Deck Joist Span Calculator
How far deck joists span: enter the species, spacing and the span from the ledger to the beam, and see the smallest 2x joist that works, with the longest span of every size at 12, 16 and 24 in apart. Overhangs past the beam are checked too, including the 220 lb load at the end the code's deck table uses.
2x10 joists 16 in apart work for a 12′ span, using 73% of their capacity (limited by bending).
Longest span, Southern Pine No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 9′ 10″ | 8′ 11″ | 7′ 7″ |
| 2x8 | 13′ | 11′ 9″ | 9′ 7″ |
| 2x10 | 16′ 2″ | 14′ | 11′ 5″ |
| 2x12 | 19′ | 16′ 6″ | 13′ 5″ |
Spans in bold reach 12′.
At a 12′ span, 16 in apart
| 2x6Fails live load deflection (span/360) | 237% |
| 2x8Fails live load deflection (span/360) | 104% |
| 2x10Works | 73% |
| 2x12Works | 53% |
Every check for the 2x10
| Bending | 73% |
| Shear | 25% |
| Live load deflection (span/360) | 50% |
| Total load deflection (span/240) | 42% |
Size the whole deck Check the 2x10 in the beam designer
10 psf dead and 40 psf live load on a deck of Southern Pine No.2, treated and wet. The beam designer link checks one joist dry and without the repetitive member factor, so it allows a little less.
Deck joist span tables
The longest span for No.2 deck joists of each species under 40 psf live and 10 psf dead load, treated and wet, without an overhang. Worked out by the calculator, not copied from the code tables.
Douglas Fir-Larch No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 9′ 7″ | 8′ 3″ | 6′ 9″ |
| 2x8 | 13′ 2″ | 11′ 5″ | 9′ 3″ |
| 2x10 | 16′ 1″ | 13′ 11″ | 11′ 4″ |
| 2x12 | 18′ 8″ | 16′ 2″ | 13′ 2″ |
Southern Pine No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 9′ 10″ | 8′ 11″ | 7′ 7″ |
| 2x8 | 13′ | 11′ 9″ | 9′ 7″ |
| 2x10 | 16′ 2″ | 14′ | 11′ 5″ |
| 2x12 | 19′ | 16′ 6″ | 13′ 5″ |
Hem-Fir No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 9′ 5″ | 8′ 7″ | 7′ 1″ |
| 2x8 | 12′ 6″ | 11′ 1″ | 9′ |
| 2x10 | 15′ 7″ | 13′ 6″ | 11′ |
| 2x12 | 18′ 1″ | 15′ 8″ | 12′ 10″ |
Spruce-Pine-Fir No.1/No.2
| Joist | 12 in apart | 16 in apart | 24 in apart |
|---|---|---|---|
| 2x6 | 9′ 8″ | 8′ 10″ | 7′ 3″ |
| 2x8 | 12′ 9″ | 11′ 3″ | 9′ 2″ |
| 2x10 | 15′ 10″ | 13′ 9″ | 11′ 2″ |
| 2x12 | 18′ 5″ | 15′ 11″ | 13′ |
Spans are from the ledger or beam to the beam, center to center of the supports.
How the calculator works out a joist span
Each joist is a simple span checked to the 2018 National Design Specification (NDS) for wood, with allowable stress design (ASD). Joists 24 in apart or closer share load, so they take the repetitive member factor of 1.15. Deck joists are treated lumber outdoors, so they also take the wet service factors, and the incising factors for species other than Southern Pine. The longest span is the longest one that passes every check:
- Bending and shear under the dead and live load, with live load on the back span alone and on the overhang too.
- Deflection: live load sag up to the span over 360, and total load sag up to the span over 240, the limits in the International Residential Code (IRC).
- Overhang: up to a quarter of the span, with its end deflecting no more than its length over 180 under a 220 lb load at the tip.
The IRC's deck joist table (R507.6) comes from the same kind of check, so its spans are close to these. A building department may ask for the code table; for a deck outside it, a calculation like this one.
Deck joist questions
How far can a 2x8 deck joist span?
Southern Pine No.2 2x8 joists 16 in apart span 11′ 9″ under 40 psf live and 10 psf dead load; 12 in apart, 13′.
How far can a 2x10 deck joist span?
Southern Pine No.2 2x10 joists span 14′ at 16 in apart and 16′ 2″ at 12 in apart.
How far can a 2x6 deck joist span?
8′ 11″ at 16 in apart and 9′ 10″ at 12 in apart for Southern Pine No.2.
How far can deck joists overhang the beam?
Up to a quarter of the back span, and less where the joist is too flexible to carry 220 lb at its end. Enter the overhang above to see the longest span with it.
Should deck joists be 12 or 16 in apart?
16 in is usual for wood decking. Many composite boards, and boards laid on a diagonal, need joists 12 in apart; check the decking maker's instructions.
What about the beam and posts?
The deck framing calculator sizes the joists, beam, posts and footings together. To check one beam on its own, use the wood beam span calculator.