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Ceiling Joist Span Calculator & Span Table

Ceiling joist span is read from the IRC table for your lumber, size and spacing: a No. 2 Douglas fir-larch 2x6 at 16 in on center spans 17 ft 8 in under an attic with no storage.

Units
What is in the attic
Ceiling joist span
ft
in

Clear distance between bearing walls or beams, not the joist length.

Joist size
Joist spacing, on center

Stamped on the board; most yards stock No. 2.

2×6 Douglas fir-larch No. 2 at 16 in o.c.

17 ft 8 inmaximum span

Your 12 ft span passes, with 5 ft 8 in to spare.
Smallest that spans yours
2×6
Max span, table
17 ft 8 in
Deflection limit
L/240
Live load
10psf
Dead load
5psf
Table
R802.5.1(1)

Ceiling joist span chart: Douglas fir-larch No. 2, attic without storage, 10 psf live

Maximum joist span by size and spacing
Joist spacing, o.c.2×4 span2×6 span2×8 span2×10 span
12 in12 ft 5 in19 ft 6 in25 ft 8 inover 26 ft
16 in11 ft 3 in17 ft 8 in23 ft 4 inover 26 ft
19.2 in10 ft 7 in16 ft 8 in21 ft 4 in26 ft
24 in9 ft 10 in15 ft19 ft 1 in23 ft 3 in

Dark figures reach your 12 ft span. Your row is shaded. IRC 2021 Table R802.5.1(1), No. 2 Douglas fir-larch, deflection L/240.

Every species and grade for 2×6 at 16 in

Maximum span of a 2x6 joist by species and grade
SpeciesSelect Str.No. 1No. 2No. 3
Douglas fir-larch18 ft 9 in18 ft 1 in17 ft 8 in14 ft 1 in
Hem-fir17 ft 8 in17 ft 4 in16 ft 6 in13 ft 9 in
Southern pine18 ft 5 in17 ft 8 in16 ft 11 in12 ft 11 in
Spruce-pine-fir17 ft 4 in16 ft 11 in16 ft 11 in13 ft 9 in

IRC span check

  • Your span 12 ft

    2×6 at 16 in o.c. spans 17 ft 8 in, IRC R802.5.1(1)

  • Deflection limit L/240

    Live load 10 psf, dead load 5 psf, built into the table

These are prescriptive tables for ordinary ceilings with No. 1, No. 2 and other visually graded lumber, uniform loads and no notches or holes. Point loads, cut joists, bearing walls above and unusual loads need an engineer or your building department to verify.
The math

  1. Table lookup

    IRC R802.5.1(1): Douglas fir-larch No. 2, 16 in o.c., 2×6 = 17 ft 8 in

  2. Your span against it

    17 ft 8 in allowed − 12 ft needed = 5 ft 8 in to spare

  3. Sag at the limit

    17 ft 8 in ÷ 240 = 0.88 in

  4. Smallest size

    shallowest 2×4 to 2×10 with a span ≥ 12 ft = 2×6

Ceiling joist span is read from the IRC table for your lumber, size and spacing: a No. 2 Douglas fir-larch 2x6 at 16 in on center spans 17 ft 8 in under an attic with no storage. Enter your span above to see whether the joist passes and which size to use if it does not.

How the ceiling joist span is found

Ceiling joists carry far less than floor joists, so the IRC gives them their own tables in Section R802.5.1. Table R802.5.1(1) is for an uninhabitable attic without storage: 10 psf of live load, 5 psf of dead load. Table R802.5.1(2) is for an uninhabitable attic with limited storage: 20 psf of live load, 10 psf of dead load. Both limit sag to span divided by 240, a looser limit than the L/360 of floors because a ceiling is not walked on. Sizes run from 2x4 to 2x10 at 12, 16, 19.2 and 24 in on center.

Sag limit: L ÷ 240. Longest span from stiffness alone: L = cube root of (384 × E × I ÷ (5 × w × 240)), where w is the live load in pounds per inch of joist.

Worked example: a 12 ft ceiling

A 12 ft 0 in span, No. 2 Douglas fir-larch at 16 in on center, attic without storage:

  • 2x4 spans 11 ft 3 in, so it is 9 in short and fails.
  • 2x6 spans 17 ft 8 in, so it passes with 5 ft 8 in to spare.

The 2x6 is stiffness-limited here. With E = 1,600,000 psi and I = 20.8 in⁴, the deflection formula gives 17.7 ft (17 ft 8 in), and the table prints 17 ft 8 in. At that span the sag limit is 212 in ÷ 240 = 0.88 in.

Ceiling joist span chart: Douglas fir-larch No. 2, attic without storage

From IRC 2021 Table R802.5.1(1), 10 psf live load, 5 psf dead load, L/240. The calculator shows the same chart for every species and grade.

Joist spacing, o.c.2x4 span2x6 span2x8 span2x10 span
12 in12 ft 5 in19 ft 6 in25 ft 8 inover 26 ft
16 in11 ft 3 in17 ft 8 in23 ft 4 inover 26 ft
19.2 in10 ft 7 in16 ft 8 in21 ft 4 in26 ft 0 in
24 in9 ft 10 in15 ft 0 in19 ft 1 in23 ft 3 in

The table stops at 26 ft. Where it prints "Span exceeds 26 feet" the calculator says "over 26 ft".

Attic with storage

Once you put boxes in the attic the loads double and the spans shrink. The same 2x6 of Douglas fir-larch at 16 in on center spans 13 ft 0 in in Table R802.5.1(2) against 17 ft 8 in with no storage, and a 2x4 falls to 8 ft 11 in. Choose "Limited storage" if the attic can be reached and used for light storage. These are still uninhabitable attics: a room over the ceiling is a floor and uses the floor joist span calculator.

Species and grade

At 16 in on center a No. 2 2x6 in an attic without storage spans 17 ft 8 in in Douglas fir-larch, 16 ft 11 in in Southern pine and in spruce-pine-fir, and 16 ft 6 in in hem-fir. With limited storage the same board spans 13 ft 0 in in Douglas fir-larch, 12 ft 10 in in spruce-pine-fir, 12 ft 8 in in hem-fir and 12 ft 0 in in Southern pine.

Ceiling joists and rafters

In a stick-framed roof the ceiling joists also tie the rafters together so the walls do not spread. The joist tables here do not replace the rafter checks: size the roof framing with the rafter calculator, and if the roof is built from trusses the truss maker sets the ceiling chord, so use the roof truss calculator for the layout instead. Extra insulation adds dead load; the tables cover the 5 or 10 psf they name, and the insulation calculator tells you how much you are adding.

Canada, the UK, Australia and New Zealand

These are US IRC tables. Ceiling joist tables for Canada, UK C24 timber and Australia and New Zealand come from separate codes and loads and are not included here.

Next questions

The IRC tables are for ordinary uniform loads and uncut joists. Have an engineer or your building department verify heavy equipment, water heaters or anything else hung from the ceiling framing.