How to Resize Window Frames

Remove drywall from around the window with a pry bar to expose the window framing and wall studs. Take out the old window by removing the screws or nails holding it to the rough frame. Reduce the width by installing new 2-by-4-inch studs on the sides. Change the depth by adding a new header or sill board at the top or bottom of the rough opening.

Expand the size for a window by replacing the header, sill and side studs between those horizontal braces. Remove drywall and wall siding at least one stud width on each side of the opening and take out the old window. Brace the wall outside the studs with 4-by-4 inch posts set between the wall top and bottom plates. Fasten these temporarily with a hammer and 16d framing nails.

Pull off "trimmer" studs on each side between the header and sill. Take down the header and sill by pulling them loose from the nails holding them in place. Cut the wall studs with a reciprocal saw to the size of the new rough opening; base this on the specifications with the replacement window. Alter any studs above or below the existing opening if necessary for the new dimensions.

Make a new header with 2-by-6-inch boards, cut with a circular saw to the width of the rough opening, and nailed into a header with a 1/2-inch spacer of plywood between them. Build a new sill of 2-by-4-inch boards, cut to width and nailed together through the 4-inch faces. Fasten new header and sill to the outside studs. Use a level to set them level and drive framing nails through the studs into the header and sill ends.

Install new "trimmer" studs, cut to fit between the bottom of the header and the top of the sill. Use doubled 2-by-4s to support the header if the new window is much wider and deeper than the old one. Connect any studs that were cut off to the header and sill. Toenail studs to header and sill by driving nails diagonally.

Set the new window in place. Use tapered wooden shims to get it square and level. Drive shims between the rough frame and the window from both sides to adjust the window and secure it. Fasten it to the rough frame with screws, typically driven through the frame into the header, sill and studs. Add metal flashing around the outside of the window and replace drywall and siding that were removed.