Deck joist spacing: 12, 16 or 24 inches?
Two rules set your joist spacing: the framing table, which decides how far the joists can span, and your decking’s spec sheet, which decides how far apart they can be. The stricter one wins.
The short answer
- 16″ on center is the default for most residential decks, both wood and composite.
- 12″ on center for diagonal decking, many composite and PVC boards on stairs or commercial jobs, thinner boards, and when you want a stiffer floor or a longer span from a smaller joist.
- 24″ on center only with decking rated for it, usually 2×6 wood decking. Most 5/4 wood and composite boards are not.
What the code table allows
IRC Table R507.6 trades spacing against span. Closer joists each carry less deck area, so each can span farther. Southern pine, for example:
| Joist | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|
| 2x6 | 9′-11″ | 9′-0″ | 7′-7″ |
| 2x8 | 13′-1″ | 11′-10″ | 9′-8″ |
| 2x10 | 16′-2″ | 14′-0″ | 11′-5″ |
| 2x12 | 18′-0″ | 16′-6″ | 13′-6″ |
Going from 16″ to 12″ spacing buys about a foot of span for a 2x8, and adds one joist for every 4 ft of deck width. Going to 24″ cuts span by 15–20% and usually conflicts with the decking rules below. Other species and the cantilever limits are on the span tables page.
What your decking requires
Decking manufacturers publish a maximum joist spacing for each product, and installing beyond it can void the warranty. These are typical patterns. Your product’s instructions are what count:
- Composite and PVC: commonly 16″ o.c. maximum for residential decks laid perpendicular to the joists, and 12″ for diagonal layouts, stairs and some commercial uses.
- 5/4 wood decking: commonly 16″ o.c.
- 2× wood decking: often allowed at 24″ o.c. on straight layouts.
- Thin hardwoods (¾″): commonly 16″ o.c., sometimes 12″.
Diagonal decking
A board at 45° crosses the joists on the diagonal, so the clear distance it spans between supports is about 1.4 times the joist spacing. At 16″ o.c. that is roughly 22.6″ of board between supports, so manufacturers typically call for 12″ joists (about 17″ of diagonal span).
Laying out joists
- Mark the ledger and rim at the same time, starting from the same end, so every joist is square.
- Pull marks from the outside of the end joist, then shift each mark ¾″ so the tape reads to the joist’s centerline. Standard tape measures highlight 16″ marks for this.
- The last bay is usually short. That is normal. Never stretch a bay beyond the spacing.
- Add a doubled joist anywhere board seams will land, and under picture-frame borders.
The planner places joists, works out where your seams fall, and adds the doubled joists to the list.