Reproduced from Codewords 39, August 2009, Department of Building and Housing.

In the past there have been timber fastener durability concerns with the use of new high copper treatment options as an alternative to the traditionally used CCA treatment (refer to Build magazine February/March 2007). In that article it was recommended to use stainless steel fasteners or durable equivalents such as silicon bronze, when using these new treatments in timber exposed to the weather.

This led to the Department co-sponsoring further research. BRANZ is now publishing details of this 3-year study confirming that significant corrosion rates are being experienced by both mild steel and galvanised nails and screws used as fasteners in Copper Azole (CuAz) or alkaline copper quaternary (ACQ) treated timber (refer to Build magazine article August/September 2009).

Market analysis shows that only a very small amount of these high copper treatments have been used to date. A review of NZS 3604, Timber-framed buildings, is under way and this research will be used to inform any amendments made to this Standard.

Identification of CuAz and ACQ treated timber

See Figure 1, or view a larger version of the image [38 kB JPG].

Timber treated with Copper Azole (CuAz) or alkaline copper quaternary (ACQ) will be marked in accordance with the requirements of NZS 3640. This Standard requires the timber to be identified – either on one end of each piece, or at 1500 mm centres along the length – by marking the treatment plant number, the preservative number, and the hazard class number.

Recommendation

We recommend the use of 304/316 grades of stainless steel or durable equivalents such as silicon bronze for all timber fasteners, including nail plates, bolts and nails, for all CuAz or ACQ treated timber exposed to the weather.

The research confirms the corrosion concerns that initiated the study. The Department's recommendation endorses that made by BRANZ in the Build magazine February/March 2007 edition and confirmed in the recent Build publication.

The department has checked, as far as possible, to confirm that these treatments have not been used to date in locations that might cause danger to the public. If owners know these treatments have been used, they should check critical connections for signs of corrosion. It is likely most have been used as decking timber and corrosion will be readily apparent.

For further advice, please contact the Department of Building and Housing on 0800 242 243.

→ Buy NZS 3640:2003 Chemical preservation of round and sawn timber

→ Buy NZS 3602:2003 Timber and wood-based products for use in building