Header Size Calculator Guide

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Tributary Load Is Why a 6-Foot Slider Is Not Always a Double 2x8

A wall header carries whatever the framing above dumps onto that opening. On an exterior bearing wall that is typically a strip of roof (and maybe a floor) whose width is half the building, measured perpendicular to the ridge. IRC calls that dimension building width and tabulates 12, 24, and 36 ft. A 36 ft-wide house puts more snow and dead load on each exterior header than a 24 ft ranch with the same opening.

Ground snow load is the other multiplier. The table uses 30, 50, and 70 psf. Footnote: if ground snow is under 30 psf and roof live load is 20 psf or less, use the 30 psf column. That is why a Florida 3-0 door and a mountain 3-0 door are not the same header even when both walls are 2x4.

Interior bearing walls ignore snow. Table R602.7(2) is keyed only to building width and whether the header supports one floor or two. Nonbearing partitions skip both tables: IRC R602.7.4 allows a single flat 2x4 up to 8 ft when the pocket above is not more than 24 in. Using a bearing table on a closet opening wastes lumber; using the partition rule on a wall that carries the roof is how headers sag.

How the Header Schedule Actually Works

The code does not say “use a double 2x10 over every door.” It lists header sizes and the maximum span each size may run under a load case. You read it backwards from the job: opening width in, smallest size whose span is still greater than that opening.

Each cell also lists NJ — jack studs required at each end. A 2-2x10 that spans 6′-10" in the 24 ft / 30 psf roof-only column needs two jacks. One jack is not a close-enough substitute; where NJ equals 1, an approved framing anchor to the king is the alternate, not a missing jack on a two-jack cell. Extra jacks do not buy a smaller header. They only add bearing and, with king studs, help the wind/king-stud count in Table R602.7.5.

IRC 2021 groups No. 2 Douglas fir-larch, hem-fir, Southern pine, and spruce-pine-fir on the same numbers. Species still matters for the grade stamp and for older code editions that tabulated SYP longer. LVL is not in R602.7; the calculator maps a conservative 2.0E equivalent so you can compare. The sticker on the LVL is the design document. Material choice: solid sawn vs built-up vs LVL.

Footnote on bracing: tabulated spans assume the top of the header is laterally braced by floor or roof framing. If cripple studs bear on the header and nothing braces the top, multiply 2x8 / 2x10 / 2x12 spans by 0.70 or have it designed. That single footnote is why a header that “made the table” still dips in a tall gable wall with a deep cripple pocket.

The header span table prints the grids so you can ask the inverse question: what opening can this 2-2x10 carry? The calculator answers the job question: what size do I need for this opening?

Worked Example (Same Opening, Two Load Cases)

A 3-0 door (36" RO) under roof-only / 24 ft / 30 psf still fits a 2-2x4 (3′-1" span, NJ 1). The “always double 2x8” habit is leftover from mixed load cases, not from this cell.

How to Apply the Result on the Job

1. Confirm the wall is the load case you typed. Pull a floor plan or look at joist/rafter bearing. If joists run parallel and do not land on this wall, it may be nonbearing even on the exterior.
2. Use rough opening as span. Door unit 32" is not a 32" header span. Add shims and the jack thickness to get RO, then that RO is the table span.
3. Cut length is longer than span. The header sits on the jacks. Add 1½" of bearing per jack each end (two jacks each side = 6" extra lumber).
4. Match plies and nail them. Built-up members are face-nailed as a unit; crown them the same way. Comparison of solid vs built-up vs LVL: which to use.
5. Install the NJ jacks, then kings, then cripples. Layout stick count is the stud wall calculator — do not use this tool to order kings.
6. If the top is a cripple pocket, apply 0.70 or step up a size / go LVL.

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