In this article:

• Preparing a rough opening
• Mounting the window
• Finishing the window

Many windows come prehung, which means they come assembled with frames. This makes the job of installing windows easier for do-it-yourselfers. If, however, you do not feel confident about tackling this task on your own, call in a qualified window installer.

Following are general instructions for how to install a prehung window. Note that directions will vary slightly from product to product, so always read your manufacturer’s instructions before beginning.

Preparing a rough opening

From inside your house, decide where you wish to place your new window, and then mark the location. You can do this by drilling at each corner and then inserting long nails through the holes so that they poke through to the exterior of the house. Then use a level and draw lines to mark the perimeter of the window opening.

Next, from outside, saw through the sheathing and siding, using your lines as a guide. Remove the drywall inside.

Place your window into the opening to verify that it will fit. Be sure to make the opening slightly larger than the actual window; check your manufacturer’s instructions to see exactly how much space you should leave. Cut through the siding and drywall again, if necessary, to provide ample space.

Install water-resistant barriers, as recommended by your manufacturer. Generally, this will mean inserting strips of moisture barrier around the edges of the opening, tucking them inside and stapling them in place, folding the strips back over the exterior area, and then fastening the fold with flashing tape.

At this point, put two small spacers at the bottom of the opening, about 1/2 inch in from the sides. Check for level, and add shim if needed. Attach the spacers and shims with 6d nails. Make sure the spacers and shims do not protrude past the outside and inside walls; cut them flush if they do.

Mounting the window

From outside your house, rest the bottom of the window on the spacers, and then slowly push the top into the opening. Make sure the window is centered. Partially drive nails through one or both upper corners to hold the window in place. Insert shims along the sides near the top and bottom of the window and at the midpoint, and adjust them as needed until the window is plumb. Check to make sure the window is operational, and then secure the window to the opening by driving nails into the brick mold or casing. Apply flashing and sealant (outside and inside) according to your manufacturer’s directions.

Inside, you can apply molding to complete your project: a stool, apron, and side casings for single- or double-hung windows.

If you have a wood window, you may need to paint the frame. Check your window’s installation instructions for directions.

1) From inside the house, drive nails through the wall to mark the corners of the window. Then, outside, draw lines to outline the opening for the window. Cut through the siding and the sheathing with a power saw, making sure to use the correct blade. Place the window up to the opening and continue trimming it, as necessary.
2) Line the opening with 8-inch-wide strips of moisture seal and tuck them into the siding. Slit the corners of the moisture seal diagonally with a utility knife and fold the pieces toward the interior. Trim the strips if necessary until they extend just inside the interior edge of the opening. Then staple them.
3) Place two pairs of shims, evenly spaced, at the bottom of the window opening and keep adjusting them until they are level. Use two 6d (2-inch) nails per pair of shims to affix them to the opening, but first drill pilot holes to keep the shims from splitting. After they are secured, cut them so they are flush with the siding.
4) Place the window in the opening and drive a nail partway through the trim at one of the upper corners. Check that the window is level, make any necessary adjustments to the shims, and then finish driving the nail. Drive nails at the other corners and finally around the perimeter.
5) Affix the trim on the outside, and caulk the space between the window and the siding, according to the manufacturer’s directions.
6). On the inside, check the window for level and add shims around the sides until the window is snug in the opening. Using 8d (2 1/2-inch) finishing nails, nail through the jamb and the shims into the trimmer studs; use a nailset to set the nailheads just below the surface. Add insulation between the jambs and the trimmer studs, cut the shims so they are flush with the wall, and then affix the interior trim.