Framing
Floor Joist
A horizontal structural member that spans between supports to carry floor loads, typically spaced 12, 16, or 24 inches on center.
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In the Field
Joists must bear a minimum 1.5 inches on wood supports and 3 inches on masonry or concrete. Notching and boring is restricted — follow IRC Table R802.4.2 for allowed locations and sizes. Never notch the tension face (bottom) of a joist in the middle third of the span.
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Rule of Thumb
A rough span limit for 2x10 floor joists at 16 OC in normal residential loading is about 15–16 feet. Use span tables for exact values — species and grade matter significantly.