How far can a 2x8 deck joist span?

At 16 in on center: 11 ft 10 in in southern pine, 11 ft 1 in in Douglas fir-larch or hem-fir, and 10 ft 7 in in redwood or western cedar. Tighten to 12 in on center and southern pine reaches 13 ft 1 in.

How far can a 2x10 deck joist span?

At 16 in on center: 14 ft 0 in southern pine, 13 ft 7 in Douglas fir-larch, 13 ft 0 in redwood and cedar. This is the size most 12 ft deep decks land on.

How far past the beam can deck joists cantilever?

The smaller of the table value and one quarter of the actual joist span, per R507.6.1. For a Douglas fir-larch 2x10 the table allows 3 ft 1 in, but on a real 12 ft span the quarter rule caps it at 3 ft 0 in. An overhang not exceeding the nominal depth of the joist is always permitted.

Should deck joists be 16 or 24 inches on center?

16 in is the usual choice. Go to 12 in if the decking manufacturer requires it, if boards run diagonally, or if you want to reduce bounce. 24 in is permitted by the code for the spans in the table but rules out most composite decking.

Does the species really change the answer?

Yes, by more than a foot in places. A 2x12 at 16 in o.c. spans 16 ft 6 in in southern pine and 15 ft 1 in in western cedar. Species is stamped on the grade mark of the lumber - check it rather than assuming.

Which IRC edition are these spans from?

Table R507.6 as published in the 2018 and 2021 editions. Adoption varies by state and county, so confirm the edition your building department enforces before you rely on it.

Home › Structural › Joist Calculator

Joist Calculator

Source: IRC Table R507.6 · cantilever rule R507.6.1

Find the maximum span for a deck joist, and the cantilever the code allows past the beam. Or work it the other way round — put in the span you actually have to cover and it names the smallest joist that does the job, and shows how far short the others fall. Values come straight from IRC Table R507.6, printed in full below.

Joist span calculator

Lumber species group — per IRC R507.6

Joist size

Joist spacing — on center

Deck width — optional, to get joist count

Or: span I need to cover — fill this to get the size instead

Actual joist span — optional, for the cantilever limit

Table values: IRC Table R507.6. Live load 40 psf, dead load 10 psf, L/360 main span, L/180 at cantilever with a 220 lb point load, No. 2 grade, wet service.

Worked example

A 12 ft × 16 ft deck in Douglas fir-larch, joists at 16 in on center running the 12 ft direction from the ledger to the beam, with a short overhang past the beam.

Read the span. Douglas fir-larch, 2×8 at 16 in o.c. → 11 ft 1 in maximum span. A 12 ft run does not fit, so this is the moment to move to 2×10 rather than discover it at inspection.
Step up a size. 2×10 at 16 in o.c. in the same species → 13 ft 7 in. The 12 ft span now fits with room to spare.
Cantilever. Table R507.6 allows 3 ft 1 in for a 2×10 in this species, but R507.6.1 also caps the overhang at one quarter of the actual span. Enter the real 12 ft span and the quarter rule gives 3 ft 0 in, so 3 ft governs — not the table.
Joist count. A 16 ft wide deck at 16 in o.c. needs 13 joists, before adding any doubled joist under a partition or a hot tub.

Leave the actual span blank and the calculator assumes the joist is run to its full table span, which is the least conservative reading. Enter the span you are really building and the quarter-span rule usually takes over: at 12 ft it gives 3 ft 0 in, at 10 ft it gives 2 ft 6 in. The calculator applies whichever limit is smaller and names the one that governed.

The formula

Table R507.6 is prescriptive — the span is looked up, not derived. The values were generated from the same NDS checks that govern any sawn-lumber bending member, and deflection rather than bending strength is usually what sets a deck joist span:

Δ = 5wL⁴ / 384EI ≤ L/360 and f_b = M/S ≤ F_b′

Δ: deflection at midspan, in
w: uniform load, lb/in = (40 + 10 psf) × joist spacing ÷ 12
L: clear span, in
E: modulus of elasticity, psi — species property from the NDS supplement
I: moment of inertia, in⁴ = bd³/12 — a 2×10 is 1.5 × 9.25, so I = 98.9
S: section modulus, in³ = bd²/6
F_b′: adjusted allowable bending stress, psi

The L/360 limit is a serviceability rule, not a strength rule — a joist at its table span is nowhere near breaking, it is at the point where the deck would feel bouncy. This is why stiffer species get longer spans at identical strength, and why doubling a joist helps deflection more than intuition suggests.

Where these numbers come from

The table below is IRC Table R507.6, “Deck Joist Spans for Common Lumber Species.” It sits in Section R507.6 (“Deck joists”) of the International Residential Code, and the cantilever rule is R507.6.1.

Design basis: 40 psf live load, 10 psf dead load, No. 2 grade lumber, wet service conditions, deflection limited to L/360 on the main span and L/180 at the cantilever under a 220 lb point load. Douglas fir-larch and hem-fir values include the incising factor; the northern species group does not.

