The span table, 40 psf live load

Allowable joist spans in feet and inches for No. 2 lumber with a 10 psf dead load (IRC Table R507.6):

Joist Southern pine, 12” o.c. Southern pine, 16” o.c. Southern pine, 24” o.c.
2x6 9’ 11” 9’ 0” 7’ 7”
2x8 13’ 1” 11’ 10” 9’ 8”
2x10 16’ 2” 14’ 0” 11’ 5”
2x12 18’ 0” 16’ 6” 13’ 6”
Joist Douglas fir-larch, hem-fir, spruce-pine-fir, 12” o.c. 16” o.c. 24” o.c.
2x6 9’ 6” 8’ 4” 6’ 10”
2x8 12’ 6” 11’ 1” 9’ 1”
2x10 15’ 8” 13’ 7” 11’ 1”
2x12 18’ 0” 15’ 9” 12’ 10”
Joist Redwood, western cedars, ponderosa and red pine, 12” o.c. 16” o.c. 24” o.c.
2x6 8’ 10” 8’ 0” 6’ 10”
2x8 11’ 8” 10’ 7” 8’ 8”
2x10 14’ 11” 13’ 0” 10’ 7”
2x12 17’ 5” 15’ 1” 12’ 4”

The span is the joist’s length between supports, from the ledger or beam to the next beam. Where the ground snow load is 50 or 60 psf, the same table gives shorter spans; a 16 inch southern pine 2x10, for example, drops to 13’ 9” at 50 psf and 12’ 9” at 60 psf.

Joist spacing and the decking

The decking limits how far apart the joists can be (IRC Table R507.7):

Decking Square to the joists, 2 joists Square, 3 or more joists Diagonal, 2 joists Diagonal, 3 or more
1 1/4 inch wood (5/4) 12” 16” 8” 12”
2 inch wood 24” 24” 18” 24”

A deck board resting on two joists is a single span; on three or more, a multiple span. Diagonal boards can run up to 45 degrees from square.

Cantilevers and bearing

  • Cantilever: joists can run past the beam by the table’s cantilever limits, which depend on the joist and its back span: a southern pine 2x10 with an 8 foot back span can cantilever 2 feet, and the table marks the longest back spans as not permitted for a cantilever (IRC Table R507.6).
  • Bearing: each joist end needs at least 1 1/2 inches of bearing on wood or metal and 3 inches on concrete or masonry; joists framed into the side of a beam or ledger hang in approved joist hangers (IRC R507.6.1).

Counting the joists

Joists are spaced along the ledger, plus one at the far end: a deck 16 feet along the house at 16 inches on center takes 16 × 12 ÷ 16 + 1 = 13 joists. The board foot calculator turns the lumber list into board feet for pricing.

Common questions

How far can a 2x8 deck joist span?

At 16 inches on center, 11’ 10” in southern pine and 11’ 1” in Douglas fir-larch, hem-fir or spruce-pine-fir, for a 40 psf live load (IRC Table R507.6).

How far can a 2x10 deck joist span?

At 16 inches on center, 14’ 0” in southern pine and 13’ 7” in Douglas fir-larch, hem-fir or spruce-pine-fir.

What is the standard deck joist spacing?

The spacing the decking allows: up to 16 inches for 5/4 wood boards square to the joists across three or more joists, 24 inches for 2 inch boards (IRC Table R507.7).

Does a deck need a permit?

The local building department decides what needs a permit; ask it before building.