Maximum allowable floor joist spans per IRC Table R502.3.1(1) for residential construction. 40 psf live load, 10 psf dead load, L/360 deflection limit. Select your species and grade to find the maximum span for your joist size and spacing.
How to Read a Joist Span Table
A joist span table tells you the maximum distance a floor joist can span between supports without exceeding code limits for deflection and strength. The span depends on four variables: wood species (determines strength and stiffness), lumber grade (#1 vs #2), joist size (2x8, 2x10, etc.), and spacing (how far apart the joists are).
What the Spans Mean
The span is measured as the clear distance between supports— from the inside face of one bearing wall or beam to the inside face of the next. It does not include the bearing length (typically 1.5" minimum on each end).
2x10 Span Chart
2x10 joists at 16" o.c. are the most common residential floor framing configuration. Depending on species, they span approximately 17 to 19 feet — enough for most rooms without intermediate support. Douglas Fir-Larch #1 gives you the longest span at about 19'-1", while Spruce-Pine-Fir #1/#2 spans about 17'-2".
2x8 Span Chart
2x8 joists are suitable for shorter spans — typically 13 to 15 feet at 16" o.c. depending on species. They're common in additions, small rooms, and as ceiling joists where loads are lighter.
2x12 Span Chart
2x12 joists handle the longest spans — up to about 23 feet at 16" o.c. with the best species. They're used for great rooms, open floor plans, and anywhere you need to minimize intermediate support.
Spacing Considerations
- 12" o.c. — Maximum spans, uses more lumber. Good for tile floors or heavy loads.
- 16" o.c. — Standard residential spacing. Best balance of span and material use.
- 19.2" o.c. — Divides a 4x8 sheet into 5 equal bays. Gaining popularity.
- 24" o.c. — Shorter spans, less lumber. Common for roof rafters, not ideal for floors.
Related Resources
Span values reference IRC Table R502.3.1(1). These tables assume standard residential loading (40 psf live + 10 psf dead) and L/360 deflection limit. Always verify with your local building code — jurisdictions may amend IRC tables. Not a substitute for professional engineering judgment.