Timber materials will have varying degrees of natural durability and this depends on the species.

 

Decay of timber will generally occur where the moisture content is above 22% for a prolonged period of time.

 

The natural durability of timber is defined as the inborn resistance of a specific timber to fungal decay or insect attack.

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Timbers are often grouped into five durability categories, Perishable, Non-Durable, Moderately Durable, Durable, and Very Durable.

 

• Perishable and Non-Durable are not suited for exterior use as they would break down with the elements.

• Moderately Durable can be used externally but must be protected from the elements by coatings (e.g paint) and above ground.

• Durable and Very Durable are suited for external use but may vary from species on their in-ground durability.

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In terms of longevity :-

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• Perishable - less than 5 years when in contact with the ground;

• Non-Durable - 5-10 years when in contact with the ground;

• Moderately Durable - 10-15 years when in contact with the ground;

• Durable - 15-25 years when in contact with the ground;

• Very Durable - More than 25 years when in contact with the ground.

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Where durability against the risk of decay or insect attack is required for a non-durable species, chemical preservative treatments may be applied.

 

Preservatives are often used as a second line ‘insurance’ against design or construction failures that could lead to moisture contents above 22%.

 

Examples where preservative treatment of timber is required are :-

 

• roof timbers,

• flat roof joists and tiling battens exposed to risk of condensation,

• sole-plates above damp proof course level,

• external wall studs.

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