Proper Estimate

Deck joist span calculator

Enter how far your joists need to span and how far apart they'll be. You get the smallest joist that works, from the deck joist table in the International Residential Code.

Joists

Ledger to beam, not counting any overhang

Joist spacing
Wood

Most pressure-treated lumber in the East and South is southern pine. Check the grade stamp.

Along the house, to count the joists

Smallest joist that works

2x10

spans up to 14 ft 0 in at 16 in

Maximum span by size

  • 2x69 ft 0 in Too short
  • 2x811 ft 10 in Too short
  • 2x1014 ft 0 in Works
  • 2x1216 ft 6 in Works
  • Joists needed13

IRC 2021 Table R507.6: No. 2 grade, wet service, 40 psf live or snow load, 10 psf dead load, no cantilever.

Deck joist span tables

Maximum joist spans in feet and inches, between supports, with no cantilever. From IRC 2021 Table R507.6, for No. 2 grade lumber in wet service, 40 psf live load or ground snow load, 10 psf dead load and L/360 deflection.

Southern pine
Joist 12 in apart 16 in apart 24 in apart
2x6 9 ft 11 in9 ft 0 in7 ft 7 in
2x8 13 ft 1 in11 ft 10 in9 ft 8 in
2x10 16 ft 2 in14 ft 0 in11 ft 5 in
2x12 18 ft 0 in16 ft 6 in13 ft 6 in
Douglas fir-larch, hem-fir, spruce-pine-fir
Joist 12 in apart 16 in apart 24 in apart
2x6 9 ft 6 in8 ft 8 in7 ft 2 in
2x8 12 ft 6 in11 ft 1 in9 ft 1 in
2x10 15 ft 8 in13 ft 7 in11 ft 1 in
2x12 18 ft 0 in15 ft 9 in12 ft 10 in
Redwood, western cedars, ponderosa pine, red pine
Joist 12 in apart 16 in apart 24 in apart
2x6 8 ft 10 in8 ft 0 in7 ft 0 in
2x8 11 ft 8 in10 ft 7 in8 ft 8 in
2x10 14 ft 11 in13 ft 0 in10 ft 7 in
2x12 17 ft 5 in15 ft 1 in12 ft 4 in

How to read the span

The span is the clear distance a joist covers between supports: from the ledger on the house to the beam, or from one beam to the next. If you need more span than a 2x12 gives, add a beam to split the span in two, or move the joists closer together.

Joists can usually run past the beam as a cantilever, but the overhang is limited too, and for smaller sizes the allowed span gets shorter when you cantilever. The cantilever columns of Table R507.6 have the numbers.

Spacing and decking

Closer joists let you use smaller lumber and make the deck feel stiffer. Most composite and PVC decking needs joists 16 in apart or closer, and about 12 in for diagonal boards. Check the decking maker's install guide before choosing 24 in.

Snow loads

The spans assume 40 psf. Where the ground snow load is higher, the 2021 IRC gives shorter spans for 50, 60 and 70 psf. Your building department can tell you the design snow load where you are.

Questions

How far can a 2x8 deck joist span?

At 16 in spacing: 11 ft 10 in for southern pine, 11 ft 1 in for Douglas fir-larch, hem-fir or spruce-pine-fir, and 10 ft 7 in for redwood, western cedar, ponderosa or red pine. At 12 in spacing, southern pine reaches 13 ft 1 in; at 24 in, 9 ft 8 in.

How far can a 2x10 deck joist span?

At 16 in spacing: 14 ft 0 in for southern pine, 13 ft 7 in for Douglas fir-larch, hem-fir or spruce-pine-fir, and 13 ft 0 in for redwood, western cedar, ponderosa or red pine. At 12 in spacing, southern pine reaches 16 ft 2 in; at 24 in, 11 ft 5 in.

How far can a 2x12 deck joist span?

At 16 in spacing: 16 ft 6 in for southern pine, 15 ft 9 in for Douglas fir-larch, hem-fir or spruce-pine-fir, and 15 ft 1 in for redwood, western cedar, ponderosa or red pine. At 12 in spacing, southern pine reaches 18 ft 0 in; at 24 in, 13 ft 6 in.

How many joists do I need?

Divide the deck length in inches by the spacing, round up, and add one for the end. A 16 ft deck at 16 in: 16 × 12 ÷ 16 = 12, plus 1 = 13 joists. Doubled joists under splices or around stairs come on top.

Is the span the same as the deck width?

Not always. The span is the distance between supports, from the ledger to the beam or from beam to beam. If the joists overhang the beam, the overhang isn't part of the span, and it has its own limits.