When framing for a window in a new location, it is usually easiest to cut an opening in the interior drywall that is quite a bit larger than the window opening. This will require you to patch the walls afterward, but installing framing—especially the header—inside a wall is difficult.
It is also usually easiest to leave the exterior sheathing and siding in place while you cut the studs and build the framing. This prevents having to replace siding. However if you will be replacing the siding anyway, you can cut a large opening in the exterior at the same time you cut the interior opening. If the window has brick molding or a flange, you will need to cut back the siding to accommodate it.
If you have a brick wall, hire a professional mason to cut an opening. When framing always work carefully to produce a structure that presents a smooth face for the drywall. Install adjoining pieces perfectly flush with each other.
Expect to spend a full day on this project—you'll need to remove interior drywall and create the frame. Though it looks daunting, this project is fairly simple. We recomment DIYers have experience measuring, marking, and cutting before attempting to frame.
Before you begin, place drop cloths on the floor and seal the doorways to prevent dust infiltration. Place a fan in a nearby window and point it outward.
Then, examine your studs. In this arrangement an existing wall stud is used as a king stud on one side, and a 2x2 or 2x4 cleat is attached to the stud on the other side to provide a nailing surface for patching the drywall after the window is installed. (This is easier and stronger than trying to cut the drywall down the middle of a stud and then using half a stud's thickness for the nailing surface.)
You will need at least one new king stud, which is the same length as the other existing studs. Cut two cripple studs; the sill will rest on their tops. (A jack stud will be fastened above so it and the cripple stud sandwich the sill.) Drive a fastener every 12 inches or so, in an alternating pattern.
Install a king-and-cripple stud combo by wedging the king stud between the top and bottom plates. If framing a cavity push the king-and-cripple stud halfway behind the drywall and drive angled 10d nails or 2-inch screws. (Fastening will probably move the studs over slightly, so start with the studs offset by 1/4 inch or so.)
Mark the bottom (sole) plate to indicate where the king studs and jack studs will go. The distance between the jack studs is the width of the rough opening.
Hold a level atop the jack stud and mark across the studs to indicate the bottom of the header. Measure upward to mark where the top of the header will be. Make cutlines about 1/8 inch higher than the measurement. If there is a gap, fill it with shims. Make the bottom cuts (step 7) about 1/8 inch lower than the measurement.
Also mark the middle studs for cutting at the bottom. They should be marked for the needed rough opening dimension plus 1 1/2 inches (for a single bottom plate) or 3 inches (for a double bottom plate).
Complete your marking by using a square to mark across each of the studs to ensure a square cut. If you have not already done so, install a temporary support for the wall before you start cutting.
Set the blade of a circular saw to full depth, and check to make sure it is square to the saw's baseplate. Cut a scrap piece to confirm that the blade is cutting square. Wearing ear and eye protection, cut the lines with the circular saw.
Finish the cuts using a handsaw or a reciprocating saw.
To build the header, cut two pieces of 2x6 or 2x8 to the width of the opening plus 3 inches. Cut strips of 1/2-inch plywood as spacers. Lay the 2xs on top of each other to determine which direction (if any) they "crown," bend upward in the middle. See that their crowns face the same way. Position a spacer every 8 or 10 inches, lay the second 2x on top, and drive two or three 3-inch screws.
Set the header on the jack studs with the crown facing up. You may have to hammer it into position. Attach the header with 16d nails or 3-inch screws.
Cut one or two pieces to length for the sill. Nail or screw the sill in place. If using two pieces fasten one piece to the top of the cripple studs, then add the second piece.
Check the opening for square. As long as the opening is 1/2 inch wider and taller than the window, it is OK if the opening is 1/4 inch or so out of square. As a final check, set the window in the opening and check it for square.
Once you are certain the opening is correct, use a drill equipped with a long 1/2-inch spade bit to bore locator holes to the outside at each corner.
If patching walls will be difficult (as when you have plaster walls), you may choose to slip the new framing in. Cut a section of the finished wall that is the correct size of the opening. Use a reciprocating saw to slice through the studs above and below, taking into account the widths of the header and sill.
Cut through the studs midway and pull them out. Carefully clean out the openings to allow room for the header and the sill.
Slip the new header into the opening and slide in a jack stud at either end to hold the header up. Slip in the sill as well. Angle-drive screws to hold the framing pieces together.
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