Size a window rough opening for new framing. Enter the window dimensions and your shim gap to get the rough opening and the header length to cut.
Rough Opening Width
— in. wide
Width of the framed opening (window width + gaps).Usage Tip
Aim for an even gap all around so the unit sits square — too tight and you cannot shim it plumb. Confirm the manufacturer rough-opening dimensions before you cut.
THE MATH
RO width = Window width + 2 × gap
RO height = Window height + 2 × gap
Header length = RO width + 3″
RO height = Window height + 2 × gap
Header length = RO width + 3″
Gap is the shim space on each side (typically ½″ to ¾″) that lets you level and plumb the unit.
Header length adds 1.5″ for each jack stud the header bears on.
Header length adds 1.5″ for each jack stud the header bears on.
These are typical shim allowances, not a fixed standard — flanged new-construction windows have manufacturer-specified rough openings, so confirm the spec sheet for your unit before framing.
Frame a rough sill between the jack studs at the base of the opening, with cripple studs below it down to the bottom plate.
Check the opening for square by measuring equal diagonals, and do not frame tight to the window.
Frame a rough sill between the jack studs at the base of the opening, with cripple studs below it down to the bottom plate.
Check the opening for square by measuring equal diagonals, and do not frame tight to the window.