How this instrument works
This is a preliminary structural estimate only, not a substitute for a licensed structural engineer or your local building department's plan review. A door or window header is a built-up horizontal beam that carries the roof and wall load from above an opening down to the jack studs on either side, and every code jurisdiction has its own prescriptive header-span tables for exactly this reason — sizing a header wrong risks sagging, cracking finishes, or worse over time.
This instrument checks the same two limit states a structural beam calculation always checks: bending (does the header's material bend past its allowable stress under the load) and deflection (does it sag more than a set fraction of its span, here L/240, the standard IRC criterion for a roof member with a ceiling below it). It solves each check backward, for the maximum span at which that limit would just start to fail, then reports the smaller — the governing — of the two as the header's estimated maximum opening width.
The reference design values used, Fb (allowable bending stress) and E (modulus of elasticity), come from the AWC National Design Specification Supplement Table 4A for two visually graded lumber species: Spruce-Pine-Fir No.2 and Douglas Fir-Larch No.2. These are base reference values only — this calculator does not apply any of the NDS Chapter 4.3 adjustment factors for load duration, wet service, temperature, or beam stability that a real design must include, and the result is not the official IRC prescriptive header-span table figure. It's a simplified engineering estimate for planning purposes; always verify against your local code's header table or a structural engineer before framing.
- Select Species / grade — Spruce-Pine-Fir No.2 or Douglas Fir-Larch No.2, each with its own AWC-published Fb and E reference values.
- Select Header plies — how many 2x pieces are built up together (single, double, or triple ply).
- Select Header size (nominal) — the depth of lumber used, from 2x4 up to 2x12.
- Enter Roof snow/live load (psf) and Roof dead load (psf) — use your local ground snow load or roof live load; there's no safe universal default, so check your jurisdiction's requirement.
- Enter Tributary width supported (ft) — typically half the building width or roof span this header carries.
- Read Governing max opening width (ft) — the smaller of the bending-limited and deflection-limited spans, a preliminary estimate to check against your local header-span table or an engineer.
Worked example — a double 2x8 header over a 6 ft tributary width
A double-ply 2x8 header (actual size 3in × 7.25in) in Douglas Fir-Larch No.2 (Fb=900 psi, E=1,600,000 psi) carries a 30 psf roof snow load plus 15 psf dead load over a 6 ft tributary width. Section modulus S = 3 × 7.25² ÷ 6 = 26.28125 in³, and moment of inertia I = 3 × 7.25³ ÷ 12 = 95.2695 in⁴. Total load w_total = (30+15) × 6 = 270 lb/ft; live-only load w_live = 30 × 6 = 180 lb/ft.
The bending check gives L_bending = √(900 × 26.28125 ÷ (1.5 × 270)) = √58.40 = 7.642 ft. The deflection check (L/240) gives L_deflection = (1,600,000 × 95.2695 ÷ (450 × 180))^(1/3) = 1881.9^(1/3) = 12.35 ft. The governing (smaller) span is 7.64 ft, bending-controlled — a realistic double-2x8 opening-width ballpark for this load, before checking it against your local header table.
Questions
Is this the same as my local building code's header span table?
No. This calculator applies the general bending and deflection formulas from mechanics of materials using AWC NDS Table 4A base reference values, but it does not apply the load-duration, wet-service, size, or beam-stability adjustment factors an official NDS design requires, and it is not the IRC's own prescriptive header-span table (IRC Table R602.7). Use this as a rough planning estimate, then confirm the actual required header size against your jurisdiction's adopted code table or a structural engineer.
Why does the deflection check use L/240 here instead of L/360?
L/240 is the IRC's standard deflection criterion for a roof member supporting a ceiling below it, which is what a door or window header typically does, while L/360 is the tighter criterion commonly used for floor framing. A header allowed to deflect a bit more than a floor joist won't crack a ceiling the way excess floor bounce would — that's the structural reasoning behind the two different standard ratios.
What roof snow load should I enter?
There's no safe universal default for this field on purpose — ground snow load and the resulting roof live load are entirely site-specific, varying enormously by region and even over short distances in mountainous terrain. Look up your local ground snow load and roof live load from your building department or a structural engineer before entering a value here; guessing a number risks meaningfully under- or over-sizing the header.
What does 'tributary width' mean for a header?
Tributary width is the portion of the roof (or floor, if applicable) whose weight actually bears down through this specific header, typically about half the building's width for a header centered under a ridge, or the distance to the nearest other support. A header carrying a wider tributary width sees a proportionally higher load per foot of its own span, which is why doubling the tributary width in this calculator noticeably shortens the resulting maximum opening.
Why does adding plies increase the maximum opening width?
Each additional 2x ply adds 1.5in to the header's width, b, and both the bending check and the deflection check scale with the header's cross-sectional stiffness — a wider header resists bending stress better (S = b·d²/6 grows linearly with b) and deflects less (I = b·d³/12 also grows linearly with b) for the same load. Going from a single to a double or triple ply is one of the most direct ways to size up a header without changing its depth.