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Fig. 1 Deformation of timber sections

Moisture movement and shrinkage

As wood dries below its fibre-saturation point, it shrinks. However, the loss in dimension is not the same in all directions. Timber shrinks in three directions:

Longitudinally, along the grain of the timber
Radially, across the growth rings, and
Tangentially, along the line of the growth rings..

The longitudinal shrinkage is small at about 1%. While discounted in milling processes, it can be an important consideration in construction with unseasoned timber. In the radial direction, depending on the species, shrinkage is around 3% to 6% when drying down from fibre saturation point (about 25 – 30% moisture) to 12% moisture content. In the tangential direction, the shrinkage is generally about twice this amount, 6% to 12% over the same moisture range.

These differential rates of shrinkage have considerable effect on methods of milling and the performance of timber in service, especially when unseasoned timber is used and allowed to dry in service. Considerable deformation can occurs. The effect of shrinkage on timber sections cut from various sections of a log is shown in Fig. 1.

If seasoned timber is used, the direction of cut is usually not as important as subsequent changes in dimension, or ‘movement’, are related to changes in the piece’s equilibrium moisture content due to environmental changes.

Reference:

NAFI Timber Manual: Timber Datafile P1 – Timber Species and Properties.