Bowed decking boards are one of the most common frustrations when building a deck in New Zealand — especially with timber that has been exposed to our variable humidity. The good news is that most bows can be corrected during installation using a few simple techniques and the right tools, without needing to replace the board.
Why Do Decking Boards Bow?
Timber naturally moves as it absorbs and releases moisture. In New Zealand's climate — which can swing from humid coastal conditions to dry inland summers — cupping, crowning, and bowing are all common, even in treated pine or hardwood decking. Boards that have been stored flat in a stack, or left in the sun before installation, are particularly prone to bowing along their length (a longitudinal bow).
Understanding the cause helps you choose the right fix. A slight bow is usually manageable; a severe or permanent set in the timber may mean the board is better used elsewhere or cut into shorter lengths.
Check Your Joist Spacing First
Before wrestling with a bowed board, confirm your joist spacing is correct. For most 90mm or wider decking timber, joists are typically spaced at 400mm to 450mm centres — closer spacing gives you more fixing points to pull a bow into line. If your joists are too far apart, straightening a bowed board becomes much harder and the fix may not hold. Check your plans and ensure your framing complies with NZS 3604 or your engineer's specifications.
Technique 1: The Timber Lever Method
This is the most common on-site approach and works well for moderate bows.
- Fix one end of the board to the first joist with your decking screws or hidden fixings.
- Walk along the board and identify where the bow peaks away from the joist.
- Cut a short offcut of 90×45 timber (a scrap piece works fine) and use it as a lever block — wedge it between the bowed board and the previously fixed board.
- Apply firm sideways pressure with your foot or knee against the lever block to push the bowed board into position.
- While holding pressure, drive your fixing screw into the joist immediately. Work your way along the board, joist by joist.
This method is faster with two people — one levers, one screws.
Technique 2: Clamps and Ratchet Straps
For more stubborn bows, clamps give you more controlled, sustained pressure.
- Use F-clamps or bar clamps to pull the bowed board tight against the neighbouring board at each joist location, then fix before releasing.
- Alternatively, loop a ratchet strap around a joist or a fixed board and use it to pull the bowed board into line. This works especially well mid-span.
- A decking tool (sometimes called a board straightener or decking wrench) is a purpose-made lever that hooks over the joist and lets you lever the board into position — these are available at Bunnings or Mitre 10 and are well worth the investment if you're laying a full deck.
Technique 3: Screwing at an Angle (Toe-Screwing)
For a bow that curves away from the previous board, toe-screwing (driving a screw at an angle through the edge of the bowed board into the joist) can draw the board in as the screw tightens. This works best as a supplement to the lever method rather than on its own.
Technique 4: Pre-Wetting or Conditioning the Timber
If a board has a pronounced bow caused by one face drying faster than the other, lightly dampening the drier face and leaving it face-down in the sun for 30–60 minutes can help it relax back toward straight before you fix it. This is a useful trick for kiln-dried timber that has dried unevenly in storage.
Fixing Methods and Spacing
| Board Width | Recommended Fixings per Joist | Notes |
|---|---|---|
| Up to 90mm | 1 fixing per joist | Centre fix |
| 90mm–140mm | 2 fixings per joist | Helps control cupping |
| 140mm+ | 2 fixings per joist | Essential for wide boards |
Always use stainless steel or hot-dipped galvanised screws rated for treated timber — standard zinc-plated screws will corrode quickly in NZ conditions, especially near the coast. Check the BRANZ guidelines for corrosion zone requirements in your area.
Preventing Bowing Before It Starts
- Acclimatise your timber on-site for at least 48 hours before installation, stacked with spacers (stickers) between boards to allow airflow.
- Store boards out of direct sun and rain.
- Buy from a reputable supplier and inspect boards before purchase where possible — PlaceMakers and Mitre 10 both stock treated pine decking.
- Reject any boards with severe or permanent set — no amount of leverage will hold them flat long-term.
When to Call a Professional
If you're finding that a large number of your boards are bowed, or the bow is so severe that you can't pull it into line without risking splitting the timber, it's worth getting advice from a qualified deck builder. Persistent bowing across a whole deck can also indicate a framing issue — joists that are themselves bowed or not level — which is a structural matter best assessed by a professional.
Building a deck over a certain size or height may also require a building consent from your local council. Always check with your council before starting — Schedule 1 of the Building Act lists exemptions, but the rules depend on your specific situation. The MBIE Building Performance website has clear guidance on when consent is needed.
You can find qualified, local deck builders on tradedirectory.co.nz to help with your project.
Licensed Building Practitioner (LBP)
