Composite decking is popular for good reason: it looks like timber, doesn't need oiling and won't rot. But here's the catch. There's no Australian Standard or deemed-to-satisfy pathway in the National Construction Code for wood plastic composite (WPC) decking boards. That means every composite deck needs evidence that the product is suitable, and many products on the market can't provide it.

If you're planning a deck, check this before you buy, not after the certifier asks.

What composite decking is

Composite decking is a blend of wood fibres and plastic. The wood fibre gives it body, and the plastic protects it from weather, insects and rot. Boards come in a range of colours and timber-look finishes.

Why compliance is a problem

Because no Australian Standard covers WPC decking, the QBCC says a performance solution is needed for these boards, supported by evidence that they meet the NCC's performance requirements. That evidence typically includes:

  • Structural calculations to the AS/NZS 1170 loading standards, showing the boards can span your joist spacing and carry the loads.
  • Slip resistance testing to AS 4586.
  • Fire performance testing to the AS 1530 series.
  • Bushfire testing to AS 1530.8, if your site is bushfire prone.

The QBCC's fact sheet on wood plastic composite decking boards (May 2023, updated for NCC 2022) sets this out. It warns that boards not designed, made and installed to meet the NCC can develop unsafe conditions such as trip and fall hazards. The QBCC has also issued a public warning about one product, DECKO ECW decking boards, as a non-conforming building product.

What can go wrong with the wrong boards

  • Sagging between joists if the boards can't handle the span.
  • Slippery when wet, especially around pools and on outdoor stairs.
  • Not allowed on bushfire-prone sites if they haven't been tested for your Bushfire Attack Level (BAL).
  • Warping and movement from heat, especially in dark colours in full Queensland sun.

How evidence of suitability works

Under the NCC, a product can be shown to be suitable in several ways, including:

  • A CodeMark certificate of conformity (the CodeMark scheme is currently under review by the ABCB).
  • A test report from an accredited testing laboratory.
  • A certificate or report from a professional engineer or other appropriately qualified person.
  • Other documentary evidence accepted under the NCC that the product meets the relevant requirements.

CodeMark is one option, not the only one. What matters is that the evidence covers your intended use: your span, your loads, your site and your BAL.

Who checks it

Your building certifier can refuse to sign off a deck built with a product that has no evidence of suitability. The QBCC can act on non-conforming building products, including issuing public warnings and directions.

Don't assume the retailer has checked. They should have, as suppliers have legal obligations too, but what should happen and what does happen aren't always the same.

Three-step check before you buy

  1. Ask for evidence of suitability. Get the test reports, engineer's certificate or CodeMark certificate in writing, and make sure they cover decking use, slip resistance and fire performance.
  2. Check the span tables against your joists. The manufacturer's installation guide should state the maximum joist spacing for your board. If your frame doesn't match, the deck won't comply.
  3. Check your BAL if you're bushfire prone. If your site has a BAL rating, the boards need test evidence for that level.

If the supplier can't or won't provide the paperwork, treat it as a red flag and choose another product.

Keep the records

Once the deck is built, keep the product details, installation guide and compliance documents with your house records. If the deck needed approval, a buyer's solicitor may ask for the final certificate when you sell, and a pre-purchase inspector may flag a deck with no paperwork.

A deck is a big investment. A few questions before you buy can save you from pulling it up later.