Joists are the support beams that sit under your deck boards, and how far apart they sit decides how solid the deck feels. Here are the common rules of thumb in plain language – plus the one piece of advice that always beats a general rule.
40–60 cm centres suit standard 21–28 mm timber decking. For thin boards or hollow composites you'll typically drop to 30–40 cm. Always check the manufacturer's spec for your board.
There's no single number that fits every deck. The spacing depends on a few things working together:
| Factor | How it affects spacing |
|---|---|
| Board type | Timber decking tolerates wider spans than many hollow composite boards. |
| Board thickness | Thicker boards (28 mm) span further than thin ones (19 mm). |
| Load | A hot tub, large planters or heavy use call for closer joists. |
| Board direction | Diagonal layouts usually need tighter joist spacing than straight runs. |
The rule of thumb: the thinner the board and the heavier the load, the closer the joists need to sit.
The numbers here are common rules of thumb – not a structural calculation. Your board's manufacturer states the maximum joist spacing in the installation guide, and that's what you should follow. If the deck has to carry something heavy (like a hot tub), is raised, or you're unsure about the ground and load, follow the supplier's guidance and your local building requirements – and check with a professional if in doubt.
Once you've picked a joist spacing, the deck calculator works out joist, board and screw counts from your dimensions – with waste included.
As a rule of thumb, 40–60 cm centre-to-centre for standard 21–28 mm timber boards. For thin boards (under 21 mm) or hollow composite boards you usually need to drop to 30–40 cm. Always follow the manufacturer's spec for your specific board.
Spacing joists too far apart makes the deck feel bouncy: the boards flex between joists and it feels unstable. Over time the boards can also cup or crack.
Often yes. Many hollow composite boards need 30–40 cm spacing, and even closer for diagonal layouts. It's stated in the manufacturer's installation guide – use that rather than a general rule.
Yes. Where two boards meet end-to-end, each board needs its own joist face to screw into. So you set two joists side by side (a doubled joist) at the join.
Joist count = (deck length ÷ spacing) + 1. The deck calculator works it out for you alongside the board and screw counts.
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No credit card required. Free sample project included.