The cantilever has two limits and the smaller one wins. R507.6.1 permits the overhang in the table, but never more than one quarter of the actual joist span. A calculator that reports only the table figure will overstate the allowable overhang on any joist that is not spanning its full table distance — which is most of them. This one applies both and names the governing limit.

Cantilevered spans not exceeding the nominal depth of the joist are permitted regardless of the table.

IRC Table R507.6 — Deck Joist Spans (ft-in), by spacing on center
Joist size	12 in	16 in	24 in	Max cant.
Southern pine
2×6	9–11	9–0	7–7	1–3
2×8	13–1	11–10	9–8	2–1
2×10	16–2	14–0	11–5	3–4
2×12	18–0	16–6	13–6	4–6
Douglas fir-larch, hem-fir, spruce-pine-fir
2×6	9–6	8–8	7–2	1–2
2×8	12–6	11–1	9–1	1–11
2×10	15–8	13–7	11–1	3–1
2×12	18–0	15–9	12–10	4–6
Redwood, western cedars, ponderosa pine, red pine
2×6	8–10	8–0	7–0	1–0
2×8	11–8	10–7	8–8	1–8
2×10	14–11	13–0	10–7	2–8
2×12	17–5	15–1	12–4	3–10

Reproduced from IRC Table R507.6, cantilever column per R507.6.1. The cantilever shown is the table maximum; the quarter-span rule may reduce it further. Verify against the edition your jurisdiction has adopted.

Deck joist spacing — 12, 16 or 24 inches

Spacing is a design choice, not a code minimum: 16 in on center is the common default, 12 in buys span and stiffness, 24 in saves lumber and loses both. Two things push a deck to 12 in that have nothing to do with the joist itself. Composite and PVC decking often requires 12 in on center, and always does when boards run diagonally — check the decking manufacturer's instructions, which the code defers to. And a 45° diagonal layout increases the effective span between joists by about 41%, which is why the same boards that pass at 16 in straight fail on the diagonal.

Read across a row in the table above to see the cost of the choice: a Douglas fir-larch 2×10 spans 15 ft 8 in at 12 in o.c., 13 ft 7 in at 16 in, and 11 ft 1 in at 24 in.

Floor joists are a different table

R507.6 covers deck joists, at 40 psf live load and wet service. An interior floor joist is governed by IRC Tables R502.3.1(1) and R502.3.1(2) instead, at 30 or 40 psf depending on whether the room is a sleeping area, and dry service. The dry-service values are longer. Do not use this page to size a floor inside the building envelope, and be wary of any floor joist span calculator that does not say which of the two tables it is reading.

Frequently asked questions

How far can a 2×8 deck joist span?: At 16 in on center: 11 ft 10 in in southern pine, 11 ft 1 in in Douglas fir-larch or hem-fir, and 10 ft 7 in in redwood or western cedar. Tighten to 12 in on center and southern pine reaches 13 ft 1 in.
How far can a 2×10 deck joist span?: At 16 in on center: 14 ft 0 in southern pine, 13 ft 7 in Douglas fir-larch, 13 ft 0 in redwood and cedar. This is the size most 12 ft deep decks land on.
How far past the beam can deck joists cantilever?: The smaller of the table value and one quarter of the actual joist span, per R507.6.1. For a Douglas fir-larch 2×10 the table allows 3 ft 1 in, but on a real 12 ft span the quarter rule caps it at 3 ft 0 in. An overhang not exceeding the nominal depth of the joist is always permitted.
Should deck joists be 16 or 24 inches on center?: 16 in is the usual choice. Go to 12 in if the decking manufacturer requires it, if boards run diagonally, or if you want to reduce bounce. 24 in is permitted by the code for the spans in the table but rules out most composite decking.
Does the species really change the answer?: Yes, by more than a foot in places. A 2×12 at 16 in o.c. spans 16 ft 6 in in southern pine and 15 ft 1 in in western cedar. Species is stamped on the grade mark of the lumber — check it rather than assuming.
Which IRC edition are these spans from?: Table R507.6 as published in the 2018 and 2021 editions. Adoption varies by state and county, so confirm the edition your building department enforces before you rely on it.

Check these numbers yourself

IRC R507.6 — Deck joists (full section text)UpCodes. Confirm the jurisdiction selector matches yours.
AWC DCA 6 — Prescriptive Residential Wood Deck Construction GuideAmerican Wood Council, free PDF. The prescriptive deck guide the IRC deck provisions track.
ICC Digital Codes — the IRC itselfInternational Code Council. Free to read online; registration required.

Figures on this page last checked against the source documents on 2026-09-05. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Estimates only. This calculator reproduces a prescriptive code table for planning purposes. It is not a structural design and it does not replace your local building department or a licensed engineer. Codes are amended locally and a deck collapse injures people. Confirm every span and cantilever against the code edition adopted in your jurisdiction before buying lumber or building.