Calculating header size is complicated. You have to take into account: (1) the length of the window or door opening; (2) the combined weight of the floors, walls and roofs above; (3) the building width; (4) the snow load in the area; (5) whether it’s a bearing wall (where joists, trusses and rafters rest) or a non-bearing wall (to which joists, trusses and rafters run parallel); (6) whether it’s an exterior or interior bearing wall; and (7) what species of wood you’re working with.
The 2000 International Building Code book contains two full pages of mind-numbing charts for calculating proper header sizes in different situations. Fig. B shows the maximum allowable spans for different size headers in just one situation. As you’d expect, the deeper the header, the longer the distance it can span. But trust me, you don’t want to wade into all the technicalities. There’s no simple formula. My advice is this:
Have an engineer or architect calculate the required header size for your window and door openings.
Ask your local building code official to help you calculate header size. It’s usually not in their job description, but the nice ones will help you out.
When in doubt, build a double 2x12 header sandwich like we explain next. In all but the most bizarre situations, they’ll easily carry the weight for 4-ft. wide window and door openings and, in most situations, be code compliant for openings up to 6 ft. wide—a common patio door width.
HEADER SIZE MAXIMUM SPAN NUMBER OF TRIMMERS
Double 2x6 4 ft., 0 in. One
Double 2x8 5 ft., 0 in. Two
Double 2x10 6 ft., 2 in. Two
Double 2x12 7 ft., 1 in. Two