Industry Moves to Allay Concerns over Timber
29 April 2005
Industry Moves to Allay Concerns over Timber Strength
The public can have confidence that timber used in residential construction is sufficiently strong and fit for purpose, said John Pfahlert, Chief Executive of the NZ Building Industry Federation.
Mr Pfahlert was commenting today following the decision by the Commerce Commission to prosecute 6 Carter Holt Harvey staff over alleged breaches of the Fair Trading Act for selling reportedly under strength timber between 1999 and 2003.
“80% of timber sold in the domestic market is graded as to strength using machine stress grading equipment. The timber grading rules operated in New Zealand were based on Australian rules and contain a sufficient margin for error that even if under strength timber were incorporated into a house there is virtually nil risk of structural failure” said Mr Pfahlert.
The strength of radiata pine in New Zealand and Australia has decreased gradually over the years as the crop rotations have shortened, and timber is harvested at a younger age.
The timber industry moved in late 2003 to review some of the strength characteristics in the tables of the timber grading rules, and to develop a New Zealand Standard for verification of machine stress graded timber. Companies such as Carter Holt Harvey now have their machine stress graded timber strengths independently verified against this Standard, said Mr Pfahlert.
This move to revise the strength characteristics of timber was driven in part by the Commerce Commission enquiry, and in part by customer demand for a simplified timber grading system in New Zealand.
Standards New Zealand is currently reviewing the New Zealand Standard for construction of light timber framed houses, in particular the span tables for construction of roof trusses. It is likely that the strength requirements of the timber to be used in roof trusses will be increased to eliminate any doubt that the timber is of sufficient strength.
This does not mean that the timber currently being used is too weak, just that there is scientific argument about whether the margins for error built into the existing span tables is adequate.