Whether remodeling, adding on or building “from scratch,” we take great pains to construct strong, stable, enduring walls. Then we proceed to cut in opening after opening to accommodate the big, sun-filled windows, entry doors and patio doors we love. Strange, yes. But if we create these openings the right way, we never have to fear our walls or houses will collapse. And that “right way” means using the structural support system that has been developed over the years to keep our houses solid and happy. Here’s why you can punch in those big openings and what you need to know the next time you add on to or remodel your home.
Headers take the weight of the materials and occupants above and transfer it via the trimmers down to the floors and foundations below. Longer openings require larger headers. Bearing walls (those that carry the weight of joists and trusses) require larger headers than nonbearing walls.
For a larger version of Figure A, see Additional Information, below.
If you were to slide on those old Marvel Comics X-ray Specs (you know, the ones that can see through anything), and look around your doors and windows, you’d see something amazing. Over each window and door you’d find a sturdy wooden bridge—and at each end you’d find support pillars. These bridges aren’t glamorous like the Golden Gate or Brooklyn bridges, but they’re darn important. They’re the structural elements that allow us to install windows and doors without weakening the walls.
In the real world of architects, carpenters and lumberyards, these bridges are called headers (Fig. A). The vertical pillars supporting each end (normally composed of 2x4s or 2x6s) are called king studs and trimmers (or jack studs). In most wood frame dwellings, headers are made of dimensional lumber installed on edge. The king studs nailed into the end of each header and the trimmers that butt just under the header combine to create a sturdy vertical support column. Together the headers, king studs and trimmers act as a system that transfers weight from above, down and around the window and door openings to the floor and foundation below. The longer the distance a header spans and the heavier the load it supports, the more substantial it needs to be. Undersized headers will bow downward, pinching windows and doors and making them difficult to operate. They will also crack drywall and distort trim. There is no simple rule of thumb to determine exactly what size a header should be. But you gotta get it right
Figure B: Example of Calculating Header Size
Header size required to support the roof, ceiling and one center bearing floor of a 28-ft. wide building; 30-lb. snow load.
|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|
(Based on the 2000 International Building Code)
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.
Calculating header size is no picnic. Here are the allowable spans for header sizes in just one of hundreds of situations. If your remodeling or construction plans weren’t drawn up by
an architect or structural engineer, work with your building code official to determine
the right header size.
Header size required to support the roof, ceiling and one center bearing floor of a 28-ft. wide
building; 30-lb. snow load.
My first job as a carpenter’s helper was to construct headers for the tract houses we were building. I’d cut and nail together “header sandwiches” consisting of two 2x12s with 1/2-in. plywood between (Fig. A). This size worked out well because:
- The finished 3-1/2 in. thick headers (1-1/2 in. plus 1-1/2 in. plus 1/2 in.) were the same thickness as the 2x4 wall framing (3-1/2 in.; see Fig. A).
- When we installed the headers even with the top of the standard 92-5/8 in. high studs, it established just the right height for windows and doors (Fig. A).
- Since the architects and engineers who designed the houses had calculated that 2x12 headers were deep enough and sturdy enough to span the longest (6-ft.) opening, it was surely strong enough to span the more numerous smaller openings. By overbuilding, we carpenters could focus on building houses—not on poring over complicated charts to determine the header size for windows and doors. That’s everything a carpenter (and a DIYer) could ask for in a header: the right thickness, height and strength.
Three more components are used to finish the work headers began:
Trimmers (Fig. A) butt under, and support, each end of the header and are nailed to the king studs alongside them. Longer headers and those supporting more weight require the support of two or more trimmers on each end, and some openings require more than one king stud. Again, consult your architect, engineer or local code official to determine when you need to install extra trimmers or king studs.
Sills (Fig. A) establish the bottoms of window rough openings (the clear opening required for installing windows and doors). With large window openings, it’s a good idea to use doubled sills (Fig. A) for strength and stability.
Cripple studs (Fig. A) fill in the space between the sills and the 2x4 nailing plate below; they carry only the weight of the window itself. Sometimes cripples are installed over a window or door (Fig. C) to fill in the space between the top of a header and top of a wall. These do carry weight. As a bonus, all this extra wood provides an ideal anchor for the nails used to install wood trim and moldings—especially the wide stuff.
Big Headers for Big Openings
Garage doors and other large openings require headers too beefy to be made from standard lumber. These fabricated wood products can span longer distances and carry greater loads.