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 Structural Timber and the Building Code

All new building work in New Zealand must comply with the NZ Building Code. This page looks at the structural requirements for compliance. For durability click here ».

A design based on New Zealand Standards, as referenced in the Compliance provisions of the NZ Building Code, must be accepted by building consent authorities as complying with the Building Code.

Standards and Structural Requirements of the building code

Note: Farm Forestry Timbers have determined the Characteristic Stresses of our No. 1 Structural grade for the following species:

With our grades, these species can be used for structural applications under the building code using NZS 3603, by involving an engineer in the design. Look for an engineer in the Marketplace

Please acknowledge the contributions made by our producers that allowed these stresses to be available, by considering their products or services:

These Standards provide Acceptable Solutions and Verification methods as referenced in the Compliance provisions of the NZ building code. Acceptable Solutions mean that a building will automatically comply with the Building Code.  Verification Methods provide means of testing so that a building complies with the Building Code.

NZS 3604 Timber Framed Buildings

NZS 3604 sets a minimum standard for the design and construction of timber-framed buildings. NZS 3604 provides methods and details for the design and construction of timber-framed structures not requiring specific engineering design. NZS 3604 is a core resource for builders and building consent authorities determining compliance with the New Zealand Building Code.

Although NZS 3604 applies to only radiata pine and Douglas fir, the standards provisions may be applicable to other timbers, provided adequate structural performance and durability can be demonstrated.

NZS 3604 also states that properties for timber grades are as specified in NZS 3603.

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NZS 3603 Timber Structures

NZS 3603 contains design stresses and methods for specific engineering design of timber structures to meet the performance requirements of the New Zealand Building Code.

NZS 3603 Timber structures Standard statesclause 2.3 that timber of species and grades not listedTables 2.2 and 2.3 are only for radiata pine and Douglas fir may be assigned characteristic stressesFor strength properties, characteristic stress or strength is an estimate of the lower 5-percentile value determined with 75% confidence, from tests on a representative sample of full size test specimens. For stiffness properties, the characteristic value is the mean value..

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NZS 3602 Timber and wood based products for buildings

NZS 3602 provides requirements for wood-based building components to achieve durability performance. For example this standard allows untreated Cypress timber to be used for structural members protected from the weather.

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NZS 3622 Verification of Timber Properties

Both VSGVSG (Visual Stress Grades) are sorted on the basis of visual characteristics. Visual characteristics are good for determining strength but not so good for predicting stiffness. In the case of visual stress grades, the verification process is more pertinent for checking the stiffness characteristics. and MSGA machine stress grader is only capable of testing the stiffness of timber but not strength. There is correlation between stiffness and strength but visual characteristics such as knots are the main determinant of strength. Therefore visual grading is also undertaken after the wood has been machine graded for stiffness. In order to determine whether the strength properties for machine stress graded product are being achieved, samples of the production must be independently tested on a static testing machine in accordance with the Verification Standard NZS 3622:2004. This tests for both stiffness and also bending strength. grading systems assign timber into Structural Grades (SG) according to their stiffness and strength.

Both grading systems are subject to the same rules and grade acceptance criteria as set out in the Verification Standard NZS 3622:2004.

The verification process described in NZS 3622:2004 is an output control system, which means that timber is continuously taken from production and tested for stiffness and bending strength by the producer. Independent auditing also takes place.

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Determining Characteristic Stresses of specialty timber species

Farm Forestry Timbers have determined the Characteristic Stresses for a range of timber species for structural use in New Zealand. The visual grades and stresses can be found here.

Determining the Characteristic Stresses of a timber species allows engineers to design timber structures with that species*(Provided that the timber also meets the durability performance requirements of NZS 3602). Both NZS 3603 and NZS 3622 allow for determining characteristic stresses for grades of any timber species, by testing a reference sample in accordance with AS/NZS 4063.1:2010.

The properties that are evaluated for the reference sample include:

  • Bending strength;
  • Modulus of elasticity;
  • Strength in tension; and
  • Strength in compression

The initial evaluation shall:

  1. Identify the reference sample;
  2. Identify the sorting procedure;
  3. Identify the grades and sizes to which the procedure relates;
  4. Evaluate the properties of the sample according to the requirements of AS/NZS 4063 using appropriately calibrated equipment and appropriately trained staff under the supervision of an independent suitably qualified organisation;
  5. State or derive target bending strength and modulus of elasticity values for the grade/size combinations that the producer proposes supplying;
  6. State or derive machine threshold values and visual characteristics where the grading method is machine grading;
  7. State or define visual characteristics where the grading method is visual grading; and
  8. Document the evaluation in a report, copies of which shall be held by the audit organisation and the producer.

A 2011 Scion quote for this work is here. The cost per species at that time was around $4200 + gst.  

Alternative solutions

Where the design is not able to meet the requirements of NZS 3603 or NZS 3602 the building code allows for whats called an "Alternative Solution". This also uses specific engineering design but must provide further evidence that the design will comply with the durability and strength requirements of the building code. more »


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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