Rough Opening (RO)
The framed opening in a wall for a window or door, sized larger than the unit to allow for shimming, leveling, and insulation. Typically 1/2" to 1" wider and taller than the window or door frame.
In Detail
Rough opening dimensions are specified by the window/door manufacturer. Common convention: add 1/2" to each side (1" total width) and 1/2" at top. Bottom may include a 1/2" gap or a rough sill, depending on installation method. The RO must be square, plumb, and level — any deviation makes the unit impossible to install correctly. Verify RO dimensions against the manufacturer's installation guide, not generic rules of thumb.