How far can floor joists span?

How far can a floor joist span?

It depends on the wood, the grade, the joist size, the spacing and the load. From 2021 IRC Table R502.3.1(2) for living areas (40 psf live, 10 psf dead load): Douglas fir-larch No. 2 at 16 inches on center spans 12 ft 9 in as a 2×8, 15 ft 7 in as a 2×10 and 18 ft 1 in as a 2×12.

Which table do I use — 30 or 40 psf?

Section R502.3.1 sends floors that support sleeping areas, and attics reached by a fixed stairway, to Table R502.3.1(1) with a 30 psf live load. Every other floor uses Table R502.3.1(2) at 40 psf (Section R502.3.2). Both tables allow a dead load of up to 20 psf and limit deflection to L/360.

What is the dead load, 10 or 20 psf?

The weight of the floor itself: the joists, subfloor, flooring and any ceiling hung below. Each IRC table has a column for 10 psf and one for 20 psf. If you are not sure your floor stays within 10 psf, the 20 psf column gives the shorter, safer span — for Douglas fir-larch No. 2 2×10s at 16 inches in a living area, 14 ft 3 in instead of 15 ft 7 in.

How is joist span measured?

As the clear distance between the faces of the supports — the American Wood Council's span tables, which the IRC tables come from, measure it that way. The joist itself is longer by its bearing at each end. The IRC also notes that lumber longer than 20 feet may be hard to get.

Does spacing or a bigger joist help more?

Both help. Southern pine No. 2 in a sleeping room (30 psf live, 10 psf dead): a 2×10 at 24 inches spans 12 ft 10 in, at 16 inches 15 ft 8 in; a 2×12 at 24 inches spans 15 ft 1 in. For grades, species or loads not in these tables, the IRC points to the AWC Span Tables for Joists and Rafters.

Worked example: choosing a joist for a 14-foot span

Suppose the clear distance between the supports is 14 ft, the room is a living area (40 psf live load), the dead load is 10 psf and the lumber is Douglas fir-larch No. 2. At 16 inches on center the smallest size in the table that reaches 14 ft is a 2×10, good for 15 ft 7 in. At 24 inches on center the smallest is a 2×12, good for 14 ft 9 in. Wider spacing saves joists but costs depth, and a deeper joist also leaves less room for pipes and ducts below. If the span is only slightly longer than a table value, the next size up or the next closer spacing is the usual fix; the span is never rounded up.

How grade and dead load change the answer

For a 2×8 at 16 inches in a living area with 10 psf dead load, Select Structural Douglas fir-larch spans 13 ft 7 in, No. 2 spans 12 ft 9 in and No. 3 spans 9 ft 9 in. The grade is stamped on each board, so check what the lumberyard actually delivers. Dead load works the same way: a 2×10 No. 2 at 16 inches spans 15 ft 7 in with 10 psf and 14 ft 3 in with 20 psf. A heavy floor covering, a mortar bed under tile or a plaster ceiling below can push a floor into the 20 psf column.

What the tables do not cover

The IRC tables assume sawn lumber in the species and grades listed, floors without large concentrated loads, and a deflection limit of span ÷ 360 under live load. They do not cover engineered joists such as I-joists or trusses, which come with the manufacturer's own span tables, nor lumber in other species. Nominal sizes are larger than real ones: a 2×10 is about 1½ by 9¼ inches. Notches and holes cut into a joist weaken it, so plumbing and wiring runs need to follow the code limits for where and how large they may be, and a cantilever or a large opening for a stair calls for a separate design. For anything outside the table, or if the building department asks for a stamped design, use the AWC span tables or an engineer.

Sources: International Code Council, 2021 International Residential Code, Section R502.3 and Tables R502.3.1(1) and R502.3.1(2); American Wood Council, Span Tables for Joists and Rafters (2024). Two cells of the online IRC table look misprinted, so the calculator sends you to the printed code for them. Building a deck? Use the deck joist calculator. How we check our calculators.