Deck joists are the parallel boards that carry the decking. Two numbers describe them: the spacing (how far apart they are, measured centre to centre) and the span (how far each joist reaches between supports — from the ledger on the house to the beam, or from beam to beam). The two are linked: the closer the joists, the less load each one carries and the farther it can span. Size, species and grade complete the picture.
Start with the decking
The decking decides the maximum spacing before structure does. Boards must be stiff enough not to bounce between joists:
- Composite and PVC decking: most brands require 16″ on center for residential decks with boards perpendicular to the joists, and 12″ when boards run diagonally. Some heavier profiles allow 24″. The product's installation guide is the rule — and its warranty depends on it.
- Wood decking: 5/4 boards are thinner than 2× boards and need closer joists. Diagonal decking spans farther between joists along the board, so it needs closer spacing too. Check your local code's decking table.
- Each board must bear on at least three joists (or blocking) according to the IRC-based Alexandria deck details — so no short board should bridge only two.
In practice 16″ on center is the default for almost every deck. 12″ is used for diagonal patterns, heavy loads and bouncy composite boards; 24″ is used with 2× wood decking on simple decks.
Joist span table (reference)
The table below reproduces the maximum joist spans from a published, IRC-based deck guide. It applies only to its stated basis: southern pine No. 2, 40 psf live load plus 10 psf dead load, wet service and L/360 deflection. Other species (Douglas fir-larch, hem-fir, SPF, redwood, cedar) span less, and areas with high snow loads need larger joists.
| Joist | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|
| 2×6 | 9′ 11″ / 6′ 8″ | 9′ / 6′ 8″ | 7′ 7″ / 6′ 8″ |
| 2×8 | 13′ 1″ / 10′ 1″ | 11′ 10″ / 10′ 1″ | 9′ 8″ / 9′ 8″ |
| 2×10 | 16′ 2″ / 14′ 6″ | 14′ / 14′ | 11′ 5″ / 11′ 5″ |
| 2×12 | 18′ / 18′ | 16′ 6″ / 16′ 6″ | 13′ 6″ / 13′ 6″ |
Spans are measured between the centrelines of the bearings: from the ledger to the centre of the beam for an attached deck. Notice the shape of the table. Going from 16″ to 12″ spacing adds roughly 1 to 2 feet of span; going one lumber size deeper adds about 2 to 3 feet. For a 12 ft deep attached deck, 2×8 at 16″ (11′ 10″) is just short, while 2×10 at 16″ (14′ 0″) works with room to spare.
Overhangs (cantilevers)
Joists can cantilever past the beam, which lets you pull the beam and posts in from the edge. The same guide allows joists to overhang past the centre of the beam by up to one quarter of the actual joist span, and lists a separate "with overhang" span limit because the cantilever loads the back span differently. A 12 ft joist span can overhang up to 3 ft. The calculator warns if your overhang exceeds a quarter of the span.
Beams and posts
The beam carries the outer end of the joists; posts carry the beam. Beam span — the distance between posts — depends on the beam size and on the joist span, because longer joists put more load on the beam. From the same source, a two-ply 2×10 beam may span 9′ 0″ between posts when it supports joists spanning up to 8 ft, but only 7′ 4″ when the joists span 12 ft. Beams can overhang past the end posts by up to a quarter of the beam span. Post size, footing size and depth, and the ledger connection to the house have their own requirements — footing size in particular depends on soil and frost depth where you live.
How many joists?
Joist count follows the same rule as wall studs: deck width ÷ spacing, rounded up, plus one. A 16 ft wide deck at 16″ on center needs 192 ÷ 16 = 12, plus 1 = 13 joists, including the two end joists. At 12″ on center the same deck needs 17. Add a rim joist across the outer ends, a ledger on the house for an attached deck, and blocking between the joists over the beam or at mid-span to stop them twisting.
A quick planning sequence
- Pick the decking and read its required joist spacing.
- Decide where the beam goes and work out the joist span (and any overhang).
- Choose a joist size whose tabulated span covers it for your species and spacing.
- Choose a beam size and post spacing from the beam table for that joist span.
- Take the plan to your building department — most decks need a permit, and inspectors check the ledger, footings and guards closely.
The deck calculator runs these checks against the cited table, draws the framing plan and lists joists, beams, posts, boards, screws and railing parts. It is a planning tool: the final design must follow your local code or a licensed professional.
Planning information only — framing and deck requirements vary by jurisdiction. Verify with your local building code or a licensed professional.