Header Size Troubleshooting

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Sagging Headers

Drywall cracks at the upper corners of a window, a slider that started binding in year two, or a crown that you can see against a string line: the header is moving. Causes that show up over and over:

• The table cell was for a lighter load. Roof-only size left in place after a second floor landed on the wall. Re-run as “roof + one center-bearing floor.”
• Unbraced top (0.70 factor ignored). Deep cripple pocket, no floor/roof framing nailing off the header top. IRC says multiply 2x8/10/12 spans by 0.70 or design it.
• Built-up plies not nailed as a unit, or crowns opposing so the member is a spring.
• Wrong species/grade story. Utility grade or a 2x6 stamped for studs used as a header ply.

String a line along the header soffit. More than about 1/4" of dip on a 6 ft opening is not “wood drying” — size or bracing is wrong. Method reminder: guide.

Undersized Headers on Load-Bearing Walls

“We always use a double 2x6” is a partition habit. On a 24 ft-wide exterior wall, 30 psf, roof only, a 2-2x6 only spans 4′-7". A 5-0 window is already past it. On roof + floor the same 2-2x6 is 3′-9". The wall did not stop being bearing because the opening is a window.

How it happens on remodels: someone treats an exterior wall as a partition because there is a hallway on the other side, while ceiling joists or a roof truss still bear on the top plate. Look at the joist direction and the roof. If in doubt, assume bearing and use Table R602.7(1), or get it engineered.

Missing Jack (Trimmer) Studs

The header must bear on jack studs (or an approved framing anchor when the table’s NJ is 1). Hanging a header on the king studs with nails in shear, or letting it rest on a single jack when NJ is 2, overloads the remaining wood and the king. Symptoms: king studs bow, nails withdraw, trimmer splits, header drops at one end only.

NJ is in the same table cell as the span. The calculator prints it. If you cannot fit two jacks, you do not get to keep the same header size — you step up plies (sometimes a 3-2x8 has NJ 1 where a 2-2x10 has NJ 2) or use a hanger rated for the reaction. King/jack counts for ordering lumber are the stud wall calculator; this page is about whether those jacks are actually under the header.

Rough Opening vs Header Length

Two different numbers get swapped constantly:

• Rough opening / table span — inside of jack to inside of jack. This is what you type in the calculator.
• Header cut length — RO plus bearing on the jacks (1½" per jack each end).

Cut the header to the RO and it has nothing to sit on. Cut it to the RO plus 3" when you have two jacks each side and you are still 3" short. The window unit size (for example 3050) is a third number and is not the RO until you add the installation gap the manufacturer lists.

On the plate layout, mark kings first, then jack faces, then RO. The header board is the distance between kings, which equals RO + jack thickness both sides. If that sentence is fuzzy, walk it with a tape on the next opening before you cut a $40 LVL.

How to Diagnose and Fix

1. Shore the opening. Do not sister a live sagging header with finish in place and no temp posts.
2. Identify load path: roof only, roof + floor, or partition. Re-run the calculator.
3. Count jacks vs NJ. Count plies vs the recommended size. Check whether the top is braced.
4. If the existing header is one size small and the dip is small, a designed sister (often LVL) can work. If jacks are missing, add them to bearing — sistering the header without bearing is theater.
5. Replace when the member is split, undersized by more than a table step, or in a shear wall. Wide openings past 4-2x12 leave the prescriptive table; that is LVL/glulam/steel and an engineer. Steel path: steel beam calculator.

Material when you replace: sawn vs built-up vs LVL. Full spans: header span table.

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