Wood Shrinkage Explained

A basic knowledge you must know before doing some wood works. Wood shrinkage is one of wood physical properties. It plays important roles in the utilization of wood. Wood is hygroscopic, it has the ability to absorb and release moisture. The shrinkage occurs at perpendicular to grain (cross-section dimension), not at longitudinal to grain.
As a rough rule-of-thumb, wood shrinkage is about 8% tangential direction, 4% radial direction, and 0.1% longitudinal direction from fresh cut/green to ovendried condition. Higher density (heavier) wood generally shrink more than lower wood density (lighter).

Wood shrinks because it loose moisture (water content) as it dries. The shrinkage at tangential cut is bigger than shrinkage at radial cut.

A fresh cut wood from trees initially contents high moisture because a living tree moves water from root to branches and leaves. Wood adapts its environment moisture by either releasing or absorbing moisture. But the normal environment is not enough to release wood moisture into its Equilibirum Moisture Contents (EMC). Most wood manufacturers use kiln to dry the timber. It uses heat and ventilation system to control the chamber humidity to release wood moisture contents.
Sawmills use different cutting method to achieve the maximum yield of timber or to get the best quality of timber toward shrinkage.

One way to avoid cumulation shrinkage (on wood flooring for example), determine the timber position and estimate wood shrinkage direction based on cutting method.
Under certain condition, wood shrinkage will create gap between timber or pressure of deformation.


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