Decking

Deck Joist Span Calculator

Find the smallest joist that covers the span you need, or the furthest a joist you already have can reach — straight from IRC Table R507.6, by species group, spacing and size, with the cantilever limit worked out. The complete deck joist span chart is below the tool if you just want to read the number off a table.

Smallest joist

2x10

spans 14 ft at 16" o.c.

2 ft of margin over your 12 ft

Cantilever up to 3 ft past the beam.

The working

Span needed12 ft (face of support to face of support)

Species groupSouthern Pine

Joist spacing16 in on centre

Smallest joist that reaches2x10 — table maximum 14 ft

Margin left over2 ft

Spacings that also work12 in, 16 in

Maximum cantilever past the beam25% of 12 ft = 3 ft

Planning estimate only. Check the result against current product instructions, local requirements, and your own takeoff. See the calculator disclaimer.

Span is measured from face to face

The single most expensive mistake on this table is measuring the wrong distance. Span means the clear distance from the face of one support to the face of the next — ledger face to beam face. Not centre of ledger to centre of beam, and not the overall width of the deck. On a doubled 2x10 beam that difference is around three inches, which is exactly the kind of margin that turns a legal 2x10 into an illegal one.

Species is not a detail

Deck joist tables are published per species group because the wood genuinely differs. A 2x10 at 16 in on centre spans 14 ft 0 in in Southern Pine, 13 ft 7 in in Douglas Fir-Larch or Hem-Fir, and 13 ft 0 in in Redwood or Western Cedars. That is a foot of difference on the same nominal board. Eastern US yards mostly stock treated Southern Pine; western yards more often carry Douglas Fir-Larch or Hem-Fir. The grade stamp on the board tells you which you have — read it rather than assuming.

The decking can be stricter than the framing

This table tells you what the joists can carry. It does not tell you what your decking will tolerate. Most composite and PVC decking requires 16 in on centre maximum, and drops to 12 in when boards run diagonally — well inside what a 2x10 could structurally span at 24 in. The decking manufacturer's instructions always tighten this table, never loosen it, so check them before you set your joist layout.

Why beams are not on this page yet

Beam spans live in IRC Table R507.5, and we have not published them here because the reproductions we could verify contradict each other — they disagree about whether Douglas Fir-Larch and Redwood share a column, they disagree on values, and one widely-cited version prints a beam span that decreases and then jumps in a way no real span table does. On a structural member holding up the whole deck, averaging two sources that disagree is not estimating, it is guessing. The joist spans on this page are published only because two independent reproductions of Table R507.6 matched on all thirty-six values.

Deck joist span chart — IRC Table R507.6

Maximum joist span, face of support to face of support, for No. 2 grade lumber at 40 psf live plus 10 psf dead load. Cantilever is separately limited to 25% of the adjacent span under IRC R507.6.1.

Southern Pine

Joist	12" o.c.	16" o.c.	24" o.c.
2x6	9 ft 11 in	9 ft	7 ft 7 in
2x8	13 ft 1 in	11 ft 10 in	9 ft 8 in
2x10	16 ft 2 in	14 ft	11 ft 5 in
2x12	18 ft	16 ft 6 in	13 ft 6 in

Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir

Joist	12" o.c.	16" o.c.	24" o.c.
2x6	9 ft 6 in	8 ft 8 in	7 ft 2 in
2x8	12 ft 6 in	11 ft 1 in	9 ft 1 in
2x10	15 ft 8 in	13 ft 7 in	11 ft 1 in
2x12	18 ft	15 ft 9 in	12 ft 10 in

Redwood, Western Cedars, Ponderosa Pine, Red Pine

Joist	12" o.c.	16" o.c.	24" o.c.
2x6	8 ft 10 in	8 ft	7 ft
2x8	11 ft 8 in	10 ft 7 in	8 ft 8 in
2x10	14 ft 11 in	13 ft	10 ft 7 in
2x12	17 ft 5 in	15 ft 1 in	12 ft 4 in

What this tool does not do

No beam sizing (IRC Table R507.5 — see above), no post or footing sizing, no ledger fastener schedule, and no snow or hot-tub loading. It covers No. 2 grade sawn lumber only — not LVL, not engineered I-joists, and not treated lumber graded below No. 2. Deck board quantities are a separate takeoff: use the deck board calculator once your framing layout is settled. It also stops at how many joists of what size — converting that framing list into the board feet a yard will quote against is a different unit again, and one where the nominal call-out rather than the dressed section drives the number: the board foot calculator handles that conversion. Stairs off the deck are governed by R311.7 rather than R507 and are laid out from the finished floor-to-floor rise, not from the joist layout — that geometry, including the 3/8 in riser tolerance that catches most flights, is in the stair calculator.

Sources

Grouped by manufacturer or publisher, then by document. Expand a group for the cited document, its verified-on date, and every value taken from it — all of it stays in the HTML either way.

