Deck Joist Spacing: 12, 16 or 24 Inches?
Spacing by decking
| Decking | Straight | Diagonal (45°) |
|---|---|---|
| 5/4 x 6 wood (1 in actual) | 16 in | 12 in |
| 2 x 6 wood (1.5 in actual) | 24 in max, 16 in better | 16 in |
| Composite, most brands | 16 in | 12 in |
| PVC and slimmer composite profiles | 12–16 in, per brand | 12 in |
| Hardwood 1x6 | 16 in | 12 in |
The governing number is the span rating published by your decking manufacturer — composite brands differ, and a warranty claim starts with whether the joist spacing matched the chart.
Why diagonal decking needs closer joists
A board laid at 45° across joists at 16 in centres actually spans about 22.6 in between bearings — the diagonal of the square, not its side. The board does not care what the joist layout says; it cares about the unsupported distance. Dropping the joists to 12 in brings the diagonal span back to about 17 in, which is what the decking was rated for.
That is also why the deck calculator's waste allowance should go to 15% for diagonal layouts: every board end is a mitre cut, and the offcuts rarely reuse.
What closer spacing fixes — and what it does not
- Fixes: bounce and sag between joists. Decking stiffness between supports scales strongly with span, so 12 in centres feels noticeably more solid underfoot than 16, especially with composite, which is less stiff than wood of the same size.
- Fixes: fastener-line sag on composite left in sun — long spans let hot boards droop between screws.
- Does not fix: an under-sized frame. Joist spacing is about the decking. The joists themselves have their own span limits by size and species — a 2x8 at 16 in centres can only run so far before it needs a beam. Closer spacing of too-small joists does not buy much; deeper joists or another beam does.
- Does not fix: soft footings. No spacing decision compensates for posts on dirt.
Cost of the upgrade
Going from 16 to 12 in centres on a 16 ft deck adds four joists — from 13 to 17 in the deck calculator's worked example, about a 30% increase in joist lumber and hangers, plus a third more screws in the decking. On the whole-deck budget that is a small premium, which is why "12 in centres under composite" is such common advice: it is cheap insurance against the most common composite complaint, a spongy feel.
Layout details that matter as much as the number
- Crown up. Sight down each joist and install with the bow upward; load flattens it.
- Consistent layout from one end — mark the ledger and the beam at the same time so joists are square, or every decking row lands off-centre on its supports.
- Butt joints need bearing. Wherever two decking boards meet end to end, there must be a joist under the joint — doubled, or with a sister block, so each end gets a full inch of bearing and its own fasteners.
- Blocking at mid-span stiffens long joists and stops them rolling, and it is required by many codes above certain spans.
Enter the deck size and your board width to get the board count, the linear footage to order, the joists underneath and the screws to fix it all down — with the gap between boards included, which is what most quick estimates forget.
Frequently asked questions
Is 16 inch joist spacing OK for composite decking?
For most brands laid straight, yes — 16 inches on centre is the standard listed spacing. Diagonal installations typically require 12 inches, and some slimmer profiles require 12 regardless. The manufacturer span chart is the authority, and the warranty depends on following it.
Why does diagonal decking need 12 inch centres?
Because a board crossing the joists at 45 degrees spans the diagonal, about 1.4 times the joist spacing. On 16 inch centres that is nearly 23 inches of unsupported board — more than the decking is rated for. Twelve inch centres brings the diagonal span back to about 17 inches.
Can deck joists be 24 inches apart?
Only under full 2x thick wood decking, and even then the deck feels noticeably bouncier than at 16 inches. Composite and 5/4 boards will sag at 24. It is a spacing to design out rather than into.
Does closer joist spacing let me use smaller joists?
Not meaningfully for the joists themselves — their allowable span is set by depth and species and improves only modestly at closer spacing. Closer spacing is for the decking above; the frame below is sized by its own span tables.