Timber Decking Board Spacing: Why Gap Sizing Matters in NZ Weather
The Gap Nobody Warns You About
You've measured twice, you've bought good timber, and you're ready to lay your deck. There's just one number nobody mentioned before you started: the gap between each board. Get it wrong, and within a year you could be looking at boards that cup, water that pools instead of draining, and gaps that either vanish shut in winter or open up wide enough to catch a jandal in summer.
It's an easy detail to rush. Most DIY guides mention "leave a small gap" and move on, without explaining why the number changes depending on where in New Zealand you're building, what timber you've chosen, or what season it is when you're laying the boards. That vagueness is exactly why so many home decks develop problems that a proper build never would.
For most New Zealand decks, leave a 3–6mm gap between timber decking boards, with wider boards (over 90mm) needing gaps closer to 6mm. A 100 x 3.75mm nail laid on its side makes a reliable, ready-made spacer. If your timber has been supplied dry, lean towards the wider end of that range, since dry boards swell once they're outside and exposed to moisture. Coastal and humid regions should also favour a slightly wider gap to allow for extra swelling.
Key Takeaways
Leave a 3–6mm gap between standard 90mm decking boards; push towards 6mm for wider boards.
A 100 x 3.75mm nail on its side makes a cheap, consistent spacer.
Timber supplied dry swells once installed outdoors — use a wider starting gap than you would for wet-supplied timber.
Leave a minimum 12mm gap where decking meets the house cladding.
Coastal and humid areas benefit from spacing towards the wider end of the range.
Let timber acclimatise on-site for several days before fixing it down.
Keep your spacing tool consistent along the whole run of boards to avoid an uneven finish.
What Board Spacing Actually Does
Board spacing is the deliberate gap you leave between each length of decking as you nail or screw it down. It looks like a small styling choice, but it's doing three jobs at once: letting rainwater drain straight through instead of sitting on the surface, giving each board room to expand and contract as it absorbs and releases moisture, and keeping air moving underneath the deck so the timber can dry out between wet spells.
Timber is a natural material, and it never really stops moving. It swells when it takes on moisture and shrinks as it dries, and in New Zealand, that cycle happens constantly rather than settling into one steady state. A gap that's too tight will squeeze shut during a wet spell and put pressure on the boards next to it. A gap that's too wide lets water splash back up, invites debris to build up, and can even become a trip hazard. Spacing correctly from day one is one of the simplest ways to protect the investment you've made in your timber and get the full lifespan out of it.
Why New Zealand's Weather Makes This Trickier Than It Looks
New Zealand doesn't do "one climate." A deck in coastal Nelson deals with salt-laden humidity, while the same timber species inland might face sharper swings between damp mornings and dry, sun-baked afternoons. Both conditions push and pull on timber in different ways, and your board spacing needs to account for whichever pattern your section experiences.
Here's the part that catches people out: the moisture content of your timber at the time you install it determines how much gap you need. Timber supplied "wet" (still holding factory or transport moisture) will shrink as it dries out on your deck, so gaps can start slightly tighter. Timber supplied dry will do the opposite — it absorbs moisture from the air and ground once it's outdoors, swells, and needs a wider starting gap so it doesn't press tight against its neighbours. If you're unsure which you've bought, it pays to ask your supplier directly, or read up on buying dry timber before you commit to a gap width.
Seasonal timing matters too. A deck built in the depths of a damp Tasman winter will behave differently to one built during a dry summer week, simply because the timber's starting moisture level is different in each case. It's worth letting your decking timber sit on-site for several days before installation, so it can begin acclimatising to your specific spot rather than the conditions it was stored in.
The Right Gap: What the Numbers Actually Mean
For standard 90mm-wide decking boards, a gap of 3 to 6mm is the accepted range used across New Zealand deck-building guidance. The simplest, most reliable way to hit that number consistently is to use a 100 x 3.75mm nail on its side as a spacer between each board as you fix it down — it's a trick tradespeople have relied on for years because it's cheap, always the same size, and sits neatly in your tool belt.
Wider boards need more room to move, because a wider piece of timber physically swells and shrinks by a greater amount than a narrow one for the same change in moisture. If you've chosen boards wider than the standard 90mm, push your gap towards 6mm rather than 3mm. It's also worth checking your timber supplier's specific guidance, since different species and mill treatments can shift the ideal number slightly.
Don't forget the gap where your decking meets the house. Building guidance calls for a minimum 12mm clearance between the decking and the cladding, which is considerably more than the gap between individual boards. That space allows water to drain away from the building line and gives you room for ongoing maintenance and cleaning.
Coastal Conditions and High-Humidity Areas
If your section sits near the coast, or you're in one of the pockets around Tasman and Nelson that hold onto humidity longer after rain, it pays to lean towards the wider end of the spacing range. Salt-laden air and consistently higher humidity both encourage timber to hold onto more moisture for longer, which means more expansion and a greater chance of boards pressing tight against each other if your gaps started out too narrow.
It's also worth thinking about airflow underneath the deck itself. A well-ventilated subframe helps timber dry out between wet periods, working alongside your board spacing rather than against it. If you're planning a deck for a coastal lifestyle block or exposed section, it's worth reading up on how different timber species handle timber movement before you finalise your material choice.
