Blog /
How to Size a Header (IRC Table R602.7)
The Problem
You’re framing a wall and need a header over a window or door. 2-2x6? 2-2x8? 2-2x10? LVL?
For exterior bearing walls in a house built under the IRC, the answer comes from Table R602.7(1) in the 2021 IRC. Interior bearing walls use Table R602.7(2). To read either one you need four things: the opening width, the building width, the ground snow load, and what the header holds up.
What the Header Does
A header carries the load from above the opening over to the jack studs on each side. It’s a beam. The wider the opening and the more weight above it, the deeper it needs to be. A header under a roof only is a very different job from one under a roof and two floors.
Reading IRC Table R602.7(1)
What it supports. Five rows of conditions: roof and ceiling; roof, ceiling, and one center-bearing floor; roof, ceiling, and one clear-span floor; and the same two again with two floors.
Building width. Measured perpendicular to the ridge. The table has columns for 12, 24, and 36 feet. Footnote c lets you interpolate between them. Over 36 feet, the table doesn’t apply.
Ground snow load. Columns for 30, 50, and 70 psf. Your building department has the design value for your site. Above 70 psf, the table doesn’t apply.
Lumber. The table covers No. 2 grade Douglas fir-larch, hem-fir, Southern pine, and spruce-pine-fir. It doesn’t give longer spans for a better species; one set of numbers covers all four.
NJ. Next to each span is the number of jack studs required at each end. Where the table calls for one jack stud, footnote d allows an approved framing anchor instead.
Real Lookups
Building 24 feet wide, 30 psf ground snow load, exterior wall, header supporting roof and ceiling only:
| Header | Max span | Jack studs each end |
|---|---|---|
| 2-2x6 | 4’-7” | 1 |
| 2-2x8 | 5’-9” | 1 |
| 2-2x10 | 6’-10” | 2 |
| 2-2x12 | 8’-1” | 2 |
Same building, header supporting roof, ceiling, and one center-bearing floor:
| Header | Max span | Jack studs each end |
|---|---|---|
| 2-2x6 | 3’-9” | 1 |
| 2-2x8 | 4’-10” | 2 |
| 2-2x10 | 5’-8” | 2 |
| 2-2x12 | 6’-8” | 2 |
The 2-2x10 that spans 6’-10” under a roof drops to 5’-8” once it also carries a floor. That’s the difference between a 6-foot window working and not working.
Why Your 2x6 Might Not Be Enough
Plenty of framers default to a 2-2x6 for anything under 4 feet. Often it’s fine. Not always.
Take a 3’-6” window in a first-floor exterior wall of a house 32 feet wide, 50 psf ground snow load, with one center-bearing floor above. The table gives a 2-2x6 3’-6” at 24 feet wide and 3’-0” at 36 feet. Interpolating to 32 feet gets about 3’-2”. The window is wider than that. A 2-2x8 (4’-5” at 24 ft, 3’-9” at 36 ft, about 3’-11” at 32 ft) works.
Building width and snow load both shorten spans quickly. A header that’s fine on a narrow house in a low-snow area can fail on a wide house up north.
The Footnotes That Matter
Unbraced tops. Footnote f: spans for 2x8, 2x10, and 2x12 headers assume the top of the header is braced, typically by being tight to the top plate. If it isn’t (cripples between the header and the plate, for example), multiply the span by 0.70.
Interpolation. Footnote c lets you interpolate between building widths. You can’t extrapolate past 36 feet.
Low snow. Where the roof live load is 20 psf or less, use the 30 psf column for ground snow loads below 30 psf.
Built-Up Headers in Thicker Walls
“2-2x10” means two 2x10s nailed together, usually with a spacer to match a 2x4 wall’s thickness. In a 2x6 wall the table also lists 3-ply and 4-ply headers, which span farther. A 3-2x10 under a roof only, 24 feet wide, 30 psf, spans 8’-7”.
When to Use an LVL
LVL, PSL, and glulam beams aren’t in the IRC header tables. Size them from the manufacturer’s span tables or get an engineer. You’ll need one when:
- The opening is wider than any lumber header in the table allows
- There’s a garage door opening
- A post, girder, or point load lands on the header
- Your conditions are outside the table: over 36 feet wide, over 70 psf snow, or loads the table doesn’t list
Nonbearing Walls
Headers aren’t required in nonbearing walls. IRC R602.7.4 allows a single flat 2x4 over openings up to 8 feet wide, as long as the nailing surface above is no more than 24 inches up. Framing a 2-2x10 header in every interior partition is wasted lumber.
The stud count calculator covers the king studs, jack studs, and cripples around each opening.
Inspection
Inspectors check headers against the plans, and the plans should match the IRC table or an engineer’s design. Get it right before the wall goes up. Checking the table takes a minute. Reframing an opening takes an afternoon.
Running Header Calcs on Site
Trussly’s Header Sizing calculator holds the full IRC Tables R602.7(1) and R602.7(2). Enter the opening, building width, snow load, and what the header supports. It interpolates between widths, applies the unbraced-top reduction when you tell it to, and shows every size in the table with its allowed span and jack studs. When nothing in the table works, it tells you to get an engineered beam instead of guessing.