When building a frame to install a window, one of the most important aspects is the window header—specifically the window header size.
Correctly sizing the header to the opening is critical not only to smooth window installation but to maintaining the structural integrity of your home. If your header is incorrectly sized, the window may fail to open and close properly or its frame or glass might crack. The worst-case scenario is a structural failure of a section of your house.
What Is a Window Header?
A header is any strong beam that horizontally spans an opening, such as a doorway. So, a window header is one that spans the top of a window opening. The header is the structural element—much like a bridge—that allows the space below to be open, not closed.
Another way to understand a window header is to consider what's being replaced. In that space ordinarily would be an exterior wall of wood-frame construction. Studs spaced every 16 or 24 inches would form a very strong wall system that bears vertical forces. The window header allows you to remove some of those studs while maintaining the strength of the wall.
Common Window Header Sizes
Window headers are usually built of two pieces of dimensional sawn lumber placed side-by-side. Like other pieces used in wall systems, window header thickness is limited by the depth of the wall.
Most wall systems use two-by-fours, which are actually 3-1/2 inches wide. Greater strength for beams can be achieved by using thicker beams. But this is not possible in walls since the walls always have to maintain that 3-1/2-inch depth. So, the solution is to go up: that is, to use wider pieces of lumber that extend vertically higher.
Window Span: 3 feet
For windows that span 36 inches or less, use two two-by-fours or one four-by-four. Since many windows are in the 24-inch to 34-inch range (width), this is a commonly used window header size.
Window Span: 4 feet, 6 Inches
For wider window spans of 4 feet, 6 inches or less, increase the side of the window header to two two-by-sixes. A large number of windows fall in the 45- to 50-inch range, so this window span is also used quite often.
Window Span: 5 feet, 9 Inches
For even wider window spans of 5 feet, 9 inches or less, use two two-by-eight pieces of lumber.
Sizing a window header individually, using span and load tables, is always the most accurate way to determine header size. This helps to address the innumerable factors that can change with windows, including sizes, shapes, and loads. Contact a structural engineer, licensed contractor, or architect.
Factors That Change Header Sizes
These window header size recommendations apply to single-story construction on a home 20 feet wide. Wider buildings decrease window header span length. Heavy snow or other factors can change window header sizes, too. That's why it's important to tailor all the building to local codes.
Some do-it-yourselfers and builders will over-build the window header in an abundance of caution. A couple of two-by-twelves side-by-side are commonly used for 6-foot patio or french doors and even for 4-foot wide window openings.
Over-building will increase the cost just slightly. One downside is that it increases thermal bridging between the outside and the inside. Less insulation is being used in this critical area.
King and Jack Stud Quantities
Each window header requires two king studs and two jack studs (one on each side). Yet, as window widths increase, a total of four jack studs (and two king studs) may be required. Adding another story on the building means that not only do window spans narrow but the likelihood of more jack studs being required increases.
Using Laminated Veneer Lumber (LVL) For Window Headers
Instead of sawn lumber, laminated veneer lumber, or LVL, can be used for window headers.
Laminated veneer lumber uses thin veneer layers to create larger beams. In the factory, the layers are glued together and held under high-pressure until bonded. Another type of LVL is glulam beams: full-sized lumber glued under high-pressure to form beams.
The strength and stiffness of LVL exceeds normal sawn dimensional lumber. LVL's value is evident with very wide spans, such as for garage door headers or french doors. Due to the higher costs of LVL, you may want to use LVL mostly for wide window header spans of 6 feet or greater, rather than smaller windows that can be easily bridged with sawn lumber.