How to use the header size calculator
- What the header supports: just the roof and ceiling, or floors above too. A center-bearing floor has its joists landing on a center wall or beam. A clear-span floor spans the full width of the house, so the exterior walls carry more of it.
- Ground snow load: your building department publishes it. Where it's under 30 psf (and roof live load is 20 psf or less), use 30.
- Building width is measured perpendicular to the ridge, which is the direction the rafters or trusses run.
- Header size: (2) 2×10 means two 2×10s on edge, normally with 1/2" plywood or OSB between them so the header is as wide as a 2×4 wall.
- Braced or not: if cripple studs sit on top of the header instead of joists or rafters bearing on it, the code reduces 2×8, 2×10 and 2×12 header spans to 70%.
Example
A one-story house 24' wide with a 30 psf ground snow load. A (2) 2×10 header in an exterior wall can span 6'-10" with 2 jack studs at each end. Over a 6' opening it works with 10" to spare. A (2) 2×8 only reaches 5'-9", so it falls short.
Jack studs and king studs
Jack (trimmer) studs carry the header down to the plate. The table gives how many are needed at each end. Where it says 1, the code also allows an approved framing anchor to the full-height king stud instead. King studs are full-height studs beside the jacks. The IRC sets how many by opening width in Table R602.7.5.
When to use an engineered header
For long openings like garage doors and big sliders, girder trusses bearing on the header, point loads from posts, or anything outside the table, use an LVL or other engineered header sized by its maker or a licensed engineer.
FAQ
What size header do I need for a 6 foot opening?
In an exterior wall carrying only roof and ceiling, with 30 psf ground snow and a 24' wide building, a (2) 2×10 spans up to 6'-10" with 2 jack studs per end. A (2) 2×8 only spans 5'-9", so it falls short.
How far can a double 2x12 header span?
Carrying roof and ceiling only, with 30 psf snow, a (2) 2×12 spans 10'-7" in a 12' wide building, 8'-1" at 24' and 6'-10" at 36', per IRC Table R602.7(1).
How many jack studs does a header need?
The IRC table lists it for each header, from 1 to 4 per end. Bigger loads and longer spans need more. Where the table says 1, an approved framing anchor to the king stud can replace the jack.
Do interior walls use the same header table?
No. Interior bearing walls use IRC Table R602.7(2), and non-bearing walls don't need a structural header. This calculator only covers exterior bearing walls.