Door Header Size Calculator

A header is a beam spanning an opening in a stud wall, carrying whatever sits above it and delivering that load through jack studs to the plates and foundation below. Required depth grows quickly with span because bending moment in a simply supported member rises with the square of the length. Sizing begins with tributary width: the header picks up half the joist span on each side, so a 12 foot joist run contributes about 6 feet of tributary width. At typical residential design loads of 10 psf dead plus 40 psf live for floors, that is roughly 300 pounds per linear foot per story, while a roof at 15 psf dead with a 20 psf snow or live load adds about 210 plf. Prescriptive tables in the International Residential Code let builders skip the structural calculation for common cases. Doubled 2x6 through 2x12 members handle most single-story bearing situations out to roughly 8 feet of clear span, with two jack studs on each side once openings get wide. In a 3.5 inch 2x4 wall the pair of members is separated by a 1/2 inch plywood spacer so the built-up header fills the wall cavity. Beyond the table limits, or where multiple stories plus a roof bear on a wide opening, engineered lumber or steel and a designer signature are required. Note also that deflection rather than strength often governs: codes commonly limit live-load deflection to span divided by 360, so a header that will never break can still crack drywall and bind the door it sits above.

Header sizing is a byproduct of light-frame construction, which displaced heavy timber framing across North America during the nineteenth century. Balloon framing, built from sawn studs and cheap machine-made nails, spread from Chicago in the 1830s and made it routine to interrupt a wall with an opening that needed a beam over it. Platform framing largely replaced balloon framing by the middle of the twentieth century and gave the modern arrangement of king studs, jack studs, and a built-up header seated on them. The numbers behind the tables came from lumber standardization and testing. Federal work on American Lumber Standards beginning in the 1920s fixed the relationship between nominal and actual dimensions, and the National Design Specification for Wood Construction, first published by the lumber industry in the 1940s and now maintained by the American Wood Council, established the allowable stresses from which span tables are derived. Those tables were folded into the regional model codes, and after the Building Officials and Code Administrators, the International Conference of Building Officials, and the Southern Building Code Congress merged into the International Code Council, the International Residential Code published from 2000 onward became the common reference in the United States. Laminated veneer lumber, commercialized during the 1970s, later extended practical header spans well past anything the sawn lumber tables allow, which is why wide openings now default to engineered beams.