Structural Timber Update
26 July 2006
There have recently been a number of changes to structural timber grading methods and grades. We are currently in a transition period between the existing system and a new one. There is currently a choice as to which system to use. This is a confusing situation for the building industry.
The timber structures standard (NZS3603) was amended and published in March 2005. This standard sets out structural timber grades and their associated structural properties. In June 2006 the amended timber framed buildings standard (NZS3604) was published to take into account the new grades in NZS3603. NZS3604 provides design solutions for timber framed buildings up to three stories high.
The amended NZS3603 introduces the following new grades:
Visually Stress Graded (VSG) 8 & 10
Machine Stress Graded (MSG) 6,8,10,12,15
G8 grade (this is a green version of a dry VSG/MSG8 grade and is verified green)
VSG8 and MSG8 have the same structural properties and there is a small difference between the VSG10 and MSG10. Therefore, technically, VSG8 can be substituted for MSG8 and VSG10 can be substituted for MSG10, although there may be a preference for one grade over the other, so any substitution should be checked with the customer/specifier.
All of the new grades (including MSG) require verification in accordance with the new verification standard (NZS3622). The verification method and pass criteria are exactly the same for both grading systems. The requirement to use a third party quality assurance organisation is mandatory.
The amended NZS3603 retains a grade called No1 Framing. This grade is visually graded as per NZS3631 but is not verified. Because this grade is not verified, its structural properties have been reduced to a conservative level. This unverified grade has the same structural properties as MSG6.
NZS3604 provides design information in the form of tables for three sets of grades. Tables are provided for:
MOE (Stiffness) 6 grades (MSG6 and Unverified No 1 Framing)
MOE (Stiffness) 8 grades (MSG8,VSG8 and green 8 grade (G8/VSG8/MSG8)
MOE (Stiffness) 10 grades (MSG10 and VSG10)
The legal situation
The Department of Building and Housing (DBH) determines what standards it cites in the Compliance Documents of the Building Code. At the moment, the existing standards are still cited in the B1 Structure Compliance Document. The DBH has proposed to cite the amended standards but this has not yet happened, even though the amended standards have been published by Standards New Zealand. The citation is supposed to occur in July 2006 and there is a proposed transition period of six months where both standards can still be used. Until an announcement is made by the DBH, the existing standards are still acceptable and the amended standards will need to be treated as an alternative solution for compliance with the building code. Standards New Zealand believe that because the amended standards are more conservative, territorial authorities will have no problem accepting the use of them.
Some are choosing to start using the amended standards now while others are choosing to continue using the existing ones. This is causing much confusion. To highlight this, No 1 Framing grade is in both standards but has different structural properties depending on what standard you choose to use. Which properties should you use?
Due to the lack of a definitive decision by the DBH, some producers have started producing the new grades while others are waiting for confirmation of the amendments citation. This means that even if you want to use the new grades, there may be difficulty in sourcing them.
Confusion will remain until such time as the DBH make an announcement. The building industry needs a decision from the DBH so that it can plan for the future. Specifiers want to know what to specify, timber merchants want to know what to stock, builders want to know what to use and sawmillers want to know what they need to produce. We await an announcement from the DBH to help set direction for the industry.