Fasteners for CCA-Treated Timber
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Bright steel nails
reacting with the timber and causing black stains |
It is most important that the correct choice of material is made in nailing and fixing CCA-treated timber.
The nails and fasteners should be compatible with the treatment type and method of connection required.
The most common material used in nail manufacture is cold drawn, low carbon steel. Other materials include aluminium, copper, brass, silicon-bronze, and stainless steel. Aluminium nails are used for fixing aluminium sheeting and fittings and the others are used in boat building and where especially corrosive environments exist e.g., swimming pools.
In exterior applications, the metal in some nails may react with extractives in the wood and form stains; e.g., uncoated steel nails can cause black stains. The solution to this problem is to use galvanised or other nail types.
Various special coatings and treatments also increase nail-holding power and provide corrosion resistance as well as improving appearance. The treatments include:
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Hot dipped galvanised nails for
fixing CCA-treated decking |
- electro-plating
- polymer and other coatings
- hot dipped galvanising
Nickel and cadmium are the most commonly used electro-plating treatments for nails. They mostly improve appearance but also provide slightly improved corrosion resistance. Electroplated finishes applied to screws include zinc, zinc-chromate, cadmium, nickel and chromium. Again, they improve appearance more than corrosion resistance.
Manufacturers also coat nails (primarily for nail-gun application) with plastics that are claimed to improve both insertion and withdrawal resistance. In addition to plastics coating, a process called phoscoating is also used, in which the nail surface is etched with an iron phosphate coating. This roughens the surface and enhances withdrawal resistance.
For higher corrosion resistance in marine, salt or chemical environments hot dipped galvanized nails and screws should be used. Hot dipped galvanising involves the application of a relatively thick sacrificial zinc coating by hot dipping in a zinc bath. The process leaves a rough surface with enhanced withdrawal and corrosion resistance characteristics.
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Hot dipped galvanised
bolts used in wharf construction |
Hot dipped galvanised nails, bolts and coach screws should be used in corrosive environments, e.g. swimming pool structures, marine structures, some farm buildings such as piggeries, and CCA-treated timber used for exterior applications.
The relatively harsh environment to fixings of weather exposure combined with CCA chemicals can lead to electrolytic cells being established in the timber structure. Thus, where the treated timber moisture content will climb above about 18%, corrosive conditions can occur. Under these circumstances hot dipped galvanised fittings are recommended. Where the moisture content will be low, and where local conditions are not conducive of corrosion, bright steel fittings may be used. Note that in the immediate vicinity of the sea-coast, where air-borne salt spray represents a very severe hazard to metal fittings and fixings only hot dipped galavanised materials are recommended.
Further information may be obtained from: