Technical information builder inspectors should know

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The timber-grading process can be summarised as:

• To meet the performance requirements of the Building Code, timber used for structural purposes on site needs to have the structural properties that have been assumed by the designer in the design of the building.
• There are three main classes of timber grading: 
• machine stress graded and verified (MSG)
• visually stress-graded and verified (VSG)
• visually graded and unverified (No. 1 Framing Grade).
• In the Building Code supporting documents these are collectively referred to as Structural Grade (SG).
• MSG and VSG may be SG6, SG8 or SG10. Unverified No.1 Framing Grade is SG6.
• Structural timber used in frame and truss must be graded in one of three ways: 
• machine stress-graded and verified
• visually stress graded and verified
• visually graded and unverified (No. 1 Framing Grade).
• To be graded 'SG', timber is passed through a stress-grading machine to determine the strength (point at which the timber breaks) and stiffness (the amount the timber will bend under a given load ie flexibility). These results are then verified by physically testing samples on a statistical basis to ensure the stress-grading machine is calibrated correctly. The whole process must be independently audited.
• Verified timber is visually graded in accordance with well-established rules that will limit the physical defects in the timber, such as knots, splits, etc. This is verified on a sampled basis by physically testing the strength and stiffness of each test sample. This ensures the visually graded timber will have adequate structural properties to meet the grading requirements. This process must also be independently audited.
• SG6 timber is visually graded according to the same visual grading rules as VSG. However, there is no subsequent verification or audit process. The structural design properties of this timber are limited to the lowest verified grade SG6.

We are aware of different approaches for non load-bearing timber. One of these we consider appropriate involves the following steps:

• removing defects and low grade sections of the timber
• re-stress grading through the stress grade machines
• verification as required by NZS 3622
• independent auditing of the process by independent verifiers.

BCA technical staff should also be familiar with the following publications which provide more information and background to the processes described above. Standards have been amended since the issue first arose.

Standards New Zealand can supply more details.

• NZS 3622 (2004): Verification of Timber Properties
• NZS 3631 (1988): New Zealand Timber Grading Rules
• NZS 3604 (2011): Timber Framed Buildings
• NZS 3603 (1993): Timber Structures
• Carter Holt Harvey bulletin - provides guidance for the identification of possible non load-bearing timber.

BCAs should ensure all inspectors sent on site are competent and are not working outside their technical limitations (regulation 9 of the Building (Accreditation of Building Consent Authorities) Regulations 2006 requires the allocation of work to competent employees or contractors).

If an inspector is in doubt, they should consult with their technical leaders or a recognised timber and Building Code Clause B1 specialist within their BCA, or a neighbouring BCA.