Easy Wood Calc

How far can a 2x8 deck joist span?

Short answer

A 2×8 deck joist at 16 inches on center can span up to 11 feet 10 inches (11 ft 10 in) in Southern pine under the IRC’s 40 psf live load. In Douglas fir-larch, hem-fir or SPF the limit is 11 feet 1 inch, and in redwood or western cedar it is 10 feet 7 inches.

How to work it out

  1. Table R507.6, 40 psf live load, Southern pine, 2×8 at 16 in on center → maximum span 11 ft 10 in (3.61 m)
  2. Margin = maximum − your span = 11 ft 10 in − 11 ft = 10 in → span OK
  3. 12 ft = 144 in ÷ 16 in = 9 → 9 spaces + 1 = 10 joists

Two-by-eights are the usual joist for a low deck that sticks out about 10 to 12 ft from the house. At 16 in on center the whole question comes down to species: Southern pine gets 11 ft 10 in, the fir group 11 ft 1 in, and the redwood and cedar group only 10 ft 7 in — a 15 in spread between the strongest and weakest lumber for the same board size.

Adjust for your project

Can’t read the stamp? “Not sure” uses the shortest of the three spans — the safe pick.
On center = middle of one joist to middle of the next. Not listed (e.g. 19.2 in)? Pick the next wider option.
ft
The open gap the joist crosses: ledger (the board on the house) to the beam, or beam to beam. Don’t count overhang. 11' 6" works.
in
How far the joists run on past the beam. Leave at 0 if they stop at the beam.
More options: Load · Deck
Leave at 40 unless you get heavy snow. Your building department (or a search for “[your county] ground snow load”) gives the number. (1 psf ≈ 0.048 kPa.)
ft
For a joist count: one joist per spacing plus one at the far end.
Will your joists work?Yes — 10 in to spare on the span
Maximum allowable span (IRC Table R507.6)11 ft 10 in
Joists along the deck
10 joists
  • If your area’s ground snow load is over 40 psf, pick that load under More options — the spans get shorter.
Check your codeCode limits vary by jurisdiction — confirm with your building department before you cut or build. (IRC R507.6)

For this job · affiliate link
25 ft tape measureRe-measure the span face to face at both ends of the ledger before you order lumber — ledgers and beams are rarely perfectly parallel.
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2×8 at 16 in on center, by species group

2×8 at 16 in on center, by species group
Species group (see the grade stamp)Maximum allowable span (IRC Table R507.6)Will your joists work?
Southern pine11 ft 10 inYes — 10 in to spare on the span
Douglas fir-larch, hem-fir, spruce-pine-fir11 ft 1 inYes — 1 in to spare on the span
Redwood, western cedars, ponderosa pine, red pine10 ft 7 inNo — the span is 5 in too long. Go up a size or close the spacing.

What to know about 2x8 deck joists at 16 inches

  • A 12 ft deck is right at the edge. When the beam sits a foot or two in from the outer edge, the clear span from ledger face to beam face usually lands around 10 to 11 ft — inside the Southern pine and fir limits, but tight for cedar at 10 ft 7 in. If your measured span is over the figure for your lumber, don’t round it down; move the beam out, close the spacing to 12 in, or step up to 2×10s.
  • Snow country shortens it. At a 50 psf ground snow load a Southern pine 2×8 at 16 in drops to 11 ft, and at 70 psf to 9 ft 10 in. Your building department can tell you which load applies where you live.
  • 5/4 decking is fine at this spacing as long as each board crosses at least three joists. A board that only bridges two joists — a short infill piece, say — needs joists at 12 in under it.
  • The overhang is extra, and limited. At the 11 ft span in the calculator above, Table R507.6 lets a Southern pine 2×8 run about 2 ft 4-1/2 in past the beam — the table lists 2 ft 6 in at a 10 ft back span and 2 ft 3 in at 12 ft, and you interpolate between them. A cedar or redwood 2×8 at the same span gets none: its 12 ft column is NP (not permitted).

Also check the grade stamp on the pile you bought: a stack labelled “2×8 treated” at the store can be hem-fir in one region and Southern pine in another, and the chart only gives you the longer span if the stamp says so. If you are weighing the next size up, compare with How far can a 2x10 deck joist span. With the framing settled, How many deck boards for a 12x12 deck counts the surface boards and screws for a deck this size.

Different size? Use the full Deck Joist Span Calculator.

What you’ll need

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Frequently asked questions

Can 2x8 deck joists span 12 feet?

Not at 16 in on center — the longest 2×8 span there is 11 ft 10 in (Southern pine). At 12 in on center, Southern pine reaches 13 ft 1 in and the fir group 12 ft 6 in, so a 12 ft clear span works; redwood and cedar top out at 11 ft 8 in and do not. Otherwise use 2×10s.

How far can a 2x8 deck joist span at 12 inches on center?

In Southern pine, 13 ft 1 in (13 feet 1 inch) under a 40 psf live load — 12 ft 6 in in the fir group and 11 ft 8 in in redwood or cedar. Closer spacing means more joists: a 12 ft ledger takes 13 joists at 12 in against 10 at 16 in.

How far can a 2x8 deck joist span at 24 inches on center?

In Southern pine, 9 ft 8 in (9 feet 8 inches) at a 40 psf live load. At 24 in on center the decking must be 2 in thick wood (R507.7) — 5/4 boards are not allowed at that spacing.

How far can a 2x8 deck joist cantilever past the beam?

With an 11 ft back span in Southern pine, up to 2 ft 4-1/2 in under a 40 psf live load, so a 24 in overhang passes (10 in to spare on the span, 4-1/2 in to spare on the overhang). At an 8 ft back span the limit is 2 ft in every species group.

How far can a 2x8 cedar or redwood deck joist span at 16 inches?

10 ft 7 in (10 feet 7 inches) for the redwood, western cedar, ponderosa pine and red pine group under a 40 psf live load. That table value does not include an incising reduction.

Other common sizes

Sources