Brett Field
Leave a gap of about 3 to 5 mm between most decking boards. WoodSolutions recommends 3 to 4 mm for seasoned timber boards up to 90 mm wide, and 4 to 5 mm for wider ones. Composite makers ask for 4 to 5 mm (ModWood, Millboard, Trex), or set the gap with their clips (Ekodeck).
Below are the side gaps for hardwood, treated pine and composite, what to do at board ends and walls, why boards shrink and swell in Gold Coast weather, and how to work out how many boards you need. These are published figures from the timber industry and the board makers, checked on 5 October 2026.
What gap should I leave between hardwood and treated pine decking boards?
For seasoned timber, 3 to 5 mm when you lay it, depending on how wide the board is. Here is what the three main Australian timber sources say.
| Seasoned board | WoodSolutions (2020) | Timber Queensland (2014) | ATFA (2020) |
|---|---|---|---|
| Hardwood, up to 90 mm wide | 3-4 mm | 3 mm minimum | 3 mm |
| Hardwood, 91-140 mm wide | 4-5 mm | - | 5 mm (130 mm boards) |
| Treated pine, 70-90 mm wide | 3-4 mm | 5 mm | 5 mm |
| Treated pine, 91-120 mm wide | 4-5 mm | 5 mm | 5 mm |
The sources split on pine. The WoodSolutions deck design guide allows 3 to 4 mm for narrower pine boards, while Timber Queensland and the Australasian Timber Flooring Association (ATFA) both say 5 mm. Timber Queensland's figure is the one written for Queensland. Cypress follows the hardwood rule: 3 mm for boards up to 90 mm wide.
WoodSolutions says 3 to 4 mm is the ideal gap over the long term, and that the gap must never get so wide it becomes a trip hazard.
Unseasoned (green) hardwood is different. It shrinks as it dries, so it goes down with a smaller gap, 2 to 3 mm in WoodSolutions' table. Timber Queensland expects unseasoned hardwood to shrink about 6%, or 6 mm for every 100 mm of width. Not sure which timber to use? Our decking timber guide compares them for Gold Coast conditions.
What gap do composite decking boards need?
Composite boards need a gap on the sides, at the ends and against walls. Each maker sets its own, and ModWood and Trex both say that not following their instructions can void the warranty.
| Brand | Side gap | End-to-end gap | Gap to a wall or post |
|---|---|---|---|
| ModWood, 88 and 137 mm boards | 4 mm minimum | 2 mm minimum | 6 mm minimum (4 mm for Flame Shield) |
| ModWood, 137 x 32 mm board | 6 mm minimum | 2 mm minimum | 6 mm minimum |
| Trex | 5 mm minimum | 3 mm (above 4.5°C) | 6 mm (above 4.5°C) |
| Millboard | 4 mm | 2 mm | Not stated |
| Ekodeck | 3 or 6 mm, set by the clip | 1-3 mm, by board length and temperature | "A sufficient gap" (no figure) |
Trex wants bigger gaps below 4.5°C. At the Bureau of Meteorology's Gold Coast Seaway station, even July's average overnight low is 12°C, so the warm-weather figures are the ones to use here. Its 2024 installation guide also asks for a 10 mm side gap for docks and heavily wooded areas, where leaves build up, and says no gap should ever be more than 13 mm.
Do decking boards need a gap at the ends?
Timber boards, no. Composite boards, yes.
Timber boards are butted tight end to end, with the join over a joist. Timber Queensland and ATFA both say to stagger the joins, so two boards side by side never end on the same joist. Timber Queensland and WoodSolutions suggest a slight back-cut on the ends, so the top edges close up tight. ATFA adds that each board should cross at least two joists, so with joists at 450 mm the shortest board is about 900 mm. Our guide to deck joist spacing covers the frame under the boards.
Composite boards grow and shrink in length with heat, so they need end gaps (see the table above). Ekodeck's gap table runs from 1 mm to 3 mm: the longer the board and the cooler the day, the bigger the gap. Ekodeck also advises against butt joins altogether, and suggests a breaker board on decks longer than 5.4 m.
Why do decking boards shrink and swell?
Timber takes up water from rain and damp air and swells, then shrinks as it dries. Composite boards move mostly with heat.
