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Home / Code Guides / Floor Joist Spans

Floor Joist Span Code — Tables, Loads, Bearing & Deflection

Joist spans are not a judgment call — the IRC publishes the answer in table form. Which table, and which row, depends on the room above, the wood, and how much bounce you will tolerate.

RequirementModel Code StandardSection
Living areasSpans per Table R502.3.1(2), based on a 40 psf live load, by species, grade, size and spacingIRC R502.3, Table R502.3.1(2)
Sleeping areas & atticsSpans per Table R502.3.1(1) at a 30 psf live load — the reason bedroom floors can span fartherIRC Table R502.3.1(1)
DeflectionFloors are limited to L/360 deflection under live load — the table values already build this inIRC R301.7
BearingMinimum 1.5 in. of bearing on wood or metal, 3 in. on masonry or concreteIRC R502.6
Deck joistsExterior deck joists have their own span table, including allowable cantilevers of up to one-fourth the backspanIRC R507.6
Engineered lumberI-joists and other engineered products span per the manufacturer’s literature, not the sawn-lumber tablesIRC R502.1

Worth knowing

  • Span is measured clear between supports, not center-to-center of bearing.
  • Moving from 16 in. to 12 in. spacing buys more span from the same joist — often cheaper than upsizing.
  • A load-bearing wall, tub, or waterbed over a long span is outside the table assumptions; that is when an engineer earns their fee.
  • Older “rule of thumb” span cards frequently predate the current tables — check the edition before trusting a card taped inside a lumberyard.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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