Fig 1 Detailing to allow shrinkage

Design for durability - Shrinkage Restraint

Shrinkage restraint at joints needs careful consideration, particularly where unseasoned timber is used. If shrinkage restraint occurs, stresses may be induced perpendicular to the grain, causing splitting and subsequent moisture ingress or loss of structural integrity.

To avoid shrinkage restraint, the following guidelines should be followed:

• Minimise moisture content changes; use appropriate finishes and end-grain sealant
• Align fasteners along the member axis
• Use a single fastener in a joint
• Use connections that allow for movement
• Use seasoned timber
• Drill holes 10% oversize in unseasoned timber

Figure 1 illustrates details which allow shrinkage without inducing excessive stress.

Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual.

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Timber Research Unit
Department of Architecture
University of Tasmania