Floor joist span table, by species

The four common softwood species don't span the same distance. Compare the maximum floor-joist span for Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir, and Southern Pine side by side — #2 grade, 16 in on-center, in feet-inches or metres.

01 How the table works

A joist span isn't a formula you run — it's a lookup. The allowable span comes from IRC Table R502.3.1(2), which fixes the maximum for each combination of joist size, species, grade, and on-center spacing under a set floor load. Four inputs decide the number, and species is the one most people skip.

The table on this page holds the #2-grade, 16 in on-center row for living areas, because that's the configuration most residential floors are framed to. The values are the same lumber, sorted into four spans by a stamp most people never read at the yard.

Species compared at #2 grade, 16 in on-center, living-area load (40 psf live, 10 psf dead, L/360):

Deeper joists span farther in the same species: an SPF joist goes from 12 ft 4 in as a 2×8 to 15 ft 5 in as a 2×10 to 18 ft 0 in as a 2×12, all at 16 in on-center. Depth buys span faster than grade or spacing does.

02 Worked example

Say you're framing a floor with a 15 ft (4.57 m) clear span between supports, using 2×10 joists at 16 in on-center. Which species clears it?

Three of the four do. Douglas Fir-Larch #2 reaches 15 ft 7 in (4.75 m), Spruce-Pine-Fir 15 ft 5 in (4.70 m), and Hem-Fir 15 ft 2 in (4.62 m) — each with a few inches to spare over the 15 ft opening. Southern Pine #2 tops out at 14 ft 0 in (4.27 m) for a 2×10, so the same nominal joist in Southern Pine lands a full foot short.

Set the widget to 2×10 and type 15 into the clear-span field: it flags Southern Pine as short and the other three as clearing. The fix isn't always a bigger joist — sometimes it's the species stamp, or dropping to 12 in on-center.

03 When this table is wrong

A span table is not a promise — it's the answer to one specific question: how far this size, in this species and grade, spans at this spacing under a 40 psf live load and a 10 psf dead load. That dead-load figure is the assumption that quietly breaks. A conventional light-frame floor — joists, subfloor, one finish — sits near 10 psf. A mortar-bed tile floor, a gypsum-concrete sound topping, or a doubled subfloor pushes past it, and then the governing row is the table's lower 20 psf dead-load column, not the one on this page. The tabulated span over-predicts for a heavy floor. The exception is the ordinary carpeted or floated floor, where 10 psf holds and the numbers above apply directly.

Where the numbers here stop applying:

• Grade changes the span. This table is #2, the common construction grade. Select Structural and #1 span farther in the same species; #3 and stud/utility grades span markedly less. A joist stamped #3 that you size off a #2 table is over-spanned. Read the row for the grade actually stamped on your lumber.
• Spacing changes it too. These are 16 in on-center values. Tightening to 12 in on-center adds span; opening to 19.2 or 24 in shortens it. For those spacings, pull the figure from the full IRC table rather than interpolating.
• Southern Pine was downgraded in 2013. The SPIB lowered visually graded Southern Pine design values effective June 1, 2013, which shortened its spans — hardest on wider members. A #2 Southern Pine 2×10 dropped to 14 ft 0 in, below Spruce-Pine-Fir (15 ft 5 in) and Hem-Fir (15 ft 2 in). Southern Pine is no longer the longest-spanning of the four for a 2×10 and wider, which older charts and rules of thumb still get backwards.
• The table is keyed to the actual member, not the nominal name. The 2×10 span comes from the real 1.5 × 9.25 in section, not a 2 × 10 in one. The nominal-vs-actual gap is the biggest single source of DIY framing estimating errors, and it applies here — picture a "2×10" as ten inches of depth and you'll overestimate its stiffness. There's no counter-case; the section that carries the load is the dressed one.
• Some floors aren't a table problem at all. Concentrated loads (a point load from a wall above), cantilevers, notched or drilled joists, and headers around stair openings fall outside the simple-span table and need an engineer or a manufacturer's chart. Local amendments also override the IRC — the span your jurisdiction's adopted table shows is the one that governs the permit.

04 What to do with the result

Read the exact cell for your real conditions — your grade, your spacing, your dead load — from the code table your jurisdiction has adopted, then frame the size that clears the opening with margin rather than the one that just barely makes it. A joist run at the table maximum meets code and can still feel bouncy underfoot, because L/360 permits a live-load deflection of the span over 360 (about half an inch on a 15 ft run). Tightening the target to L/480 for a stiffer floor shortens the allowable spans; going up a joist size or dropping to 12 in on-center is the usual cure.

When you're close to the limit, compare the levers before switching species: a deeper joist buys the most span, tighter spacing the next most, and a higher grade the least. And because the span is set by the actual dressed section, order and cut to the real member — a 2×10 is 1.5 × 9.25 in, and that 9.25 in of depth is what the table is really rating. For anything the table doesn't cover — a point load, a cantilever, a long opening — get the size confirmed by the AHJ or an engineer before you frame it.

05 Common questions

What is the maximum span for a 2×10 floor joist?

For #2 grade at 16 in on-center in a living area (40 psf live, 10 psf dead, L/360): Douglas Fir-Larch 15 ft 7 in (4.75 m), Spruce-Pine-Fir 15 ft 5 in (4.70 m), Hem-Fir 15 ft 2 in (4.62 m), Southern Pine 14 ft 0 in (4.27 m).

How far can a 2×8 floor joist span?

At #2 grade, 16 in on-center, living areas: Douglas Fir-Larch 12 ft 7 in (3.84 m), Spruce-Pine-Fir 12 ft 4 in (3.76 m), Hem-Fir and Southern Pine 12 ft 0 in (3.66 m).

Does joist spacing change the maximum span?

Yes. Closer spacing shares the floor load among more joists, so each one spans farther. The values here are 16 in on-center; 12 in on-center spans farther, 19.2 and 24 in shorter. Read those columns from the full IRC table.

Which wood species spans the farthest?

Of the four common species at #2 grade, Douglas Fir-Larch spans the farthest. Southern Pine used to lead, but the 2013 SPIB design-value revision cut its spans, and a #2 Southern Pine 2×10 now spans 14 ft 0 in — less than Spruce-Pine-Fir and Hem-Fir at the same size.

Is this the same as a deck joist span?

No. Deck joists are exterior, wet-service members and use the separate IRC deck table (R507), which gives shorter spans for the same size. This page is interior floor framing under R502. Don't size a deck off it.