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 Natural Durability

Natural durability is a feature some specialty timbers are endowed with. This can be variable both within and between species. Only heartwood can be naturally durable. Age of the tree and where the wood comes from within the tree influence durability. Wood from nearer the centre of the tree tends to be less durable than outer heartwood.

Species Heartwood durability rating*The natural resistance to attack to any species of timber by wood destroying fungi and wood destroying insects is variable. The sapwood of all timber species has poor resistance and so natural durability applies only to the heartwood of a timber species.
Totara Very durable
Silver beech Moderately durable*Non-durable in ground contact
Red beech Durable
Hard and Black beech Durable
Eucalyptus muelleriana Durable
Eucalyptus pilularis Durable
Eucalypus globoidea Durable
Eucalyptus microcorys Very durable
Eucalyptus saligna Moderately durable
Eucalyptus botryoides Durable
Eucalyptus obliqua Moderately durable*Natural durability variable
Macrocarpa Moderately durable
Redwood Moderately durable*Natural durability highly variable
Cedar Moderately durable
Hazard classes
H1.1 Timber used in situations protected from the weather, dry in service and where resistance to borer only is required.
H1.2 Timber used in situations protected from the weather but where there is a risk of moisture exposure conducive to decay.
H3.1 Timber used outdoors above ground, exposed to the weather – generally in non-structural applications; i.e. fascia boards, weatherboards.
H3.2 Timber used outdoors above ground, exposed to weather or protected from the weather but with a risk of water entrapment; i.e. structural applications, decking, fencing and pergolas.

NZS 3602 Timber and Wood-based products for use in building

To be recognised under Acceptable Solutions within the context of the NZ Building Code, naturally durable timbers must comply with NZS3602:2003 Timber and wood-based products for use in building. Durability performance requirements under NZS 3602 are listed in table 1 and table 2. These tables list various wood-based building components and the level of treatment for the Hazard Class, to comply with the building code. Some timbers do not require treatment depending on their level of natural durability. Some components have a requirement to achieve a durability performance of 50 years (e.g. structural timber) while other components are required to achieve a durability performance of 15 years (e.g. decking, weatherboards).

50-year durability performance for structural timber inside the building envelope

Currently cypress and macrocarpa heartwood is listed as meeting durability performance requirements for structural uses within the building envelope under NZS 3602. Unfortunately, other well known naturally durable timber species such as totara, eucalypt, Southern beech, redwood and Japanese cedar are overlooked in NZS 3602 table 1 and therefore are not code-compliant for structural use. By disregarding these species the prescriptive scope of this standard is unnecessarily restrictive in its "requirements to achieve a 50 year durability performance". 

A range of further inconsistencies and omissions in NZS 3602:2003 work against code-compliance for naturally durable timber species. Currently radiata pine LVL requires no treatment for structural elements in dry conditions within the building envelope, yet naturally durable timbers of many species by omission are not allowed for the same internal structural purposes. Furthermore radiata pine treated to H1.2 boron (0.4 % BAE) is deemed adequate for structural situations within the building envelope, yet this same level of treatment level is not allowed for most other species simply because they are not listed. There needs to be inclusion rather than prescriptive restriction, along with recognition of natural durability as a quality. Farm Forestry Timbers are working hard for a revision of the standard.

Such restrictive treatment and prescription-based conditions of compliance have led to disputes between those practicing reasonable use of naturally durable timber and building consent authorities. This has led to several Department of Building and Housing Determinations. The result is that when installed to the exterior of buildings, cypress and macrocarpa structural elements such as posts and beams can be code-compliant, provided :

  • they are heart timbers
  • they are not in contact with the ground
  • they are relatively easy to replace
  • their surfaces are treated with an appropriate preservative
  • their cut ends are similarly treated or suitably capped.

Components requiring a 15 year durability performance

Currently heartwood from cypress species, larch, redwood and Western Red Cedar can be used for cladding (weatherboards) under NZS 3602. 

Exterior joinery including window frames, sills, sashes, exterior door frames, sills and doors and reveals for aluminium windows can be made from redwood, western red cedar or cypress heartwood.

External stairs, stair handrails and balustrades, verandah doors and decking can be in eucalyptus, cypress, silver, red or hard beech heartwood.

For more on building components see NZS3602:2003 (tables 1 and 2).

Disclaimer: While every effort is made to ensure the accuracy of the information provided on this site, Farm Forestry Timbers Society do not accept liability for any consequences arising from reliance on the information published. If readers have any doubts about acting on any articles they should seek confirming, professional advice.

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