Header Size Calculator
Header sizes for an opening, with jack studs and bearing — and why the plywood in a sandwich header is a spacer rather than a structural member.
Every option, and how hard it is working
The bar is the fraction of bending capacity used. Anything past 100% fails, and anything that doesn't fill the wall needs packing out even if it is strong enough.
The plywood isn't doing anything
The half inch of plywood in a sandwich header is a spacer. Two 2× plies make 3″; a 2×4 wall is 3½″. The plywood closes that gap so the header finishes flush with the studs and the drywall lies flat. Three plies make 4½″ against a 5½″ 2×6 wall, and take two spacers for exactly the same reason. The arithmetic is that tidy because that is the whole purpose. Every published header table and every design value counts the sawn lumber and nothing else.
And it would barely matter if it did count. Credit the plywood generously — as though it were equivalent graded lumber running the full depth — and it adds 16.7%. Going one size deeper adds between 80% and 129%. So even at its most flattering the spacer is worth a fraction of a single size, and in reality much less, because plywood is not graded for bending along a span. If a header is short of capacity, the answers are depth, then engineered lumber, then steel. More plywood is not on the list.
A header carries half of what bears on it. The tributary width is half the span of the joists or rafters landing on that wall — the other half goes to the support at the far end. Put the building width where the tributary width belongs and every number downstream doubles. This catches people out particularly often because the published IRC tables are indexed by building width and do the halving internally, so a table lookup and a hand calculation that look like they used the same input have not.
- Opening width is squared; tributary width is linear. Double the opening and the bending moment quadruples. Double the tributary and it merely doubles. Widening an opening costs far more than it looks like it should.
- Jack studs are part of the answer. The reaction at each end has to pass through the bearing without crushing the header across its grain, which happens at a fraction of the stress timber takes along the grain. A header sized correctly and set on one jack has just moved the failure to the bearing.
- A third ply adds exactly half again — one size deeper adds much more. And the deeper header uses less timber doing it.
- Two LVL plies fill a 2×4 wall with no spacer at all, since 1¾ × 2 is exactly 3½. That, plus roughly three times the bending value of No. 2 sawn lumber, is why LVL took over the header market.
- Follow the load down. It is common for the header to be right and for the reaction to land on a rim joist with nothing solid beneath it. The path has to reach a footing: jack studs, plates without crushing, floor framing, and down.
- A point load changes everything. A girder truss, a beam bearing or a post from above is a different calculation, and a uniform-load check will underestimate it.
How to use
- Enter the clear opening width and the building width.
- Say what the joists do — clear span puts twice as much on the header as a centre bearing wall does.
- Pick what the wall carries, since adding a floor roughly doubles the load.
- Read the jack stud count alongside the header — the bearing is part of the answer.
Frequently asked questions
Does the plywood in a header add strength?
No — it is a spacer. Two 2x plies make three inches and a 2x4 wall is three and a half, so the half inch of plywood closes that gap and lets the header finish flush with the studs. Three plies make four and a half against a five and a half inch 2x6 wall, and take two spacers for the same reason. The arithmetic is that tidy because filling the wall is the entire purpose. Every published header table counts the sawn lumber and nothing else.
How much would the plywood add if it did count?
About 16.7 per cent, and that is being generous — it assumes the plywood behaves like equivalent graded lumber running the full depth, which it does not, because plywood is not graded for bending along a span. For comparison, going one size deeper adds between 80 and 129 per cent. So even at its most flattering the spacer is worth a fraction of a single size. If a header is short of capacity the answers are depth, then engineered lumber, then steel.
What tributary width does a header carry?
Half the span of the joists or rafters landing on that wall, because the other half goes to the support at the far end. On a 28 foot clear span that is 14 feet; if the joists land on a centre bearing wall it is seven. Entering the building width where the tributary width belongs doubles every number downstream, and it catches people out especially often because the published IRC tables are indexed by building width and do the halving internally.
Does opening width or tributary width matter more?
Opening width, and by a wide margin, because it enters squared while tributary width is linear. Double the opening and the bending moment quadruples; double the tributary and it merely doubles. That is why widening an opening by a couple of feet often costs a whole header size, and why a wide window in a narrow house can be harder than a narrow one in a wide house.
How many jack studs does a header need?
As many as the reaction at that end requires. The load has to pass through the bearing without crushing the header across its grain, and timber is far weaker across the grain than along it — a few hundred psi against a couple of thousand. Wide or heavily loaded openings routinely need two or three at each end. A header sized correctly and set on a single jack has simply moved the failure to the bearing.
Is a third ply as good as going one size deeper?
No. A third ply adds exactly half again, because width enters linearly. Going one size deeper adds 80 to 129 per cent, because depth enters cubed. The deeper header also uses less timber to do it. Adding plies is what you do when depth is not available — a shallow header space above a tall window, say — not what you do first.
Why is LVL used for headers now?
Two reasons that compound. Its bending value is roughly three times that of No. 2 sawn lumber, and two plies of the standard 1¾ inch thickness come to exactly three and a half inches, so they fill a 2x4 wall with no spacer at all. Deeper LVL is proportionally slightly weaker in bending, which manufacturers publish as a depth adjustment, so a 16 inch beam gets about four per cent less than a 12 inch one.
What deflection limit applies to a header?
L/240 is the usual figure for headers generally, tightening to L/360 where the header carries a floor or supports a brittle finish like plaster or stucco. Note that a tighter limit never changes how much the header actually deflects — only how much is permitted — so it can push you up a size without anything about the loading having changed.
Do I need a header in a non-bearing wall?
Something has to span the opening, but it is not carrying anything beyond itself and the wall above. A single flat 2x is normally enough, and its job is to give the drywall and the trim something to fasten to. The important part is being certain the wall really is non-bearing, which is not always obvious from inside a finished house.
What happens below the header?
The part people forget. The reaction at each end has to reach a footing — through the jack studs, through the plates without crushing them across the grain, through the floor framing and down. It is common for the header itself to be right and for the load to land on a rim joist with nothing solid underneath. Following the load all the way down is as much a part of the job as sizing the beam.
Does this handle point loads?
No, and that matters. This is a uniformly loaded, simply supported check. A girder truss landing on the header, a beam bearing, or a post from above is a different calculation, and a uniform-load check will underestimate it — sometimes badly, since a point load at mid-span produces twice the moment of the same total load spread out.
Can I build from these numbers?
Check them against the span table your jurisdiction has adopted first. LVL design values in particular vary by manufacturer and their published tables govern rather than a generic calculation. Anything unusual — a point load, an opening near a corner, a wall carrying a concentrated roof load, or an existing wall being altered — wants an engineer rather than a calculator.
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