Getting Consistent Spacing: Tools and Technique
Consistency is what separates a deck that looks professionally built from one that looks slightly off, even if every individual gap is technically "close enough." Uneven spacing catches the eye immediately, even for people who couldn't tell you why something looks wrong.
The nail-as-spacer method remains the most accessible option for most home builders, since it costs almost nothing and gives you a repeatable width every time. If you want a bit more precision, purpose-made plastic or aluminium decking spacers are widely available and simply clip or sit against the edge of the previous board while you position the next one. Whichever method you choose, the key is picking one tool and sticking with it along the entire run of boards, rather than eyeballing gaps by hand partway through.
Before you start fixing boards permanently, it's worth dry-laying a run of several boards with your spacers in place to check how the pattern looks and confirm your joist spacing lines up with where you'll need to fix each board down.
Common Mistakes That Shorten a Deck's Life
The most common mistake is copying a gap size from an overseas guide or a generic DIY article without adjusting for New Zealand conditions or the specific moisture state of the timber being used. A number that works for a dry inland climate elsewhere in the world doesn't automatically translate to a humid coastal New Zealand summer.
The second common mistake is inconsistency along the length of the deck — starting with a careful 4mm gap and drifting wider or narrower as the job goes on, usually because the spacer was set down and picked back up rather than kept in place throughout. The third is skipping the acclimatisation period altogether and fixing timber down the moment it arrives on site, before it's had any chance to adjust to local humidity.
Getting your timber from a supplier who mills and grades locally, and who can tell you honestly whether your boards were supplied wet or dry, removes a lot of this guesswork before you've even picked up a spacer. That's exactly the kind of detail worth discussing when you're sourcing wood products for your build.
Key Benefits of Getting Spacing Right
A properly spaced deck drains rainwater straight through instead of pooling on the surface, which slows down the timber decay that pooling water encourages over time. It also gives the timber the breathing room it needs to expand and contract naturally, reducing the risk of boards cupping, splitting, or pushing tightly against their neighbours during wet spells. On top of the practical benefits, evenly spaced boards simply look better — the clean, consistent lines are one of the easiest ways to make a home-built deck look like a professional one.
Your Next Step: Build It Right the First Time
A deck is a long-term investment in how you use your outdoor space, and the spacing decision you make in an afternoon of building will affect how that deck looks and performs for years afterwards. If you're still choosing your decking timber, it's worth exploring Plankville's full range of timber options and talking through which species and supply condition (wet or dry) will suit your specific site.
If you'd like advice tailored to your section, whether that's a coastal Tasman property or an inland Richmond build, get in touch with the Plankville team directly. You can also browse Plankville in action to see how locally sourced timber performs once it's out in real New Zealand conditions, or head to the blog for more building guides written specifically for Richmond and Tasman conditions.
Small Gap, Big Difference
Board spacing is one of the smallest measurements in the entire decking process, and one of the easiest to get wrong when you're focused on the bigger picture of joists, fixings and finish. But that few millimetres of gap is doing real work every time it rains or the temperature swings, and New Zealand's changeable weather means it's worth taking seriously rather than eyeballing. Get the spacing right, choose timber suited to your conditions, and your deck will reward you with years of solid, good-looking use.
FREQUENTLY ASKED QUESTIONS
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For standard 90mm boards, leave a 3–6mm gap. Wider boards need more room, closer to 6mm, since they expand and contract more with changes in moisture. A 100 x 3.75mm nail on its side is a simple, consistent spacer most builders use.
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The typical gap is 3–6mm for standard-width boards. The exact figure depends on whether your timber was supplied wet or dry and how humid your local conditions are, so check with your supplier before you start fixing boards down.
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Reputable local timber suppliers who mill and grade their own stock, like Plankville in Richmond, can tell you the moisture condition of your boards and recommend spacing to suit. Always ask whether your timber was supplied wet or dry.
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New Zealand building guidance for timber decks generally recommends a 3–6mm gap between boards for drainage and movement, plus a minimum 12mm gap between decking and any adjoining house cladding for drainage and maintenance access.
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Coastal areas experience higher humidity and salt exposure, both of which increase timber movement. Lean towards the wider end of the 3–6mm range, and make sure the space underneath your deck has good airflow to help timber dry between wet spells.
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A 100 x 3.75mm nail laid flat is the classic low-cost spacer. Purpose-made plastic or aluminium decking spacers are also widely available and clip against the previous board for a more precise, repeatable gap along the whole deck.
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Decking spacers are available from hardware and building supply stores throughout New Zealand. Many timber suppliers can also point you towards suitable spacers when you purchase your decking boards, so it's worth asking at the same time.
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Adjustable plastic and aluminium spacers are stocked by most major hardware chains and specialist decking suppliers across New Zealand, including online retailers that ship nationwide, so availability isn't limited to any single region.
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Start with 3–6mm as your baseline for 90mm boards. Widen it if your timber was supplied dry, if you're in a coastal or humid area, or if your boards are wider than standard. Narrow it slightly only if your timber was supplied wet and will shrink further.
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Yes. Different species absorb and release moisture at different rates, so the ideal gap can shift slightly depending on what you've chosen. Ask your supplier about the specific movement characteristics of your timber species before setting your spacing.
