Rough Opening Size
If you’re installing a new window, the manufacturer will include specs for the exact measurements of the rough opening. If you’re installing the window your friend gave you or the one you bought at an auction, however, it may not come with specs. When this occurs, measure the height and the width of the window, from outside frame to outside frame, and build the rough opening 1/2 inch wider and 5/16 inch longer.
Unless you live in a community with strict design covenants, there may be no rule telling you where to put your window. If you look around at other home exteriors, however, you’ll notice that most windows have one thing in common. The top edges of the windows are all about the same height. This is because the top board in the rough opening frame is usually 82 inches above the subfloor. Whether your window is tall and narrow or short and wide, the standard height for the top of the rough opening shouldn’t change.
Rough Opening Components
Wall studs sit 16 inches apart between a wood floor plate and a top plate. Every stud in the wall shares in supporting the wall structure. When you cut out one or more studs to build the frame for the window, you’ll have to transfer the weight load from the cut studs to the full-length studs on either side. A horizontal header, usually cut from larger-dimension wood, serves as the top of the rough opening for the window, while bracing the side studs. A typical header for a standard 2-by-4 wall might be two 2-by-10s, nailed together and turned so the broad side is flush with the stud wall. A sill board, usually another 2-by-4, serves as the bottom board in the rough opening.
Setting the Window
Even if your rough opening seems perfectly level, you’ll probably have to adjust the window for level. This is because the window, especially if it’s not new, may have some dimensional discrepancy. By building the rough opening slightly larger, you can adjust the window by inserting shims on the sides or beneath the window to level. Always level the window before nailing the flange to the framing. Before trimming the window, cut thin strips of fiberglass insulation to tuck into the small gap around the window.