How far can a 2x8 floor joist span?
A 2x8 floor joist at 16" on centre spans 12 ft 9 in in Douglas fir-larch No. 2, from the IRC's own span table. Every joist size at every spacing, and what the table does not cover.
A 2x8 floor joist at 16" on centre can span 12 ft 9 in — Douglas fir-larch No. 2, under a living-area floor.
In a bedroom, where the code assumes 30 psf rather than 40, the same joist goes 14 ft 2 in; in southern pine No. 2 it is 11 ft 10 in.
How that is worked out
- Nothing here is calculated: the figure is read straight out of 2021 IRC Table R502.3.1(2), floor joists for residential living areas at 40 psf live load and 10 psf dead load, limited to L/360 deflection.
- Find the row for the species, grade and spacing — Douglas fir-larch No. 2 at 16" — and read across to the 2x8 column: 12 ft 9 in.
- That is a clear span, measured face to face between the supports, not the length of the joist and not centre to centre of the bearings.
Maximum clear span, Douglas fir-larch No. 2, living areas (40 psf live, 10 psf dead)
| Spacing | 2x6 | 2x8 | 2x10 | 2x12 |
|---|---|---|---|---|
| 12" o.c. | 10 ft 9 in | 14 ft 2 in | 18 ft | 20 ft 11 in |
| 16" o.c. | 9 ft 9 in | 12 ft 9 in | 15 ft 7 in | 18 ft 1 in |
| 19.2" o.c. | 9 ft 2 in | 11 ft 8 in | 14 ft 3 in | 16 ft 6 in |
| 24" o.c. | 8 ft 3 in | 10 ft 5 in | 12 ft 9 in | 14 ft 9 in |
Worth knowing
- These are the 2021 IRC's prescriptive figures and they are defaults, not law: what applies is whatever your jurisdiction adopted, and the building department has the last word. Check with them before you frame.
- The table covers simple spans under a uniform floor load only — no point loads, no bearing walls on the joists, no cantilevers, and no notches or holes beyond what the code allows. Anything else is a question for an engineer.
- A heavier finish changes the answer. Tile or a stone floor pushes the dead load to 20 psf, and the same 2x8 then spans 11 ft 8 in.
- A floor that passes the code can still feel bouncy: L/360 is a minimum. Going a size deeper or a spacing closer is the usual fix for a floor you will notice underfoot.
Sources: 2021 International Residential Code, International Code Council.
Work out your own numbers
- Floor joist spans
The 2021 IRC's maximum span for a 2x6 to 2x12 floor joist by species, grade and spacing, and what would reach when yours falls short.
- Deck and fence takeoff
Boards, joists, posts, pickets, screws and bags of concrete, with the gaps between boards properly counted.
- Wall framing stud count
Field studs at 16" or 24" on centre, corner studs, and the king and jack studs every opening adds.
Related questions
- How far can a 2x8 span at 16 inches on center?
- 12 ft 9 in for Douglas fir-larch No. 2 under a living-area floor, from IRC Table R502.3.1(2). Under a bedroom, where the code assumes a lighter load, it is 14 ft 2 in.
- Is a 2x8 big enough for a 12 foot span?
- At 16" on centre in Douglas fir-larch No. 2, yes — the table allows 12 ft 9 in. At 24" on centre it allows 10 ft 5 in, which is not, so the answer depends on spacing as much as size.
- How far can a 2x10 floor joist span?
- 15 ft 7 in at 16" on centre in Douglas fir-larch No. 2 under a living-area floor. The table above gives every size at every spacing.