Fig 1 Preservative treatment

Is Preservative Treatment Necessary?

When deciding whether to specify preservative treatment, the following factors must be considered:

• The presence of hazards (moisture, insect, decay, fire, etc.)
• The degree of structural reliability required (is the system load bearing or non-loadbearing, the cost of failure, the potential for death or injury resulting from failure?)
• The desired or expected service life of the structure
• The natural durability of the timber (resistance to decay or insect attack)
• The type or design of the building or component
• The presence of sapwood (generally, only sapwood can be effectively treated unless the timber is mechanically incised)

• The sapwood of some species cannot be effectively treated, e.g. cypress pine and Douglas fir.
• Plantation softwoods and some hardwood species have wide sapwood bands and are ideal for preservative treatment, particularly in round form

Figure 1 provides guidance in determining if preservative treatment is required for permanent structures. Professional judgement is required as other factors such as the design life of the structure and specific local hazards may also need consideration.

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