Decking board spacing: how big should the gap be?
Last reviewed 1 July 2026
Short answer
For timber, aim for a 5–6 mm (about 3/16 in) finished gap. If the boards arrive wet from treatment they will shrink, so lay them closer — 3 mm or even butted — and let them open up. If they are kiln dried they will swell, so lay them at the full gap or wider. Composite gaps are set by the manufacturer and are not negotiable; they are usually larger, with a separate and bigger end gap.
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The gap has three jobs
- Drainage. Water has to get off the deck quickly. A blocked or absent gap holds water against the board edge, which is where rot starts.
- Ventilation. Air moving under the deck dries the underside. Boards that only dry on one face cup.
- Movement. Timber moves across its width with moisture content. Composite moves along its length with temperature. The gap absorbs it.
Timber: the gap depends on how wet the board is today
This is the part most guidance skips. A pressure-treated softwood board delivered soon after treatment can be well above 25% moisture content. Over its first summer it will drop towards 15% and shrink across its width by roughly 1–2%. On a 144 mm board that is 1.5–3 mm of shrinkage per board.
So the gap you lay is not the gap you end up with. Lay wet boards tight — 3 mm, sometimes butted — and they will open to a comfortable 5–6 mm. Lay wet boards at 6 mm and you will end up with 8–9 mm gaps that drop keys, coins and small tools straight through.
Kiln-dried or air-dried boards work the other way: they will take up moisture and swell, so lay them at the full gap.
| Board condition on delivery | Lay at | Expect to settle at |
|---|---|---|
| Wet, heavy, recently treated | 3 mm | 5–6 mm |
| Slightly damp, stored outside covered | 4–5 mm | 5–7 mm |
| Dry, stored indoors or kiln dried | 6 mm | 5–6 mm |
A cheap moisture meter costs less than a single board and removes the guesswork.
Composite: follow the manufacturer, not a rule of thumb
Composite and WPC boards barely move across their width but move noticeably along their length with temperature — the opposite of timber. A 4 m board can grow several millimetres between a cold morning and a hot afternoon.
That is why composite systems specify two different gaps: a side gap, often 5–8 mm, and a much larger end gap where boards butt or meet a fixed edge. End gaps are frequently specified as a function of the installation temperature, with a bigger gap required when installing in cold weather.
Hidden fastener clips usually set the side gap for you, which is one of their genuine advantages. They do not set the end gap.
Important: Closing the gaps up is the most common composite failure
What the gap does to your board count
A wider gap means fewer boards, because each board covers more deck. Going from a 3 mm to a 6 mm gap on a 3 m span of 144 mm boards takes you from 21 rows to 20 — one row saved, and one row's worth of screws.
It also shifts where the last board lands. Always check the final row: if it comes out at 20 mm wide you will want to redistribute the gaps slightly or rip the first and last boards to match, rather than finishing with a sliver.
Related questions
- Can I lay decking boards with no gap at all?
- Only with boards that are visibly wet and will definitely shrink. Butting dry boards leaves nowhere for water to drain and no room for swelling, and it is a reliable way to produce a cupped, slippery deck.
- What gap should I leave at the ends of boards?
- For timber, a few millimetres where a board meets a wall or fascia. For composite, follow the manufacturer's end-gap table, which is usually considerably larger and depends on the temperature on the day you install.
Safety notice: Material planning only