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Window headers are supports found on the upper portion of the window casing. Generally, the supports are at least twice as thick as the framing components found around the opening. The header will span horizontally across the top of the window casing, offering added support that prevents the full weight of the wall from resting on the window casing itself.
Found in both residential and commercial buildings, the window header helps to add stability to the wall while making it possible to insert an opening for the window frame. A traditional window header will be located near two joists, rafters, or studs, depending on how the windows are placed. By placing this thicker and shorter wood beam under the supporting joist or rafter but over the opening for the window, there is no added stress placed on the joist. This means that after the window installation takes place, the window frame will not have to bear any load in order to keep the wall section in line with the rest of the wall.
Along with maintaining uniform support for the wall components and preventing additional load-bearing stress on joists and rafters, the window header also helps the overall structure to support and distribute the weight of the roof across the interior supporting walls as well as the exterior walls. The window header effectively allows the sections of wall above and around openings such as doors and windows to support the same amount of load as any other section. As a result, the load of the roof is borne equally, with no weakened areas that must be compensated for at another point along the wall. From this perspective, a window header can be seen as a good way to maintain the integrity of the whole structure and allow the building to settle properly over the years.
A window header is normally constructed with the same types of wood used in the design of the wall frame. It is not unusual to form a header by simply cutting two sections of the framing wood to the appropriate lengths, then nailing or gluing them together to form a thicker body.
It is not unusual for a window header to be replaced during part of a home improvement renovation. This is especially true when new window casings are inserted as part of the renovation project. Once the old frame is removed, the rafters or joists can be temporarily supported while the old header is removed and a new header is installed.
I have always noticed the extra wood around the top of my windows, but I never really thought about why it was there. I tend to think more in an aesthetic vein than in a physics vein. However, it makes perfect sense that glass would need something to hold the load of the house off of it.
My friend has decorative window headers over every window in her home. They form arches and have carvings in them. This could explain why most people just think they are purely ornamental.