What Is A Window Header And How To Determine What Size You Need
A window's header is one of its most important structural components. It's the beam that runs horizontally across the top of the window frame. The header is what helps keep the structure of the window in place, supporting the window that's right below and the wall above it. Determining the right size for a window header is essential if you're building a window into your home, and ultimately, the wrong size and placement will only serve to weaken the entire wall.
The window header size is dependent on the proportions of the window you plan to install. You'll need to figure out how many two-by-four pieces of wood you'll need to create the proper header, as well as how wide they should be. Most of the time, headers will have two pieces of lumber that are placed right next to each other, but this thickness is directly dependent on the size of the wall (the window header shouldn't be any deeper than the wall's thickness).
The height of the window header is more complex, requiring careful calculations based on the structure. If you're adding a window to your home, there are a few other key aspects to consider about this structural component. If you're replacing a window and need to replace the header, replicate what's already there unless you plan to change the sizing.
Think of a window header like a bridge
When framing a window, consider the header like the span of a bridge, which will ultimately enable the installation of the window below it. The header helps to maintain the structural integrity of the wall and lets you add a beautiful view of the outside without weakening the actual wall structure.
That's why framing windows out properly is essential, but calculating the proper size of a window header isn't so easy. You'll need to consider the opening size of the window but also how heavy the structure of the home is above, including the floors, all of the walls, and even the roof. Then, you must also consider the snow load (depending on where you live), if the wall is considered a weight-bearing wall or not, and even the type of wood being used. All of these factors ultimately determine the strength that must be created to support the window structure.
One option is to look up one of the many (and sometimes complicated) charts offered by the International Code Council to learn more about the proper sizing of the window header based on all of these factors. You can also turn to your local building department to inquire about the codes in your particular area. The building code inspector may be able to offer insight into the proper strategies as well, though they may not calculate the actual sizing for you.
The most common sizes in window headers
The heavier the weight of the structure above, the thicker the window header beam needs to be. Using a wider piece of lumber that's higher vertically allows you to add more strength to the header when the actual building wall is the standard two-by-four width.
Typically, if you have a window span that's 3 feet, you'll need to use two two-by-fours to construct the header. When that increases to a 4-foot window, you'll likely need to choose two two-by-sixes. And when you increase this to a 5-foot window, you'll need to go further by using two two-by-eights to create the header. Remember, though, that these are basic measurements, and you really do need to consult with a building engineer or use a structural chart to gain more insight into the proper sizing.
Why not just add extra weight, just in case, then? Doing this could seem logical, providing a safer solution, but the problem is 1) the added cost, and 2) it can increase thermal bridging. That means the area will have less insulation because of the wood surrounding it, and that's not ideal in a window area.
Also, be sure to check with the window manufacturer about the requirements. Specialized windows, like a bay window, will cost more to build out and require a very different window header design to support the weight.