Cut an opening in a bearing wall and the studs that used to carry the roof no longer exist — the header is the short beam that bridges the opening and collects their load. For one-story exterior bearing walls carrying roof and ceiling only, 2021 IRC Table R602.7(1) sizes it prescriptively: built-up dimension lumber in 2 or 3 plies of 2x6 through 2x12, with spans based on No. 2 grade minimum properties, and the 2021 table is species-independent — the same values apply to Douglas fir-larch, hem-fir, southern pine, and SPF alike.
Two geometry facts drive the lookup. Building width, measured perpendicular to the ridge, sets how much roof load reaches the wall — the table columns run 12, 24, and 36 ft, and spans shrink as width grows. Ply count buys reach: three members span roughly 25% farther than two, though the table only publishes 3-ply rows for 2x8 and up. In a 24-ft-wide house a 2-ply 2x10 spans 6 ft 10 in, enough for a 6-ft patio door with margin.
The table also prescribes the support: NJ, the number of jack studs required under each header end (2 for the 2-ply 2x10 example; where NJ = 1 an approved framing anchor may substitute per R602.7.5). Headers carrying a floor above, higher snow loads, or spans beyond the table leave prescriptive territory and need the full code table or an engineered design.