Joist Span
framing
The clear distance a joist can cross between supports without exceeding allowable deflection — governed by species, grade, size, spacing, and load.
Span is not a property of a board on its own; it is the answer to a question involving five variables at once. Two 2×10 joists of identical size but different species — Southern Yellow Pine and Spruce-Pine-Fir — have materially different allowable spans, because SYP is stiffer. This is why span tables always name species and grade, and why "a 2×10 spans 14 feet" is never a complete statement. Deflection rather than strength usually governs residential spans: a floor that meets the strength requirement can still feel unacceptably bouncy, which is what the L/360 deflection limit exists to prevent.
Example
A No. 2 SYP 2×10 at 16 in OC spans further than the same size in SPF. Always check the AWC table for your species, grade, spacing, and load rather than a remembered figure.
Common mistake
Using a span figure without its species and grade. It is the most common way a deck or floor ends up bouncy — or, at the extreme, unsafe.
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Open Deck Calculator →Related Terms
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