How far apart should deck joists be?
16" on centre for most decking and 12" for a diagonal layout — the decking decides it, not the joists. What each board type carries, and joist counts by deck size.
16" on centre suits composite and 5/4 decking; 24" is the limit for a full 1 1/2" board.
A 12' × 16' deck at 16" centres takes 13 joists of 12 ft — 156 linear feet of framing.
How that is worked out
- The decking sets the spacing, not the joists. What matters is how far the board you are walking on can span without flexing underfoot.
- A thinner board spans less: Composite (5 1/2" wide) and 5/4 stock want 16" centres, while a full 1 1/2" board manages 24".
- Laying the boards diagonally makes every span longer by about 1.4×, so a deck that would take 16" centres straight needs 12" for a diagonal.
Joists by deck size, at 16" on centre
| Deck | Joist length | Joists | Linear feet of joist | At 12" centres |
|---|---|---|---|---|
| 8' × 10' | 8 ft | 8 | 64 ft | 11 |
| 10' × 12' | 10 ft | 10 | 100 ft | 13 |
| 12' × 12' | 12 ft | 10 | 120 ft | 13 |
| 12' × 16' | 12 ft | 13 | 156 ft | 17 |
| 14' × 20' | 14 ft | 16 | 224 ft | 21 |
| 16' × 20' | 16 ft | 16 | 256 ft | 21 |
Worth knowing
- This is about what the *decking* spans, which is the question people are usually asking. How far the joists themselves can span between beams is a different question entirely, answered by a span table for your species, grade and load — and it is a structural one.
- The decking manufacturer's figure beats any general rule, especially for composite. Some composites want 12" centres even for a straight layout, and going to 16" voids the warranty as well as bouncing underfoot.
- A diagonal layout makes every decking span about 40% longer, so the joists have to come closer. Plan it before framing — it is an expensive thing to discover after the joists are hung.
- Stair treads and anywhere a hot tub or planter sits are separate problems. Both are concentrated loads that a deck framed to walk on was not designed for.
Work out your own numbers
- Deck and fence takeoff
Boards, joists, posts, pickets, screws and bags of concrete, with the gaps between boards properly counted.
- Fastener calculator
Screws or nails for a sheet good on an edge-and-field pattern, with the box count to buy.
Related questions
- Can deck joists be 24 inches apart?
- Only with full 2x decking, and even then it is the outer limit rather than a good idea — a 2x6 deck board at 24" centres has noticeable bounce. 16" is the normal spacing and what nearly all 5/4 and composite decking is rated for.
- Why do composite decks need closer joist spacing?
- Because composite is less stiff than wood for the same thickness — it sags between supports where a wood board would not, and it gets softer as it warms in the sun. Many makers specify 16" for straight layouts and 12" for diagonal, and their figure governs both the performance and the warranty.
- How many joists do I need for a 12x16 deck?
- 13 at 16" centres, running the 12-foot way across a 16-foot deck. The count is the run divided by the spacing plus one, since there is a joist at both ends. Blocking between them at mid-span is extra and worth doing — it stops the joists twisting and stiffens the whole frame.