TREATING A CUT THE RIGHT WAY

We’re not talking plasters or bandages – this article looks at how to achieve the best finish on cut timber

The end grain of cut timber is a vulnerable spot, where water can be more readily absorbed and rot can start if the timber remains wet. When you’re on site and making cuts to painted or stained timber that will be exposed to moisture – such as weatherboards, bargeboards and fascias – treating the cut the right way is crucial.

Under the New Zealand Building Code, claddings must be durable for at least 15 years with normal maintenance. Homeowners will naturally expect much longer than that.

NZS 3602:2003 Timber and wood-based products for use in building requires radiata pine weatherboards, external fascias and trims to be treated to hazard class H3.1 as a minimum, and protected with three coats of alkyd or acrylic paint.

These H3.1 treated timber products usually arrive on site pre-primed on all surfaces. After they are cut to length, cut ends must always be reprimed, preferably with two coating applications because of the greater porosity of timber end grain. Repriming after cutting is a requirement of NZS 3602:2003 (and also Acceptable Solution E2/AS1). Notches and holes cut in the timber should also be fully sealed.

Because primers have a short life, primed weatherboards not immediately installed may need repriming – check the supplier’s installation instructions for the maximum time primed surfaces can be exposed to the weather.

While sealing cut ends is especially important for H3.1 timber weatherboards and trims, it also applies to weatherboards made from H3.2 treated timber or cedar that are painted or stained, fibre-cement, compressed hardwood fibres and some other materials. Check the manufacturer’s requirements – they typically require cut ends to be sealed with an acrylic sealer. Some proprietary products (including H3.1 radiata pine products) require a proprietary sealant to be used on cut ends.

There are also treatment requirements for other types of cladding cut on site. For example, E2/AS1 requires that:

• Where fibre-cement sheet claddings are cut on site, cut edges are sealed with paint including 100mm across the back face from each edge.
• Where plywood sheet claddings are cut after treatment, the cut edges are brushed with a solution of 12.5% copper naphthenate in white spirits or mineral turpentine.

H5 TREATED TIMBER PILES

Timber piles must be durable for at least 50 years under the Building Code. Where piles are cut on site, whether cutting to length or notching and rebating, the cut ends must always be treated. The cut surface should be dry to the touch and then have a liberal application of timber treatment brushed on. Manufacturers will recommend treatment in their documentation, but it will usually be something like zinc naphthenate, TBTO or TBTN.

The cut ends of piles must not be placed in the ground. Some manufacturers go further and specify that cut ends should not be closer than 150mm or 300mm to the ground.

Product warranties may no longer apply if the requirements around treating cut ends are not followed.

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