Header Size Calculator

A header is a short, heavily loaded beam that carries the wall, floor, and roof loads that would otherwise pass straight through a window or door opening, and routes them into the jack studs at each side. The design load per linear foot is the sum of the contributing surface loads times the tributary width: a one-story wall with a 30 psf roof load, roughly 15 psf of wall and ceiling dead load, and a 12-foot tributary width produces 540 plf. For a simply supported member under uniform load the maximum moment is M = w L squared divided by 8, and the required section modulus is S = M divided by Fb. Standard built-up options run from a double 2x6, with a section modulus near 15 cubic inches, to a quadruple 2x12 at roughly 127 cubic inches, and the smallest section that satisfies both bending and deflection governs. Header deflection is normally checked at L/240 for total load rather than the L/360 used for floor live load, since a header carries mostly permanent load. Two checks are easy to overlook. Bearing perpendicular to grain is limited to about 625 psi for Douglas Fir-Larch, so a 1.5-inch-wide jack stud can only accept so much reaction before the fibers crush, which is why wider openings call for two or three jacks per side. And a point load from a post landing over the opening roughly doubles the moment relative to the same force spread uniformly, putting the case outside prescriptive tables entirely.

Wood headers are the framed descendant of the masonry lintel, the stone or timber beam laid across a doorway in construction going back to ancient Egypt and Greece. The specifically modern form arose with light wood framing in nineteenth-century North America, where balloon framing and later platform framing replaced heavy timber posts and beams with repetitive small-dimension studs. Once a wall was a row of identical studs rather than a frame of posts and infill, an opening had to be bridged by a purpose-built member, and the header plus jack stud plus king stud assembly became the standard solution. Prescriptive header sizing entered American practice through the regional model codes of the mid-twentieth century and was consolidated when the International Code Council, formed in 1994 from the merger of BOCA, ICBO, and SBCCI, published the first International Residential Code in 2000. Its girder and header span tables let a builder select a size without running calculations, provided the loads fall within the tabulated assumptions about roof span, snow load, and number of stories. The other major change came from engineered lumber: Trus Joist introduced laminated veneer lumber under the Micro-Lam name in 1971, and LVL headers with allowable bending stresses in the 2,600 to 3,100 psi range now routinely handle the wide openings that solid sawn lumber cannot reach at ordinary depths.