Getting the spacing between decking boards right is one of those details that makes a big difference to how your deck performs over time. Too tight and water pools, timber swells, and rot sets in. Too wide and it becomes a trip hazard — or your kids lose their toys through the gaps. Here's what you need to know to get it right the first time.
The Recommended Gap for Timber Decking Boards
For most solid timber decking in New Zealand, a gap of 5–8 mm between boards is the accepted standard. This range allows for:
- Drainage — water runs away freely rather than sitting and causing rot
- Ventilation — airflow under and between boards helps timber dry out after rain
- Timber movement — wood expands and contracts with changes in moisture and temperature
The BRANZ Good Practice Guide for decks recommends allowing for timber movement, particularly with unseasoned or green timber, which can shrink considerably as it dries. If you're laying green timber, you may want to butt boards closer together (around 3–5 mm) knowing they'll open up as they dry.
Does Timber Species or Treatment Affect the Gap?
Yes — different timbers behave differently, and this affects how you space them.
| Timber Type | Condition When Laid | Suggested Starting Gap |
|---|---|---|
| Treated pine (H3.2/H4) | Green / unseasoned | 3–5 mm |
| Treated pine (H3.2/H4) | Seasoned / dry | 5–8 mm |
| Kwila (Merbau) | Dry hardwood | 5–6 mm |
| Macrocarpa | Dry | 5–8 mm |
| Composite decking | N/A | Manufacturer spec (often 5–6 mm) |
Kwila and other dense hardwoods move less than pine, but still need a gap for drainage. Always check the specific product data sheet from your supplier.
Composite and PVC Decking Gaps
Composite decking (a mix of timber fibre and plastic) has become popular in NZ for its low maintenance. These boards still expand and contract — often more than solid timber in the heat — so the manufacturer's specified gap is critical. Most composite products call for around 5–6 mm side gaps, but always follow the product instructions exactly. Brands like James Hardie and various imported composite ranges have specific fixing systems designed to maintain consistent spacing automatically.
NZ Building Code and Standards
Decks over a certain height or attached to a house are building work under the NZ Building Code, and may require a building consent. The relevant standard for timber-framed construction is NZS 3604, which covers deck framing, bearer and joist sizing, and fixing requirements.
While NZS 3604 doesn't prescribe an exact board gap, it does require that decks drain freely and don't create slip or trip hazards. A gap wider than 10 mm can become a trip hazard, particularly for people in heels or with mobility aids, and may not comply with the Building Code's safety requirements. Keep gaps between 5–8 mm as a practical rule.
How to Maintain Consistent Gaps
Even spacing makes a deck look professional and ensures even drainage. Here are a few practical methods:
- Spacer offcuts — cut a piece of timber or use a nail/screw shank as a consistent spacer between each board as you fix them
- Composite clip systems — many composite decking products use hidden clips that automatically set the gap
- String line — run a line along the length to keep boards straight and parallel
- Start from a straight edge — begin at the house wall or a fascia board and work outward
For a standard 5 mm gap, a 5 mm drill bit or a piece of 5 mm flat bar works well as a quick spacer. Pick up spacers or suitable offcuts at Bunnings or Mitre 10.
Fixing Methods and Their Effect on Gaps
How you fix your boards also affects spacing:
- Face screwing — most common, gives full control over gap width; use two screws per board per joist to prevent cupping
- Hidden clips — used with grooved composite or some timber boards; gap is set by the clip design
- Stainless steel screws — essential near the coast or in exposed conditions to prevent rust staining
Pre-drilling near board ends prevents splitting, especially in hardwoods like Kwila. PlaceMakers stocks a good range of treated timber fixings suitable for outdoor use.
Drainage and Ventilation Under the Deck
The gap between boards is only part of the drainage picture. The space under the deck matters too. BRANZ recommends adequate clearance between the ground and the underside of joists to allow airflow and prevent moisture build-up — typically at least 150–200 mm above ground level where possible. Poor subfloor ventilation is a leading cause of premature deck failure in NZ's damp climate.
When to Call a Professional
If your deck is attached to the house, over 1 metre above ground, or requires a building consent, you should seriously consider engaging a qualified deck builder or Licensed Building Practitioner (LBP). Structural work on decks — including bearer and joist sizing, connections to the house, and balustrade installation — must meet the Building Code, and an LBP can ensure the work is compliant and sign off the documentation.
You can find qualified, local deck builders on tradedirectory.co.nz and verify LBP registration at lbp.govt.nz.
Safety note: Working at height carries real risk. Use appropriate scaffolding or edge protection when building decks above ground level, and follow WorkSafe NZ guidelines for working safely at height.