“Rough Opening” refers to the framed opening of a window. Framed openings have a header across the top, a sill plate across the bottom and vertical trimmers between the header and sill at each side.Replacement windows are available in a variety of sizes that are listed according to width and height.or example, a window that’s 4-feet wide and 3-feet high is listed as 4/0 - 3/0. But the actual measurements of windows vary depending on a particular manufacturer and the installation system. For this reason, use a proven method to measure the rough opening before purchasing a replacement window.