Once you have prepared a rough opening, installing a pre-hung window is relatively easy. The exact methods depend on the type of window you are putting in and the manufacturer you have chosen. The directions offered here are intended as helpful guidelines—be sure to follow the specific instructions that come with your new window.

Pre-hung windows are blocked and braced to prevent damage during shipping; in most cases, you should not remove the bracing until the window is fastened in place. After you have installed the unit, be sure the sash operates freely (for an operable window) before you install the interior trim or caulk the exterior. As always, contact a window installation contractor if you feel the job is over your head.

Installing a pre-hung window
A pre-hung window comes complete with a sash, a frame, a sill, hardware, and most of the trim. When ordering a window, you will need to know the exact depth of the wall it will fit into so that the jambs will be the proper size to fit flush with the wall.

It is best to install the window from the outside, although you can maneuver an operable window through the opening from the inside, open it, and lean through to fasten it. If you want to install the window from the inside, make sure you are safely anchored so you don’t fall out.

Cutting the exterior opening
After laying out where you want to put the new window from inside the room, drill holes at the corners and then drive nails through the holes so they are visible from the outside. With a carpenter’s level, mark lines on the outside connecting the corners.

From the outside, using a circular saw, jigsaw, or reciprocating saw, cut through the siding and the sheathing along the lines; lay the cut material aside. For stucco, use a circular saw fitted with a masonry-cutting blade. Always wear eye protection.

Inside, remove drywall. If necessary, assemble the mounting flanges and flashing of your window; make sure to follow the window manufacturer’s instructions carefully.

After holding up the window and marking its placement on the siding, take the window down and widen the cuts through the siding, but not the sheathing, to allow for the brickmold or flange that runs around the outside perimeter of the frame.

Mounting the window
To prepare the opening, apply 8-inch-wide strips of moisture seal around the perimeter of the window opening and slide them behind the siding. Make diagonal slits at the corners, using a utility knife; fold the moisture seal back over the rough framing and then staple it. The strips should stop just before the inside edge of the opening.

Place pairs of shims at the base of the rough opening, and adjust them until they are level. Drill pilot holes (to prevent the shims from splitting), nail them with two 6d (2-inch) nails per pair, and then cut them flush with the exterior wall. Make sure the pairs of shims are not so thick that they will prevent the window from fitting into the opening.

Inside, trim the shims flush with the jambs. For nailing the flange or brickmold to the sheathing, use 4d (1 1/2-inch) roofing nails (for a flange) or 8d (2 1/2-inch) galvanized casing nails (for brickmold).

Trimming a window
To complete your window project, you may need to install slivers of wallboard to fill the gap around the edges of the window; fit the new pieces as close as possible to the jambs on all sides. You will also need to finish the inside edges of the window with moulding. On a single- or double-hung window, this involves fastening a stool to the windowsill, attaching an apron to the wall underneath the sill, and fastening casings to the sides and top to cover joints between the wall and window. Other types of windows—casement, awning, and hopper—are usually finished just with casings on all four sides.