What is the maximum span of a 2x10 joist?

It depends on species, grade, spacing and dead load — there's no single answer. For No.2 Douglas Fir-Larch at 16" spacing with a 10 psf dead load, a 2x10 spans about 15 ft 5 in under IRC Table R502.3.1(2). Use the calculator above for your exact combination.

How far can a 2x10 roof rafter span without support?

For No.2 Douglas Fir-Larch at 16" spacing, 10 psf dead load, and 20 psf roof live load, a 2x10 rafter spans about 22 ft 3 in under IRC Table R802.5.1(2). Species, spacing and load all change the number — check your exact case above.

What size joist do I need to span 12 feet?

At 16" spacing with common species/grades, a 2x8 or 2x10 typically covers a 12 ft span, depending on species and dead load. Enter 12 in the "span this far" field above for the exact minimum size for your combination.

Does closer joist spacing let me span further?

Yes — going from 24" to 16" or 12" spacing meaningfully increases the allowable span for the same size, because each joist shares less of the floor's total load. It's usually cheaper to upsize the lumber than to add joists, but both work.

Joist & rafter span calculator

Pick a floor joist or roof rafter, the species, spacing and dead load — get the maximum allowable span from the IRC span tables, or work backwards from a span you need to cover.

Member

SpeciesSpacing (on-center)

SizeDead load (psf)

Max span — 2x10 Douglas Fir-Larch, 16" o.c.

15 ft 5 in

Live load 40 psf, L/360 (2012 IRC Table R502.3.1(2)), No.2 grade, 10 psf dead load.

Or: what size do I need to span this far? (ft)

For a 12 ft span at 16" o.c., use at least 2x8 Douglas Fir-Larch No.2 (max span 12 ft 7 in).

Covers the No.2 grade only — the grade specified for most standard residential framing. Select Structural, No.1 and No.3 spans differ; for those, other species, or spans beyond this table, consult the full IRC span tables or a structural engineer. Planning reference only — always confirm against the code edition your jurisdiction has adopted.

Common questions

What is the maximum span of a 2x10 joist?: It depends on species, grade, spacing and dead load — there's no single answer. For No.2 Douglas Fir-Larch at 16" spacing with a 10 psf dead load, a 2x10 spans about 15 ft 5 in under IRC Table R502.3.1(2). Use the calculator above for your exact combination.
How far can a 2x10 roof rafter span without support?: For No.2 Douglas Fir-Larch at 16" spacing, 10 psf dead load, and 20 psf roof live load, a 2x10 rafter spans about 22 ft 3 in under IRC Table R802.5.1(2). Species, spacing and load all change the number — check your exact case above.
What size joist do I need to span 12 feet?: At 16" spacing with common species/grades, a 2x8 or 2x10 typically covers a 12 ft span, depending on species and dead load. Enter 12 in the "span this far" field above for the exact minimum size for your combination.
Does closer joist spacing let me span further?: Yes — going from 24" to 16" or 12" spacing meaningfully increases the allowable span for the same size, because each joist shares less of the floor's total load. It's usually cheaper to upsize the lumber than to add joists, but both work.

Want the full method and tables? Read the joist span guide → or the rafter span guide →

Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.

Last reviewed 2026-07.

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