Durability and the control of moisture change in timber structures

If a structure is to be durable, the design of all timber joints and surfaces needs to recognise the factors that influence the moisture content of timber and changes that occur due to variations in that moisture content. Controlling the moisture content of timber is crucial as:

• High moisture content can lead to decay
• Varying moisture content leads to timber shrinking or swelling
• The physical properties of timber vary with its moisture content.

Architectural and structural detailing and specification are critical to the durability of timber structures. Some of the key factors that must be considered are:

• Shielding
• Isolation
• Moisture traps and contact areas
• Ventilation
• Condensation
• Avoiding restraint due to shrinkage
• Use of corrosion resistant metals
• Allowance for shrinkage or differential movement
• Use of correct fasteners and installation techniques

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