8 May 2006

New Zealand's main Standard for the design and construction of timber framed buildings, NZS 3604:1999, is being amended.  The Standard provides practical guidance to designers and builders and the broader building industry on meeting the requirements of the Building Code.  The amendment will result in better quality houses and increased consumer confidence in timber houses.  It will also ensure the Standard is consistent with the new timber properties and grades introduced in 2005 by Amendment 4 to the Timber Structures Standard, NZS 3603:1993.

"Amendment 2 to NZS 3604 updates the Standard with the new verified grades and engineering properties defined in Amendment 4 to NZS 3603," says Ian Garrett from Spencer Holmes Ltd, a member of the Institute of Professional Engineers NZ.  "The Timber Structures Standard (NZS 3603 Amendments 1 and 2) is cited in the Department of Building and Housing Compliance Document, which is used by engineers to design timber structures, and which forms the basis for the design solutions given in NZS 3604," says Ian, who is also a user of the Standard.

Amendment 2 to NZS 3604 incorporates the new verified timber grades and amended engineering properties into the design tables by providing new tables for those grades likely to be most commonly used.  The amendment also revises the existing tables for No. 1 framing to be consistent with the new values given for visually graded unverified timber.

Amendment 4 to NZS 3603 introduced new grades of timber.  An associated Verification Standard (NZS 3622:2004), provides sawmills with a means of verifying the actual properties of the timber.  The new properties given unverified timber reflect a change in the profile of timber properties of current timber production.

These changes were required as research showed that some of the timber being milled today is not as strong as stiff as that of past decades.  Factors such as forests being grown on fertile ex-farm sites and forests being milled at a younger age are though to be the main reasons for this change.  The changes will place timber in the same position as other construction materials, such as steel and concrete, where engineering properties are consistently verified.  This will provide more certainty about the engineering properties of timber and should contribute to improving the image of New Zealand timber in export markets.

"The new provisions relating to the verification of timber in NZS 3603 will provide certainty to users of the Standard that key design properties are being achieved and that the resulting buildings comply with the Building Code," says Dennis Monastra, Senior Technical Adviser, Regulatory at the Department of Building and Housing and a member of the committee reviewing NZS 3604.

The amendments to NZS 3603 and NZS 3604:

• Address industry concerns that sole reliance on visual grading is not a reliable means for establishing timber engineering properties.
• Provide the ability to recognise, through verification, the performance of higher grades of timber.
• Define and require verification of engineering properties for new visual grades VSG8 and VSG10 for dry timber and G8 for green timber.
• Provide the engineering properties for new machine stress grades (called ‘MSG’ grades), which have had their engineering properties verified.
• Retain the option to use the existing visually graded and unverified No.1 framing, but provide new design tables for No. 1 and downgrade its engineering properties.
• Give design information, including tables, in terms of the actual minimum dried size to be used instead of the timber’s nominal size.  This change will make matters simpler for users of the Standard and will align the sizes with those used in Australia.
• Specify that where timber is verified, the verification meets with the provisions of the Verification Standard (NZS 3622:2004).
• Require the use of a lower bound modulus of elasticity (Elb)for members that do not act as part of a group of four or more members.

Subject to analysis of public comment and a suitable introductory period, both Amendment 2 to NZS 3604 and Amendment 4 to NZS 3603 are intended to be cited by the Department of Building and Housing in the Compliance Document B1 Structure to the New Zealand Building Code.