For most composite decking boards installed in a standard straight pattern, 400 mm joist centres is the recommended spacing — but this is not a universal rule. Composite decking products vary significantly between manufacturers, and the correct spacing depends on your specific board, how it's laid, and the load requirements of your deck. Always read your product's installation guide before you frame up.
Why Joist Spacing Matters for Composite Decking
Unlike solid timber, composite decking boards are made from a blend of wood fibre and plastic (or sometimes fully cellular PVC). This gives them excellent weather resistance and low maintenance, but it also means they can behave differently under load — particularly when it comes to deflection (sagging between supports). If joists are spaced too far apart, composite boards can flex noticeably underfoot, feel spongy, and may void the manufacturer's warranty.
Getting the framing right from the start is far easier than pulling up a finished deck to re-space joists later.
Standard Joist Spacing Guidelines
Here's a general guide to typical composite decking joist spacing requirements:
| Installation Pattern | Typical Joist Spacing |
|---|---|
| Straight (parallel to house) | 400 mm centres |
| 45° diagonal pattern | 300 mm centres |
| Picture frame / border boards | May need additional blocking |
| High-traffic or commercial areas | Check manufacturer specs — often 300 mm |
These are general industry figures. Your product's technical data sheet is the definitive reference — always follow it.
Always Check the Manufacturer's Specifications
This is the single most important rule with composite decking. Brands such as Trex, Millboard, Futurewood, and others each publish specific installation guides that detail:
- Maximum joist spacing for their boards
- Minimum end bearing (how much board must sit on each joist at joins)
- Expansion gap requirements between boards and at ends
- Fastener type and spacing (hidden clip systems vs face-fixing)
- Any requirements for ventilation beneath the deck
Using the wrong joist spacing can void your product warranty and may result in a deck that fails to meet the NZ Building Code, particularly the structural performance requirements.
NZ Building Code and Consent Considerations
In New Zealand, decks are subject to the NZ Building Code and are often governed by NZS 3604:2011 (Timber-framed buildings), which covers framing for residential decks. Whether your deck needs a building consent depends on its height, size, and proximity to boundaries.
As a general guide:
- Decks 300 mm or less above ground may be exempt under Schedule 1 of the Building Act
- Decks higher than 1.5 m above ground almost certainly require consent
- Any deck attached to the house structure typically requires consent
Always confirm consent requirements with your local council before starting work. Rules vary by region and your specific site conditions. You can find general guidance at MBIE Building Performance.
If your deck requires consent, the framing — including joist spacing — will need to comply with the approved plans and relevant standards.
Framing Material: Timber vs Steel
Composite decking can be installed over treated timber or steel subframes. In New Zealand's varied climate — from the humid north to the wet West Coast and frosty southern regions — the subframe material matters.
- Treated timber joists (H3.2 minimum for above-ground, H4 if in contact with or near the ground) are the most common choice. Check BRANZ guidance for treatment levels appropriate to your exposure zone.
- Aluminium or steel subframes are increasingly popular with composite decking systems and can allow slightly wider spacing in some products — again, check the manufacturer's guide.
Ensure there is adequate ventilation and drainage beneath the deck to prevent moisture build-up, which can affect both the subframe and the underside of the composite boards.
Practical Tips for Getting the Spacing Right
- Snap chalk lines across your bearers before installing joists to keep spacing accurate and consistent.
- Use joist hangers rated for the load and compatible with your joist size — this keeps joists plumb and at the correct height.
- At board end joins, ensure both board ends bear fully on a joist (typically a minimum of 25–40 mm bearing per board end — check your product guide).
- For diagonal installations, calculate your joist layout carefully before you start — the 45° angle means boards span a greater distance between joists, which is why closer spacing (300 mm) is usually required.
- Double up joists or add blocking at picture-frame borders and around any deck penetrations (posts, drains, etc.).
When to Call a Professional
While experienced DIYers can tackle deck building, there are good reasons to bring in a qualified deck builder:
- If your deck requires a building consent, a Licensed Building Practitioner (LBP) must carry out or supervise the restricted building work.
- If the deck is elevated or complex in design, structural calculations may be needed.
- If you're unsure about ground conditions, drainage, or subframe design, a professional can save you costly mistakes.
- Any electrical work on or near the deck (lighting, outdoor power points) must be done by a registered electrician — this is restricted work under the Electrical Workers Registration Board (EWRB).
You can find qualified, local deck builders on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