ICC(1 document, 36 values)

IRC Table R507.6 — Deck Joist Spans for Common Lumber Species (2021 IRC), No. 2 grade, 40 psf live + 10 psf dead load, L/Δ = 360, wet service. Span is measured face of support to face of support, not centre to centre. ICC’s own viewer blocks automated retrieval, so the table is cited via reproduction; every value here was confirmed identical across two independent reproductions (up.codes and advantagelumber.com). A third (permitdeck.com) was rejected: it reported Southern Pine 2x12 at 24 in o.c. as 13-4 rather than 13-6 and mislabelled the Redwood/Western Cedars column as "SPF". · verified 2026-08-01
36 values from this document
- Southern Pine — 2x6 joist at 12 in o.c., maximum span: 119 inches
- Southern Pine — 2x6 joist at 16 in o.c., maximum span: 108 inches
- Southern Pine — 2x6 joist at 24 in o.c., maximum span: 91 inches
- Southern Pine — 2x8 joist at 12 in o.c., maximum span: 157 inches
- Southern Pine — 2x8 joist at 16 in o.c., maximum span: 142 inches
- Southern Pine — 2x8 joist at 24 in o.c., maximum span: 116 inches
- Southern Pine — 2x10 joist at 12 in o.c., maximum span: 194 inches
- Southern Pine — 2x10 joist at 16 in o.c., maximum span: 168 inches
- Southern Pine — 2x10 joist at 24 in o.c., maximum span: 137 inches
- Southern Pine — 2x12 joist at 12 in o.c., maximum span: 216 inches
- Southern Pine — 2x12 joist at 16 in o.c., maximum span: 198 inches
- Southern Pine — 2x12 joist at 24 in o.c., maximum span: 162 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x6 joist at 12 in o.c., maximum span: 114 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x6 joist at 16 in o.c., maximum span: 104 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x6 joist at 24 in o.c., maximum span: 86 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x8 joist at 12 in o.c., maximum span: 150 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x8 joist at 16 in o.c., maximum span: 133 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x8 joist at 24 in o.c., maximum span: 109 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x10 joist at 12 in o.c., maximum span: 188 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x10 joist at 16 in o.c., maximum span: 163 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x10 joist at 24 in o.c., maximum span: 133 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x12 joist at 12 in o.c., maximum span: 216 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x12 joist at 16 in o.c., maximum span: 189 inches
- Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir — 2x12 joist at 24 in o.c., maximum span: 154 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x6 joist at 12 in o.c., maximum span: 106 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x6 joist at 16 in o.c., maximum span: 96 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x6 joist at 24 in o.c., maximum span: 84 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x8 joist at 12 in o.c., maximum span: 140 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x8 joist at 16 in o.c., maximum span: 127 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x8 joist at 24 in o.c., maximum span: 104 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x10 joist at 12 in o.c., maximum span: 179 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x10 joist at 16 in o.c., maximum span: 156 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x10 joist at 24 in o.c., maximum span: 127 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x12 joist at 12 in o.c., maximum span: 209 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x12 joist at 16 in o.c., maximum span: 181 inches
- Redwood, Western Cedars, Ponderosa Pine, Red Pine — 2x12 joist at 24 in o.c., maximum span: 148 inches

IRC(1 document, 1 value)

IRC R507.6.1 — deck joist cantilever is limited to one-fourth of the actual adjacent joist span. This is a ceiling, not a licence: the table span must still be satisfied by the backspan itself. · verified 2026-08-01
1 value from this document
- Maximum joist cantilever as a fraction of the backspan: 25 percent of joist span

Frequently asked questions

How far can a 2x8 deck joist span?

At 16 inches on centre: 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 or Western Cedars. Tighten to 12 in on centre and Southern Pine reaches 13 ft 1 in; widen to 24 in and it drops to 9 ft 8 in. Species and spacing move the answer by feet, which is why a single "a 2x8 spans 12 feet" rule of thumb gets decks built wrong.

How far can a 2x10 deck joist span?

14 ft 0 in for Southern Pine at 16 in on centre, 13 ft 7 in for Douglas Fir-Larch/Hem-Fir/SPF, and 13 ft 0 in for Redwood and Western Cedars. The 2x10 at 16 in on centre in treated Southern Pine is the single most common deck framing combination in the US, and 14 ft is the number worth remembering.

What is the maximum deck joist spacing?

IRC Table R507.6 publishes spans at 12, 16 and 24 inches on centre, so 24 in is the widest the prescriptive table covers. But the decking you install can require tighter spacing than the joists structurally need — most composite decking calls for 16 in on centre maximum, and 12 in when run diagonally. The decking manufacturer instructions override this table downward, never upward.

How do you measure deck joist span?

Face of support to face of support — from the face of the ledger to the face of the beam. Not centre to centre of the supports, and not the overall width of the deck. Getting this wrong in the generous direction is the most common way a deck ends up under-framed, because centre-to-centre on a doubled beam can read several inches longer than the true clear span.

How far can a deck joist cantilever past the beam?

IRC R507.6.1 limits the cantilever to one quarter of the adjacent joist span — so a 12 ft backspan allows a 3 ft overhang. Two things people miss: the quarter is of the actual backspan you built, not the table maximum for that joist, and the backspan itself must still satisfy the span table on its own. A cantilever is not a way to stretch an undersized joist.

Does this cover deck beams as well as joists?

Not yet, deliberately. Beam spans come from IRC Table R507.5, and the published reproductions we could verify disagree with each other on both the species groupings and the values — one widely-cited source has a beam span printed that is physically impossible. Rather than average two tables that conflict on a structural member, we have left beams out until we can cite a single authoritative source. Joist spans here are confirmed identical across two independent reproductions.

Is this the same as a floor joist span table?

No. Deck joists in IRC Table R507.6 are sized for 40 psf live plus 10 psf dead load in wet service conditions, because a deck is outdoors and carries people standing close together. Interior floor joists use IRC Table R502.3.1 with different assumptions, and a sleeping-room floor is designed for 30 psf live load. Do not read an interior floor table for an exterior deck.