Window / Door Header Sizing (IRC R602.7)
Smallest built-up dimension-lumber header (double / triple 2x6-2x12) from tributary load, span, snow, and species, with the AWC NDS bending / L-360 deflection check and IRC R602.7.5 jack-stud count.
Formula and source
Tributary uniform load w (plf) = total area load (psf) × supported width (ft); roof = ground snow + 15 psf dead, each floor adds 50 psf. Smallest built-up member with min(L_bending, L_deflection) ≥ span, where Fb is adjusted by load-duration C_D and size factor C_F. Jack studs each end = ceil(end reaction / (F_c-perp × 5.25 in²)).
IRC 2021 (International Residential Code). §R602.7 (headers). AWC NDS-2018 (National Design Specification for Wood Construction). reference design values, load-duration C_D and size factor C_F.
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