Deck Joist Span Chart: Joists, Beams & Post Spacing

Deck joist span chart

Find your lumber’s species group and joist size, then read across to the spacing you plan. The code limits the spacing by the decking you lay on top, and the spans assume No. 2 grade lumber with a 10 psf dead load. You may interpolate between spacings, but not go past the table.

Deck beam span chart

The beam carries the joists’ outer ends, and its span is the distance between the posts under it. Read the beam table by the effective joist span, not the actual one: multiply the joist span by a factor that grows with the cantilever past the beam.

The same beam, with and without a cantilever

Take joists spanning 12 ft from the ledger to the beam with no cantilever. The effective joist span is 7.9 ft, so read the 8 ft column, where a double 2x10 southern pine beam spans 9 ft. Let the joists run 2 ft past the beam and the ratio is 1/6, the factor 0.90 and the effective span 10.8 ft; in the 12 ft column the same beam spans only 7 ft 4 in. The overhang puts more of the deck on the beam, so the posts move closer together.

Fasten the plies of a built-up beam with two rows of 10d nails at 16 in on center along each edge. A beam may cantilever past its end posts by up to one-fourth of its span, and a flush beam must be at least as deep as the joists that frame into it.

Deck post spacing and height

Post spacing is the beam span: set the posts no farther apart than the beam table allows for your beam and effective joist span. How tall a post may be depends on its size and on how much deck it holds up, its tributary area. Height runs from the underside of the beam to the top of the footing or pier.

Posts that stand on concrete footings need lateral restraint: a manufactured connector, or at least 12 in of embedment in the surrounding soil or a concrete pier.

Joist spacing for composite decking

The code sets no joist spacing for composite boards. Each board, or its packaging, carries a label with its allowable load and maximum span under ASTM D7032, and the boards go down to the maker’s instructions. Trex asks for joists no more than 16 in on center, and 12 in when the boards run diagonally. TimberTech’s limit is also 16 in, with 12 in for diagonal boards, except its MAX boards at up to 24 in.

Wood decking has its own limits in the code, set by board thickness and direction:

Each wood board is fastened to every joist with two 8d threaded nails or two No. 8 wood screws.

Deck with 2x6 joists

2x6 joists are the shortest reach in the table. At 16 in on center they span 9 ft in southern pine, 8 ft 4 in in Douglas fir-larch, hem-fir or spruce-pine-fir, and 8 ft in redwood and the cedars and pines, which suits a small or low deck.

They also cantilever the least: at most 1 ft 6 in, and none at all once the back span passes 8 ft.

Joist cantilever

Joists may run past the beam, but the overhang is limited by the back span, the length of joist between its supports. The limits come from a deflection check with a 220-pound load at the tip; where the table says not permitted, that size cannot cantilever on that back span.

Joist spans for snow loads

Where the ground snow load is 50, 60 or 70 psf, the code gives its own spans; snow and live load are not added together. The table shows joists at 16 in on center.

Bearing and joist hangers

A joist end needs at least 1½ in of bearing on wood or metal and 3 in on concrete or masonry. Joists that frame into the side of a beam or ledger sit in approved joist hangers.

Deck joist span FAQ

Sources & method

1. International Code Council. 2021 IRC Chapter 5 Floors, Section R507 Exterior Decks. 2021 IRC
2. Trex Company. How to Install and Space Deck Joists for Composite Decking. retrieved Nov 14, 2026
3. TimberTech. Deck Joist Spacing & Blocking. retrieved Nov 14, 2026