How to size a header for a window or door
A header carries the load over an opening — a window, a door, a garage — and hands it down through the trimmers to the floor. Get it right and the wall doesn't care there's a hole in it. Get it wrong and you see it two winters later in a door that won't latch or a cracked corner. Here's how I size them on a standard stick-framed wall.
What a header actually does
The header spans the opening and picks up whatever's bearing above it — roof, floor, or another wall. It carries that load sideways to the jack studs (trimmers) at each end, which run down to the plate and carry it to the floor. King studs run full height alongside and nail the whole assembly plumb and tight to the plates.
If nothing lands on the wall — a gable end, most interior partitions — you barely need a header at all. If something does, it's a bearing wall, and the header earns its keep.
Rules of thumb, by span
For a standard single-story bearing wall in snow country, built-up dimensional headers land in this range:
- Up to 3 ft — (2) 2x6
- 3 to 5 ft — (2) 2x8
- 5 to 7 ft — (2) 2x10
- 7 to 8 ft — (2) 2x12
These are starting points, not gospel. Two stories above, a heavy snow roof, or a point load from a girder or post changes everything. Use them to sanity-check, then confirm against your local code or an engineer for anything that's carrying real weight.
Ply and width
Two plies of 2x with a 1/2" plywood spacer sandwiched between makes the header 3-1/2" thick — flush in a 2x4 wall. Glue and nail (or bolt) the plies so they act as one member, not two boards leaning on each other. For a 2x6 wall or heavier loads, go to three plies.
King and jack studs
- One jack stud each side up to about 6 ft; two per side past that or under heavier loads.
- King studs run full height beside the jacks and tie the header into the top and bottom plates.
When to stop guessing
Big spans, garage-door openings, point loads from a beam or post above, anything carrying two stories or a serious snow roof — that's engineer territory. A header is cheap. A callback to fix a sagging one isn't.
Common questions
What size header do I need for a 6-foot opening?
In a standard single-story bearing wall, a doubled 2x10 is a common call for about a 6-foot span. The real answer depends on what's bearing above and your snow and roof load, so treat it as a starting point and confirm against your local code or an engineer.
Do I need a header in a non-bearing wall?
Often just a flat 2x or a single 2x4 to nail the opening off — nothing is landing on it. If anything above bears on that wall, treat it as bearing and size a real header.
Why put plywood between the plies?
A 1/2-inch plywood spacer between two 2x plies makes the built-up header 3-1/2 inches thick, flush with a 2x4 wall, and ties the plies together so they carry the load as one member.
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