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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.
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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