Building Code requirements

The Building Code clause B2 Durability sets out minimum durability requirements for building materials and components. The expected life depends on the location of the material or element in the building and its function.

Generally, timber components must have a minimum durability of not less than:

• 5 years for components that are easy to access and replace, such as interior finishing timber
• 15 years for components that are moderately difficult to replace, such as cladding, exterior trim and exterior joinery
• the life of the building, but not less than 50 years, for structural components such as bearers, joists and studs.

Where timber may be subjected to moisture in use or is used externally, it must be:

• a naturally durable species, or
• treated with a wood preservative to make it resistant to decay fungi or wood-boring insects (borer) and render it sufficiently durable.

Untreated non-durable timber at less than 18% moisture content and protected from wetting is not particularly susceptible to borer attack or fungal decay but is not permitted for use as framing (the exception being Douglas fir used in low-risk building and meeting the requirements of B2/AS1 paragraph 3.2.2.2).

Structural timber such as these studs must typically have a 50-year durability.

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Timber use and preservative treatment standards

The requirements for timber use in buildings are defined by NZS 3602:2003 Timber and wood-based products for use in building. This standard also specifies the minimum preservative treatment levels for given end uses. Using timbers or treatments not referred to in NZS 3602:2003 is outside the scope of Acceptable Solution B2/AS1, so evidence of compliance with the Building Code would be to be provided through a different path.

The requirements of specific treatment regimes are contained in NZS 3640:2003 Chemical preservation of round and sawn timber. It contains detailed treatment specifications, the types of chemicals that may be used, the minimum preservative retention and penetration into the timber, identification of treated timber and quality control requirements.

The Ministry of Business, Innovation and Employment has sponsored free access to both of these standards. You can download NZS 3602:2003 here and NZS 3640:2003 here.

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Hazard classes

When specifying timber treatment, the most important requirement is that an appropriate treatment level is specified for the particular situation. This can be done by reference to the appropriate hazard class as defined by NZS 3640 and called up in NZS 3602 Tables 1, 2 or 3 (as modified by B2/AS1 Amendment 7).

Table 1 summarises the treatment options, and Table 2 gives the permitted end uses, primarily for radiata pine.

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Untreated timber usage

B2/AS1 includes an option for people who want to minimise use of chemically treated timber in their home. Provided certain conditions (given in 3.2.2.2 of the Acceptable Solution) are all met, untreated solid Douglas fir framing can be used for:

• roof members protected from the weather
• floor members protected from the weather and not exposed to ground atmosphere
• internal and external wall framing protected from the weather.

Untreated radiata pine and Douglas fir can be used for framing in unlined farm buildings where framing is protected from direct wetting, not in ground contact and the building is not used for living accommodation.

For some applications, NZS 3602:2003 lists a limited number of species that are considered durable enough to meet the requirements of the Building Code without the need for treatment.

Examples of timber in which the heartwood may be used untreated are:

• externally (above ground with a not less than 15-year durability) – heartwood of larch, macrocarpa, redwood, western red cedar, vitex, kwila, selected eucalypts, beech, rimu
• internally – Douglas fir (see above), European larch, radiata pine (limited applications), heartwood of macrocarpa, rimu, matai, tawa, beech, selected eucalypts.

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Preservative treatment options

Table 3 summarises the preservative type, the hazard class that can be achieved, the identification numbers and colours.