Decking guide
Deck joist spacing: how far apart should joists be?
Joist spacing is the distance between the centres of neighbouring deck joists, often written as on centre (o.c.). It controls how stiff the deck surface feels and how much framing you need to buy, and it changes with the decking material and the board direction.
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Open the deck plannerShape your deck, preview the board layout and estimate the boards and perimeter materials.
Typical joist spacing
| Board layout | Typical maximum joist spacing |
|---|---|
| Boards perpendicular to the joists (standard) | 16 in (about 400 mm) on centre |
| Boards at 45° to the joists (diagonal) | 12 in (about 300 mm) on centre |
| Boards at about 30° | Some manufacturers allow about 14 in; check the instructions |
| Picture-frame borders | Extra support under the border as well as the standard spacing |
| Stair treads and landings | Often closer support is required; follow the manufacturer |
Why diagonal boards need closer joists
A board laid on the diagonal crosses the joists at an angle, so the unsupported length between joists is longer than the joist spacing. Closing the spacing to 12 in keeps the board's effective span the same and avoids a bouncy or sagging surface.
Joist spacing is not joist span
Spacing is the distance between joists. Span is how far a joist can travel between supports such as beams or posts. Allowable spans depend on the joist size, timber species and grade, the spacing and the local code, so use the span tables that apply to your jurisdiction rather than a rule of thumb.
How to count the joists
Number of joists = (length of the deck across the joists ÷ spacing) + 1, rounding the first part down. The extra joist is for the end of the run.
- Metric: a deck 4.8 m wide with joists at 400 mm: 4,800 ÷ 400 = 12, so 13 joists.
- Imperial: a deck 16 ft wide with joists at 16 in: 192 ÷ 16 = 12, so 13 joists.
- The same 16 ft deck with diagonal boards and joists at 12 in: 192 ÷ 12 = 16, so 17 joists.
Before you build
- Read the installation guide for your decking, especially for composite boards.
- Check your local code for maximum spacing and required joist sizes.
- Plan the board direction first, because it decides the joist spacing.
- Replacing old decking with composite? Existing joists at 24 in o.c. often need an extra joist added between each pair.
Frequently asked questions
How far apart should deck joists be?
Most decks use joists 16 inches (about 400 mm) apart on centre for boards laid across the joists, and 12 inches (about 300 mm) for diagonal boards. Always follow the decking manufacturer's instructions and your local code.
What joist spacing do composite decks need?
Many composite and PVC decking brands specify a maximum of 16 inches on centre for straight boards and 12 inches for 45 degree diagonal boards. Some stair and border details need tighter support.
What does on centre mean?
On centre (o.c.) is the distance measured from the centre of one joist to the centre of the next.
Can deck joists be 24 inches apart?
Only for materials rated for it, such as some thick hardwood boards, and only where local code allows. Many decking products and many jurisdictions limit spacing to 16 inches.
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