Fasteners and treated wood

The choice of an appropriate fastener material and type needs to be considered in line with the structural and performance requirements for where and how the timber product is used in a building.

Table: Fasteners and treated wood - Printable PDF

Corrosion by an electrochemical process will occur when susceptible metals are in contact with wood in the presence of moisture. The corrosion risk is dependent on factors such as the wood species and type of preservative treatment and concentration. The rate of corrosion is influenced by factors like the conductivity of the wood and the length of exposure. 

If wood is dry, i.e. <20% moisture content, then the corrosion rate is very low. When the moisture content is > 20%, the corrosion rate for CCA-treated timber in contact with mild steel as an example, is double that of untreated timber. Timber should also be ‘dry’ to avoid the potential corrosive effects on some metals where there is a high concentration of copper from the preservative.

Generally hot dipped galvanised fasteners are required for copper-based preservatives. Note: electroplated galvanised fasteners can be expected to have a lesser life than hot dip galvanised fasteners.  

Laboratory tests of alkaline copper quaternary and copper azole treatments which have higher copper concentrations in the timber than CCA, show results in higher corrosion rates of  mild steel than the rates for CCA treatment. However hot dip galvanised and stainless steel fasteners and fixings can be used.

When the end-use environment means damp wood or exposure is within a salt spray zone, then stainless steel fittings are required. This is particularly relevant to treated timber used in maritime locations where sea spray contributes significantly to the atmospheric environment. The 316 stainless steel is the best option when using a high concentration copper treatment. Requirements for metal fastenings used in these locations are covered in NZS 3604.

When using preserved wood with copper-based preservatives in close proximity to aluminium products, such as aluminium siding and  door and window frames, a 6mm minimum space must be allowed for between the treated wood and the aluminium products. Direct contact of alkaline quaternary ammonium, copper azole  or CCA treated wood with aluminium is not recommended and should be avoided.  

Polyethylene or nylon spacers can be used to maintain the 6mm spacing.  Another option is to use a polyethylene barrier, with a minimum thickness of 250 microns, between the treated wood  and the aluminium product to prevent direct contact of the wood and the aluminium.

Further Reading

Baker, A. J., 1987. ‘Corrosion of metals in preservative-treated wood’. In Hamel, Margaret (ed) Wood Protection Techniques and the Use of Treated Wood in Construction: Proceedings 47358. Memphis, TN: Wood Products Research Society: 99-101.

Molly, D. 2005. ‘Exposed sub-floors galvanised bolts? Not any more’. February/March 2005, pp 26-27.