If you've ever pulled up a warped deck board or watched a "perfectly" laid deck develop odd little humps after its first summer, there's a good chance the culprit wasn't the timber itself. It was the gap, or lack of one, between the boards.

Board spacing is one of those details that looks trivial on paper and turns into a genuine headache when you get it wrong. Too tight, and your deck will cup, bow or push itself apart the first time Melbourne decides to have a 40-degree week. Too wide, and you're left with a deck that catches heels, drops things through the cracks, and looks like it was laid by someone in a hurry. So let's get into what the right gap actually is, and why it changes depending on what you're building with.

Why Decking Needs a Gap at All

Timber is not static. It's a natural material that draws in moisture from the air and the ground, swells, dries out again, and shrinks, sometimes by several millimetres across the width of a single board. Composite decking moves too, just differently: instead of swelling across its width like solid timber, capped composite and polymer boards tend to expand and contract along their length as temperatures rise and fall.

The gap between your boards exists to give all of that movement somewhere to go. Without it, boards butted hard against each other will eventually fight for the same space, and something has to give. Usually that's a cupped or split board, or fasteners working themselves loose. The gap also does double duty as a drainage and airflow channel, letting water run through instead of pooling on the surface, which is exactly what you want to avoid if you'd rather not be dealing with mold, algae or rot in a couple of years.

Solid Timber: Treated Pine and Hardwood

For solid timber decking (treated pine, merbau, spotted gum and similar hardwoods), the industry rule of thumb is a 3mm gap once the timber has had a chance to acclimatise to local conditions. That's roughly the thickness of a standard tent peg or a couple of stacked coins, which is why so many tradies just carry a handful of nails or a purpose-made spacer wedge in their tool belt on decking day.

Here's the catch a lot of first-time renovators miss: if you're installing timber straight from the yard, particularly freshly treated pine that's still carrying moisture from the treatment process, you actually want to lay it tight, with little to no gap at all. As the board dries out over the following weeks, it will shrink naturally and open up its own gap. Do the maths backwards from a 3mm target and you can end up with an uncomfortably wide gap once the timber has fully dried.

The safer approach, especially with hardwoods like merbau, is to let your timber sit stickered and covered on site for a week or two before you fix a single board down. That acclimatisation period lets the timber settle to something close to its in-service moisture content, so the gap you set on installation day is the gap you'll actually be living with.

Composite Decking: A Different Set of Rules

Composite and capped polymer boards play by different physics. Because most of their movement happens lengthwise rather than across the board, manufacturers typically specify a spacing range of 3 to 6mm between boards, with the exact figure depending on the fastener system you're using: hidden clips, face-fix screws, and grooved-edge systems all have slightly different tolerances.

This is genuinely worth checking against the specific product you've bought rather than assuming "decking is decking." A gap that's correct for one composite range can be too tight or too generous for another, and getting it wrong can void the manufacturer's warranty on some product lines. If you're weighing up composite against a timber like merbau for your build, it's worth a read of our earlier comparison on composite versus merbau decking, which digs into cost and durability differences beyond just the spacing question.

Getting Consistent Gaps Without Guesswork

A tape measure and a lot of patience will get you there, but most people find it faster and far more consistent to use spacer blocks, either purpose-bought decking spacers or improvised ones cut to the right thickness from scrap material. Set two spacers per join, run your board up against them, fix it off, then move the spacers along to the next join. It sounds slow, but it's actually faster than eyeballing it and re-measuring every second board, and it removes the creeping error you get when small inconsistencies add up across a 4-metre run.

A couple of practical extras worth keeping in mind:

• Run a string line down the outer edge of your deck before you start, so any small spacing variation gets absorbed in the middle of the deck rather than throwing your final board out of line with the fascia.

• Leave a slightly larger gap (closer to the top end of your range) around board ends that butt into a wall, post or another fixed structure, since end-grain timber tends to move more than the board's face.

• Check your decking screws or hidden fastener system's own spec sheet before you commit, since some systems are engineered around a specific gap and won't seat properly outside that range.

Getting Your Numbers Right Before You Start

Spacing decisions tie directly into how much decking you'll actually need, since every gap you leave is timber you're not buying. If you haven't already worked out your board count, our guide on calculating how much decking you need walks through a simple formula that factors gaps into the total.

Whether you're working with treated pine, a hardwood like merbau, or a composite range, our team at A Class Building Materials can talk you through the right fastener and spacing combination for the specific product you've chosen, and make sure you're leaving with enough boards, screws and spacers to get the job done in one trip. Pop into the Ravenhall yard or give us a call before you start marking out, and we'll help you avoid the gaps-related headaches altogether.