Deck joist spacing: 12, 16 or 24 inches on center

Spacing is decided by the decking

The decking is a beam that spans from joist to joist, so the boards you choose limit how far apart the joists can be. Three cases cover most decks:

• Composite and PVC decking: most manufacturers require 16 inches on centre for boards laid square to the joists, and 12 inches for boards laid on a 45 degree diagonal. Some products require closer spacing on stair treads. Always read the installation sheet for the exact board, because the warranty depends on it.
• Nominal 5/4 wood decking: the residential code table (IRC R507) allows 16 inches on centre for boards perpendicular to the joists and 12 inches for diagonal.
• Nominal 2x wood decking: the same table allows 24 inches on centre perpendicular and 16 inches diagonal.

Check your local code edition, because jurisdictions adopt different versions. If you are re-decking an older frame built at 24 inches, read the repair or replace guide: moving to composite usually means adding joists between the existing ones.

How many joists each spacing needs

The count is the deck length divided by the spacing, rounded up, plus one for the last joist. Here it is for common lengths, measured along the ledger:

Going from 16 to 12 inches on a 16 ft deck adds four joists, plus four hangers. Going from 24 to 16 adds four as well. The extra framing is a small share of the total job, and it buys a stiffer, bounce-free surface. The deck joist calculator counts them with linear feet and cost.

Spacing is not the same as span

Spacing is how far apart the joists are. Span is how far each joist reaches between its supports, and it is limited by the joist size, the species and grade of lumber, and the spacing. Closer spacing lets a given joist span a little further, because each one carries less of the load. Use the joist span chart or the deck joist span calculator to check a size against the span you need, and size the beam from the beam span chart.

If a joist is too long for its size, closing the spacing is not the first fix. Add a beam partway out, which shortens the span and usually lets you drop a joist size.

Special cases that call for 12 inches

• Diagonal board patterns, as above. The board patterns guide shows what each layout needs.
• Picture-frame borders and breaker boards, which need blocking or a doubled joist underneath whatever the spacing.
• A hot tub or other concentrated load, which needs extra framing: see hot tub on a deck.
• Heavy surface loads such as tile or stone pavers on a pedestal system.

Layout tips

Mark the joist positions on the ledger and the beam before you cut anything, and keep every joist on the same centre line so the decking fastens cleanly. Start the layout from one end so any short bay lands at the far end, where it is hidden by the rim joist. Hangers go on the ledger side, and the deck fastener calculator counts the screws or clips for the decking above.