happens along the line from the bark to the center of the tree, while longitudinal shrinkage is up-and-down along the length of the trunk. Tangential shrinkage takes place along a line perpendicular to radial shrinkage. When you're looking at quartersawn lumber, radial shrinkage is change in thickness, longitudinal shrinkage is change in height and tangential shrinkage is change in length. Flatsawn lumber is a little different because it forms a cross-section of the trunk, so the directions on a piece of flatsawn wood are the same as in the original tree.

Patterns

• Wood can actually shrink in all three directions -- longitudinal, radial and tangential. The vast majority of the shrinkage, however, is tangential. Generally wood only shrinks half as much in the radial direction as it does in the tangential, and longitudinal shrinkage is negligible -- ranging from 0.1 to 0.2 percent on average in many kinds of wood. Juvenile wood is more likely to exhibit longitudinal shrinking.

Shrinkage

• The reasons why wood shrinks mostly along the tangential and radial lines has to do with its structure at the cellular level. Plant cells in the tree trunk have sturdy cell walls made primarily from cellulose microfibrils, which are like tiny cables made from chains of sugar molecules. The microfibrils of the thick secondary cell wall are aligned parallel to each other running in the longitudinal direction. The cell wall can absorb a considerable amount of water, and only once the moisture content falls below the saturation point does it begin to shrink.

Directionality

• The secondary cell wall loses most of the moisture, so the bulk of the shrinkage takes place here. Loss of moisture causes the microfibrils to move closer together, which doesn't affect the thickness in the longitudinal direction, just as bringing two parallel cables closer together does not affect their lengths. The microfibrils are not all perfectly parallel, however, so the wood usually exhibits an extremely small amount of longitudinal shrinking. Bringing the microfibrils closer together makes the cells narrower, causing shrinkage in both radial and tangential directions. Shrinkage in the tangential direction is more severe because the heartwood at the center of the tree retains less moisture than the outer sapwood, so it shrinks to a lesser degree.