Selecting and using timber

Timber is the main structural material used in houses, and regularly forms the structural frame in low-rise buildings. The natural biodegrading progress of wood affects its durability and, while some species are naturally more durable, introduced softwoods are more available and can be chemically treated with preservatives to enhance their durability when used as building timber.

Underpinning all initiatives for weathertightness is the need for framing timber to have sufficient durability to be able to withstand a period of minor leaking without rotting. This durability requirement for framing is to allow sufficient time for a leaking problem to be detected and repairs carried out without the building suffering structural damage from rotting.

In addition to durability issues, timber is naturally hygroscopic, so that even after is has ‘dried’ it is liable to absorb moisture (and consequently swell) or lose moisture (and consequently shrink) in relation to its immediate atmosphere or environment. This increases the risk of cracking at joints, moisture penetration and retention that can lead to decay.

Timber durability

The Standard covering durability of timber framing was amended in late 2003 with the publication of NZS 3602: 2003 Timber and Wood-based Products for Use in Building (available from Standards New Zealand ), which gives the requirements for certain timbers and wood-based products, in defined potential risk locations within a building, to achieve a stated durability.

The Standard was cited in April 2004 under Acceptable Solution B2/AS1 for Clause B2 Durability of the New Zealand Building Code, which requires structural components to last the life of the building (being not less than 50 years). Non-structural building components and systems must last a minimum of 15 and 5 years, depending on their location, ease of replacement and under what circumstances a failure would be detected.

Timber Treatment: Builders' Notes - February 2005

Timber Treatment Requirements: Notes for Builders is designed as a quick guide to the general use of treated radiata pine and Douglas fir in buildings where buildings have been designed to the provisions of the Standard for Timber and Wood-based Products for Use in Building NZS 3602: 2003 (cited in B2/AS1).

This is an abridged outline to the full standard NZS 3602, and is to be used for guidance only. Anyone who produces, sells or specifies timber should obtain full copies of the relevant New Zealand Standards.

The booklet helps builders understand what is the right level of treated timber in the most common situations, and includes how to identify timber treatments and which type of treated timber needs to be used where.

There is also important information on the safe handling of treated timber, as potential health risks are associated with some timber treatment preservatives.