Durability of timber

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In this subject overview

This is a high-level introduction to the concepts surrounding the durability of timber and what sort of careful planning, design and construction is required to maximise longevity and sustainability in New Zealand buildings.

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Timber is used in residential, commercial, and industrial construction. The durability of timber depends on the species, construction detailing, heat & mass transfer (building physics), installation and exposure during construction, treatment, and maintenance. Ensuring the longevity and sustainability of timber structures requires careful planning, design, and construction.

Durability is the ability of a material to resist decay and degradation over time. To be durable and sustainable in buildings, materials must:

  • Maintain structural integrity and ensure safety for the designed lifetime.
  • Maintain aesthetic appeal and comfort.

Poorly executed timber construction can pose a risk of structural failure, promote mould and or rot growth, and require frequent maintenance or replacement. 

Durable timber construction will provide long-lasting, aesthetically pleasing, and eco-friendly solutions, reducing the environmental impact.

It is important to differentiate between mould and decay. Mould may be harmful to human health but does not affect structural performance, whereas fungal decay (also known as rot) reduces the structural integrity of the material itself.

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How to achieve natural durability

Achieving natural durability in timber design and construction requires careful selection, handling, and detailing of the wood components. Some species, such as Totara, Redwood, or Lawson Cypress, have inherent natural durability and are more resistant to decay and insects than others. Choosing species that have high natural or inherent durability can enhance the durability and sustainability of timber structures. However, even durable species can degrade over time if not properly handled and detailed. Species that are less durable but well-detailed and constructed are less susceptible to decay.

Intelligent detailing

Detailing for durability should apply best practice techniques in both design and construction to prevent or reduce the exposure of timber to moisture, sunlight, or other damaging factors. It is important to provide detailing that avoids excessive moisture content in timber components. Decay will not occur if the moisture content of the wood does not exceed 18% for a prolonged period. 

Detailing to prevent water infiltration includes:

  • Adequate flashings
  • Careful drainage
  • Protection of end grain exposed to the weather

Detailing to avoid condensation includes:

  • Correct placement of insulation and building envelop wrap
  • Adequate and well-located ventilation
  • Correct placement of vapour barriers (depending on the climate zone in New Zealand – NIWA identified 19 climate zones. Passive House Institute New Zealand suggests considering these when choosing the correct building physics for your actual location and build). 

Similarly, avoiding direct contact between timber and soil, concrete, or other wet materials is essential to prevent moisture uptake and fungal growth.

Timber Unlimited is presently developing an industry best practice document that will provide further guidance on moisture content management during transport, construction and building occupation phases. This is expected to be available in March 2024.

Building physics - the basic principles

Adhering to the basic principles of building physics is best practice to achieve durability in timber structures. Building physics is the science of heat and mass transfer in buildings and is an important tool for predicting how buildings behave regarding heat and moisture transfers and airflows. 

Building physics can be used to predict and avoid moisture accumulation and thermal stress by designing buildings with:

  • Adequate thermal resistance and reduction of thermal bridges
  • Reduced air leakage through the building envelope
  • Correct placement of vapour-permeable membranes
  • Controlled ventilation

Careful design of these features will lead to better indoor air quality, as the risk of mould is significantly reduced, and durability of the timber can be guaranteed. 

Understanding the environmental conditions of the site, climate, and occupancy can help designers to provide more durable and sustainable timber structures using building physics tools and resulting predictions of transient and steady-state conditions.

Surface coatings

Surface coatings are usually applied to Engineered Wood Products (EWPs) prior to leaving the manufacturing facility. The purpose of the coatings is to provide temporary protection for the members for a limited period (typically 6 – 10 weeks) by repelling moisture and surface water. Limiting free water on timber elements during on-site storage and erection is important as it assists with maintaining the moisture content of the timber at a level below that which is required to support mould growth (typically 19%).

Coatings are typically grouped under one of two categories:

  • Penetrating sealers that are non-film forming and penetrate to a limited depth into wood fibres below the surface.  These can be either clear in finish or inclusive of a stain. They are effective at repelling moisture from the timber surface layers for a limited period.
  • Film-forming sealers that effectively repel moisture via a continuous thin film formed on the coated surfaces of the timber members.

It's important to recognise that both categories of coatings are temporary, and whilst they can assist with controlling the moisture content of timber during storage and erection on-site, they are not a long-term solution and should be used in conjunction with other construction management measures such as adhesive films, plastic wraps and physical protection from the elements such temporary site tents.

Care should also be taken to ensure that the temporary sealer coating used is compatible with final finishing regimes that may specified for the timber members.

New Zealand building code compliance for durability

Building consent authorities (BCAs) are responsible for enforcing the building code and ensuring the durability and safety of buildings. Failing to comply with the building code can result in fines, delays, or even legal liabilities.

BCAs will request evidence of compliance with the performance requirements of Section B2 of the New Zealand Building Code (NZBC) when issuing a building consent. In broad terms, meeting the building code durability requirements can be achieved either through adhering to the minimum requirements of Clause B2 / Acceptable Solution 1 (B2/AS1) or through the development and provision of an Alternative Solution.

Compliance with Functional Requirements B2.2 Building Materials and Performance, B2.3.1 Building elements can be demonstrated via B2/AS1 acceptable solutions or B2/VM1. 

Acceptable Solutions in accordance with Clause B2 can be achieved by conforming with the requirements of Standard NZS 3602 Part 1 (as modified by paragraph 3.2.2 of B2 Durability), NZS 3640 (as modified by paragraph 3.2.3 of B2 Durability) or through Standard NZS 3604 with reference to NZS 3602 (and NZS 3640). 

In cases where durability demonstration is outside the scope of an Acceptable Solution, it may be possible to demonstrate compliance via verification method B2/VM1. Proof of performance under B2/VM1 through consideration of the expected in-service conditions includes service history, laboratory testing and comparable performance of similar buildings. If neither of the above is applicable to the building or parts of the building, an alternative compliance can be achieved by the means of an Alternative Solution. This includes evidence such as certification from a registered treatment provider, a building physics study, product labelling, or inspection reports. It is up to the Building Consent Authority (BCA) to request a peer review or to accept the evidence provided as sufficient to demonstrate compliance. A first, pre-application meeting is, therefore, often very helpful to discuss and agree upon the durability path of compliance and expectations of the BCAs.

Further compliance information