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Home / Code Guides / Header Spans

Header Sizing — Span Tables, Jack Studs & Rim Board Headers

Header sizing under the IRC is table work: find your building width, snow load, and the loads the header carries, and the tables in R602.7 hand you a size and a jack stud count. The 2021 tables include single-ply options that can save lumber on smaller openings.

RequirementModel Code StandardSection
Exterior bearing wallsHeaders and girders in exterior bearing walls are sized from the exterior header table, entered by ground snow load, building width, and the number of floors and roof loads supportedIRC R602.7, Table R602.7(1)
Interior bearing wallsHeaders in interior bearing walls use their own table, entered by building width and loads carried — no snow column, since the roof load path matters only where it landsIRC Table R602.7(2)
Jack studsThe tables specify the required number of jack (trimmer) studs under each header end — more load or longer span means more jacks, and the header must bear fully on themIRC R602.7, Tables R602.7(1)–(2)
Single vs. built-upTables include both single-member and multi-ply built-up headers — a single 2x can be enough over small openings in light-load walls, while big openings under two floors may demand triple-ply or engineered lumberIRC Table R602.7(1)
Rim board headersA structural rim board at the floor line can serve as the header for openings below it, within the limits of the rim board header provisions — handy for windows tight to a floorIRC R602.7.2
Nonbearing wallsOpenings in nonbearing walls need only a minimal header — a single flat 2x member is permitted for openings up to 8 feetIRC R602.7.4

Worth knowing

  • Know your building width before opening the table — it is the span of the floor or roof framing the header supports, and guessing high is conservative but wasteful.
  • Engineered lumber (LVL, PSL) headers are sized from the manufacturer’s tables or an engineer’s design, not the IRC tables — the IRC tables cover sawn lumber.
  • Every header needs a load path down: jacks to plate, plate to studs, studs to foundation. Adding a big opening during a remodel without checking what is under the new jacks is how floors sag.
  • In high-seismic and high-wind areas, the wall bracing rules interact with big openings — a wall can have a legal header and still fail bracing. Check R602.10 before you commit to a wall of glass.

Common questions

What does the code say about exterior bearing walls for Header Spans?

Headers and girders in exterior bearing walls are sized from the exterior header table, entered by ground snow load, building width, and the number of floors and roof loads supported. (IRC R602.7, Table R602.7(1))

What does the code say about interior bearing walls for Header Spans?

Headers in interior bearing walls use their own table, entered by building width and loads carried — no snow column, since the roof load path matters only where it lands. (IRC Table R602.7(2))

What does the code say about jack studs for Header Spans?

The tables specify the required number of jack (trimmer) studs under each header end — more load or longer span means more jacks, and the header must bear fully on them. (IRC R602.7, Tables R602.7(1)–(2))

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.
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