Treating timber on site: making sure your timber work is going the distance
 

Timber is a popular choice for cladding, fences and decking for its durability, natural beauty, sustainability and generally lower upfront costs.

To ensure timber performs well over time, and doesn’t end up costing homeowners more in the long run, it’s essential that it’s treated and finished properly.

Under the New Zealand Building Code, cladding must last for at least 15 years with normal maintenance – and homeowners will often expect much longer than that. One area that often gets overlooked is the treatment of cut ends, which can significantly impact performance if they’re not dealt with correctly.

Treated timber products when cut on site, must have cut ends coated with a proprietary timber preservative to reinstate the treatment envelope. Any notches and holes should also be sealed. Repriming after cutting is a requirement of NZS 3602:2003 (Timber and Wood-based Products for use in Building) and also Acceptable Solution E2/AS1.

Exposed weatherboards, external fascias and trims should comply with at least H3.1 standards, but the cut end rule applies even when timber has been treated to the higher H3.2 standards, as well as fibre-cement, compressed hardwood fibres and some other materials. We recommend you check the manufacturer’s guidelines.

There are several general rules of thumb to follow.

• Cut ends should be protected with three coats of alkyd or acrylic paint.
  
• Timber products when delivered to site should be kept dry and clear of ground contact.
• Any nail holes should be sealed with a waterproof acrylic filler before being painted or stained.
• Before any sealants are applied, timber should be dry.
• If you’re leaving primed timber exposed to the weather without sealing it for more than a few weeks, you may need to reprime it. Check your manufacturer’s instructions on how long is allowed.

For fibre-cement sheet cladding, cut edges should be sealed with paint, including a 100 mm border around each edge. For plywood cladding, the cut edges are brushed with a proprietary timber preservative.

Beyond treating the cut ends, timber should be properly coated or sealed to prevent moisture ingress. For exterior timber, a coating that offers UV protection and weatherproofing is essential in New Zealand’s tough conditions.

Resene has some useful tips around which products offer the longest lifespans, which could be handy when discussing finishes with the homeowner. For CCA (Chromated Copper Arsenate)-treated timber, coatings that allow the wood grain to show through, such as clear finishes (or water repellents) and stains, allow ultraviolet rays through to the surface. These products have a shorter life than opaque paint systems and may be harder to clean.

The life span of any product depends on making sure there are no gaps in your structure’s armour. By ensuring all timber is correctly treated and weatherproofed from the outset, builders can help extend the life of timber products and avoid issues with moisture and decay down the track.