Deck joist span calculator and chart

Joist = the boards that run under the decking. New to this? Words explained.

How to read a deck joist span

A joist span is the clear distance the joist bridges between its supports: from the face of the ledger (the board bolted to the house) or rim to the face of the beam. Any part of the joist that runs past the beam is a cantilever, and it is not part of the span. Measure the span, then pick the column for your lumber — the species group is printed on the grade stamp — and the row for your joist size and spacing. If your spacing falls between two columns, use the wider spacing, which gives the shorter span.

Deck joist span chart (40 psf live load)

What the table assumes

• No. 2 grade lumber. That is the usual grade for treated deck framing. Check the stamp on every joist, not just the first one.
• Wet service. The spans already allow for the wood being outdoors and damp.
• A 10 psf dead load (the weight of the framing and decking) on top of the live load or snow load. Heavy extras such as a hot tub, stone pavers or a planter wall are outside the table.
• A deflection limit of L/Δ = 360. Under full load, the joist may sag no more than its span divided by 360 — about 3/8 in on an 11 ft 10 in span.
• Single-span joists, measured face to face of the supports.
• Incising. The Douglas fir-larch, hem-fir and SPF spans include the strength reduction for incising (the small slits cut into treated lumber so the preservative soaks in). The redwood, cedar, ponderosa and red pine spans do not — if your lumber from that group is incised, ask your building department how to treat it.
• Not sure of the species? The redwood and cedar group has the shortest span in almost every row, but not all: at 70 psf, a 2×6 at 24 in reaches 5 ft 10 in in that group and only 5 ft 9 in in the fir group. The calculator’s “Not sure” option takes the shortest of the three for your exact row, so it is safe either way.

How far joists can overhang the beam (cantilever)

Joists may run on past the beam, but Table R507.6 caps that overhang by the back span — the span behind the beam, which is the span you enter above. At short back spans the limit is a quarter of the back span (1 ft at 4 ft, 1 ft 6 in at 6 ft, 2 ft at 8 ft for most rows). Longer back spans leave the joist less reserve, so the limit levels off, can drop, and becomes NP (not permitted) near the joist’s span limit. It doesn’t change with joist spacing. Between the listed back spans you may interpolate; outside them you may not — the calculator does both for you.

Snow loads

Section R507.1 has you design a deck for the 40 psf live load or your area’s ground snow load, whichever is greater. The table does not assume snow and people at full load at the same time. Above 40 psf, pick the snow-load row — spans drop by up to a couple of feet:

Limits the span table does not cover

• No extrapolation. Spans longer than the table, overhangs with back spans outside 4–18 ft, loads above 70 psf and other lumber sizes need an engineered design. Don’t stretch a number from the chart.
• Decking sets the spacing too (R507.7). 5/4 (1-1/4 in) wood decking laid square to the joists needs joists at 16 in on center or closer where each board crosses three or more joists, and 12 in where it only spans between two. 2 in wood decking allows 24 in. Diagonal decking needs closer spacing, and composite or plastic decking follows the span on its label.
• Beams, ledgers, posts and footings have their own requirements in Section R507. A joist that passes here says nothing about what holds it up.
• Local rules win. Many jurisdictions amend the IRC, and an engineered design or a manufacturer’s engineered joist replaces this table entirely.

Common span mistakes

The costly mistakes are reading the wrong column: using the Southern pine figure for hem-fir or cedar lumber, using the 12 in figure while framing at 16 in, or forgetting that a snowy area needs the snow row. Measuring to the centre of the beam or to the deck’s outer edge only makes your number longer, which errs on the safe side. Two worked examples show where the common sizes land: How far can a 2x8 deck joist span and How far can a 2x10 deck joist span. Once the framing is settled, count the surface boards — for example How many deck boards for a 12x16 deck.

Older span charts can disagree. Design values for visually graded Southern pine were lowered in 2013, and the IRC deck joist table has been reworked since (this page uses Table R507.6 of the 2021 IRC). Charts from the 2015 IRC and earlier, older deck guides and how-to books can show longer Southern pine spans than the figures here. Use the table from the code edition your building department enforces, and check your local code.