Seasoned decking leaves the mill fairly dry. Timber Queensland notes the Australian Standards allow 10 to 18% moisture in hardwood decking and 10 to 15% in softwood, and it recommends no more than 15% for home decks. Outside, the boards settle to match the air around them. Timber Queensland says a well-ventilated deck in coastal Australia settles at about 16%, and decks close to or over water run higher, at 18 to 21%.
So whether a board swells or shrinks after it goes down depends on how dry it was when it was laid. That is why seasoned boards go down with a gap and green boards with a smaller one. Timber Queensland also warns that gaps may close up during long wet spells.
Composite works a little differently. ModWood's questions and answers sheet says its wood fibres are very dry when made, so the boards swell slightly after laying, which is why it asks for a bigger side gap than timber. Composite boards also grow in length with heat. ModWood says a board left in full sun on a warm day runs about 20 to 25°C hotter than the air, and suggests fixing boards when it is under 25°C and the boards are out of the sun.
Does Gold Coast weather change the gap?
It makes the gap matter more, because summer here is wet and humid. At the Gold Coast Seaway weather station, February is the wettest month, with 183.8 mm of rain on average, and September the driest, with 42.8 mm. Humidity at 9am averages 70 to 72% from January to March, against 61 to 62% in August and September.
So timber laid now, in a dry spring, will take up water as the summer rain arrives. Keep the gaps clear of leaves and dirt so water can drain, and keep air moving under the deck. ModWood says not to block airflow by sealing off the deck ends with garden beds or solid objects, and ATFA says decks are not to be boxed in. Without air flow, ATFA says, boards can swell until the gaps close, and then cup.
Low decks need bigger gaps. For decks close to the ground, WoodSolutions asks for a long-term gap of 5 to 6 mm, and ATFA says to use boards no wider than 90 mm with at least a 5 mm gap. ATFA gives the same rule for decks built on battens over a concrete slab, which our guide to a deck on a concrete slab covers.
How big a gap should I leave at the wall?
WoodSolutions says to keep timber decking boards 10 mm clear of the house wall, so water can drain down between them. Composite makers set their own gap to walls and posts: 6 mm for most ModWood boards (4 mm for Flame Shield), and 6 mm for Trex in warm weather.
The frame needs room as well. Where a deck relies on the house's termite barrier, WoodSolutions says AS 3660.1 needs the deck kept at least 25 mm off the building, so it can be inspected.
How many decking boards do I need?
Add the board's width to the gap. Then divide the width of the deck, measured across the boards, by that number and round up. That gives you the number of rows. Multiply the rows by the length of the deck to get the total length of board.
Say your deck is 5 m long and 4 m out from the house, with the boards running the long way. Using 86 mm hardwood boards (one of Timber Queensland's standard cover widths) and a 3 mm gap:
- Each board and its gap takes up 86 + 3 = 89 mm.
- 4,000 mm ÷ 89 mm = 44.9, so 45 rows of boards.
- 45 rows × 5 m = 225 m of decking board.
With 137 mm ModWood boards and a 4 mm gap, the same deck takes 4,000 ÷ 141 = 28.4, so 29 rows and 145 m of board. The last row is usually cut down its length to fit.
The gap barely changes the count. At 5 mm, the hardwood deck needs 4,000 ÷ 91 = 44 rows instead of 45. Order extra for offcuts and staggered joins, and ask your supplier how much to allow.
We build timber and composite decks across the Gold Coast, and do deck repairs and restoration. You get a timeline with the proposal, before any work starts. See our deck building page for what we build.
Where do these figures come from?
Checked on 5 October 2026: WoodSolutions Technical Design Guide 21, Domestic Timber Deck Design (revised October 2020); Timber Queensland Technical Data Sheets 4 and 7 (both revised March 2014); ATFA's Residential Timber Decks (version 2, April 2020); ModWood fixing instructions (September 2021 and January 2026) and questions and answers (November 2025); the Trex 2024 Decking Installation Guide; the Millboard installation guide (June 2022 edition, from Australian supplier Modinex); Ekodeck's Designer Series installation guide (version 11.25) and installation pages (2025); and Bureau of Meteorology climate statistics for Gold Coast Seaway (prepared 1 October 2026). The board counts are our sums. We will check the figures again every three months.
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