Joist count by deck dimension
Use this table for preliminary layout and material planning. It shows approximate joist counts across the listed deck dimension at three common on-center spacings.
| Dimension crossed | 12 in O.C. | 16 in O.C. | 24 in O.C. |
|---|---|---|---|
| 8 ft | 9 joists | 7 joists | 5 joists |
| 10 ft | 11 joists | 9 joists | 6 joists |
| 12 ft | 13 joists | 10 joists | 7 joists |
| 14 ft | 15 joists | 12 joists | 8 joists |
| 16 ft | 17 joists | 13 joists | 9 joists |
| 18 ft | 19 joists | 15 joists | 10 joists |
| 20 ft | 21 joists | 16 joists | 11 joists |
| 24 ft | 25 joists | 19 joists | 13 joists |
What does on-center joist spacing mean?
On-center spacing is the distance from the centerline of one joist to the centerline of the next. A 16-inch O.C. layout repeats joist centerlines every 16 inches; it does not leave a 16-inch clear gap between the boards.
Joist spacing vs. joist span vs. joist size
These three measurements describe different parts of the framing system. Joist spacing is the distance between neighboring joists. Joist span is the unsupported distance a joist runs between supports. Joist size refers to the member dimensions, such as a nominal 2×8 or 2×10. A spacing that works for one combination of size, species, grade and span may not work for another.
What affects how far apart deck joists can be?
Allowable spacing depends on the complete framing and decking system, not just deck width. Important factors include joist species and grade, joist size, unsupported span, design loads, beam and ledger configuration, connections, local requirements and the decking manufacturer's installation instructions.
12, 16 and 24 inches O.C.
These are common layout intervals used in deck planning, but they should be treated as layout choices to evaluate—not universal recommendations. Closer spacing increases the number of joists and can provide more frequent support for decking, while wider spacing uses fewer joists but may not be permitted by the selected decking product or structural design.
Decking material can control maximum joist spacing
Wood, composite and PVC decking products can have different maximum-support-spacing requirements. Some products permit one spacing when installed perpendicular to the joists and require closer spacing when installed diagonally. Check the current installation instructions for the exact decking product before finalizing the framing layout.
Diagonal decking and joist spacing
Installing deck boards at an angle changes how each board crosses the framing and can increase the unsupported distance between joists along the board. Because of that, diagonal layouts may require closer joist spacing. Use the decking manufacturer's diagonal-installation requirements rather than assuming the perpendicular-layout spacing still applies.
How to choose a joist layout safely
Start with the deck dimensions and decking product, then identify the joist span and candidate joist size. Check applicable span tables, product installation instructions and local requirements for the exact material and loading conditions. Use this page and the PlanDIYBuild calculators for planning quantities and layout—not as a substitute for structural design or required approvals.
Calculate your deck layout
Enter your project dimensions and selected on-center spacing to visualize joists and estimate decking, fasteners and preliminary footings.
Open Deck Materials Calculator →See the Deck Planning Hub for the dedicated joist, screw and footing calculators.
For a focused framing estimate, use the Deck Joist Calculator with your selected spacing and joist length.
Frequently asked questions
How far apart should deck joists be?
Deck joists are commonly laid out at regular on-center spacing such as 12, 16 or 24 inches, but the correct spacing for a specific deck depends on joist size, species and grade, span, loads, decking product, installation angle and applicable requirements.
What does 16 inches on center mean for deck joists?
It means the centerline of one joist is 16 inches from the centerline of the next joist. It does not mean there is a 16-inch clear gap between joists.
Is joist spacing the same as joist span?
No. Joist spacing is the distance between adjacent joists, while joist span is the unsupported distance a joist runs between supports such as a beam and ledger or between beams.
Does diagonal decking affect joist spacing?
It can. Decking installed diagonally may require closer joist spacing than boards installed perpendicular to the joists. Always follow the decking manufacturer's installation requirements.