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.
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.
Common questions
What does the code say about living areas for Floor Joist Spans?
Spans per Table R502.3.1(2), based on a 40 psf live load, by species, grade, size and spacing. (IRC R502.3, Table R502.3.1(2))
What does the code say about sleeping areas & attics for Floor Joist Spans?
Spans per Table R502.3.1(1) at a 30 psf live load — the reason bedroom floors can span farther. (IRC Table R502.3.1(1))
What does the code say about deflection for Floor Joist Spans?
Floors are limited to L/360 deflection under live load — the table values already build this in. (IRC R301.7)
Adopted edition by state
Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.
Related sections and guides
This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.