How to Install or Replace a Window in a Rough Opening
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How to Install or Replace a Window in a Rough Opening

how to install a window in a rough opening

Window Installation

Windows are one of the most important elements in your home as they not only add to the character of the house, they also let in light and air, and frame views from inside. When they are not installed correctly, they can become a source of endless problems that culminate with water damage and rot. Rot can be seen even with in relatively new remodeling and construction. Most of the windows today are constructed of finger joint pine which creates a more stable material to resist warping and expansion, but is more prone to damage if the frame becomes wet. Manufacturers depend on vinyl or aluminum cladding to protect the wood from moisture and to create a more maintenance free fixture. However, if the window is incorrectly installed, all of the trim, flashing, and caulk won’t keep the rain out.

One of the best ways to prevent water infiltration is to seal the perimeter of the opening with sticky sheets of self-adhering waterproof membrane, strips of metal flashing, and caulk. Any rain that gets by one of these layers will be stopped by another. The final step, once the opening is watertight and the window is set plumb and square, is to fill the gap between the window and the framing with foam insulation.

Tools Required

20-ounce hammer

2-foot and 4-foot levels

Staple Gun for stapling builder's felt

Small Crow bar or flat bar to lift corner of window when leveling

Utility knife

Measuring tape

Caulk gun




Minimally-Expanding Foam Insulation

15-lb Builder’s Felt (alternative to membrane for flashing rough opening)

Waterproof Membrane

Metal Flashing

Fiberglass Insulation

1 ½ inch Roofing Nails for fastening nailing fins to the sheathing


Rough opening

Measure the width and height of the rough opening at each corner and the middle. If the difference between the three width measurements or the three height measurements of the rough opening is more than 1 inch, cut tapered filler strips from studs that are the same width as the wall framing, usually 2 by 4 or 2 by 6, and nail them to the sides of the opening that are out of level or out of plumb.

Make sure the outside dimensions of the window are at least ¾-inch narrower and ½-inch shorter than the smallest width and height measurements, respectively. If they're not, you'll have to either reframe the opening or order a new window.

Flashing to prevent water infiltration

Cut a strip of self-adhering waterproof membrane 6 to 8 inches wide, or use a 9- to 12-inch-wide strip of 15-pound builder's felt that is about 24 inches longer than the window's width. Center the membrane under the rough opening and adhere it to the existing builder's felt or house wrap. Make sure its top edge doesn't extend above the edge of the opening. Cut two more strips of membrane or felt 12 inches longer than the height of the opening. Center and attach them along each side of the opening, overlapping the strip under the window.

Cut the final strip of membrane about 12 inches longer than the window’s width and center and attach it across the top of the rough opening so that it overlaps the two side strips. By working from the bottom up, you ensure that the open edge of the seams are on the bottom and that the membrane will shed water.

Setting the window

Check with your installation instructions, but in many cases the window sashes should remain in the unit to maintain the unit dimensions and shape. This obviously adds to the weight of the unit, so have someone help you when setting the window.

Attach the window's nailing fins so they are perpendicular to the sides of the window frame. Some units come with the nailing fins already attached, so be careful when unpacking, carrying, and setting the window so you do not damage them. The side nailing fins carry most of the weight of the window and the top and bottom fins secure the window to the framing to maintain stability. Set the window's sill into the bottom of the rough opening, and tip the frame into the opening until all the nailing fins are tight against the wall. You should have someone stand inside and tell you when the gaps between the sides of the window and the jack studs are equal on both sides. Tack the nailing fin to the sheathing at one upper corner with a 1 ½-inch roofing nail, but do not drive it all the way in.


Set your 2-foot level on the windowsill to determine the high side and hold your 4-foot level against the window jamb on the high side. You will then need to shift the sill left or right until the level shows the jamb is plumb. Once the unit is plumb, tack a nail into the fin at the lower corner on the same side as the first nail.

Now with your 2-foot level on the sill, adjust the un-nailed bottom corner up until the sill is level. Tack the fin in this lower corner.

Checking for square

To check that the window is square, you will need to measure the frame diagonally from corner to corner in both directions and compare the measurements. They should be within 1/16 inch. If not, recheck the frame's side for plumb and the sill for level. You may have to pull out the last two temporary nails and adjust the frame with your flat bar.

When the sill is level and the frame square, drive home the nails at each corner. If the sill is level and the diagonal measurements are identical, the sides of the window are plumb. To make sure that the window is not bowed, measure the width of the window at several points and verify that they are equal. If the jambs are bowed, push the frame in or out at the center of the bow and nail the fin at that point. Tip: You can use your 4-foot level as a straightedge to make certain you don’t create a bow in the opposite direction. When completed, nail the sides, top, and bottom of the nailing by placing a nail through every other hole in the fin. Operate the window sashes to make sure that they operate smoothly. If they hit or bind at any point on the jamb, check the window jambs for bowing again. Many window units have a plastic strip to keep the unit from bowing during installation; remove this now.

Sealing the perimeter

Follow the same process as “Flashing to prevent water infiltration” to cover all of the nailing fins with waterproof membranes or builder’s felt by cutting 6-inch-wide strips 12 inches longer or wider than the window dimensions. Make sure that you maintain the layering of the seams to shed water by starting at the bottom, then working up the sides and finishing with the top strip.

Flashing the top of the window

Some windows come with snap-in metal flashing to cover the top of the window frame; if this is the case then apply a bead of caulk to the top edge of the window casing, then press the flashing in place. If no flashing has been supplied, cut and bend a piece of metal flashing so it overhangs the front and sides of the casing by at least 1/4 inch and extends 3 inches up the wall above the window. Fasten the top edge of the flashing to the wall with 1 ½-inch roofing nails and cover it with a strip of waterproof membrane long enough to cover the top ends of the two side strips. You can also buy window flashing at lumber yards, home centers, or hardware stores; you should bring a piece of the brick molding or exterior window casing that you are using to case the outside of the window to determine the width of the flashing required. If you aren’t using exterior casing, measure the window frame that extends past the sheathing.


Inside the house, apply a single thin bead of minimally expanding polyurethane foam to the gap between the window and the framing. Allow the bead to expand and cure before adding more. Note: Don’t overfill the gap when using the expanding foam, otherwise the jamb can bow in and prevent the window sash from operating properly. Repeat until the cavity is completely filled. As an alternative, you can use fiberglass insulation, cut un-faced batting a little larger than the space between the window and the framing, and push the batting in with a putty knife or paint stirrer.

If the gap is too narrow for either foam or fiberglass insulate on, seal it with a bead of caulk.

When the unit is installed and insulation completed, you can now install the interior casing and window sill inside your home.  Depending on the wall thickness and how the window was ordered, you may need to add jamb externsions to the unit before you can install the casing.  Typically jamb extensions can be purchased from where you purchased the window, but some times they will need to be special-ordered.  Most windows are constructed for 2 by 4 framing studs.  The jamb extensions are screwed into the existing window jamb with wood or drywall screws though their edges, and usually have pre-drilled holes.  A bead of wood glue should be applied to the window jamb and the extension before installing.

Good Luck.

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Comments (1)

Fortunately, or unfortunately, I replaced all of the windows in my house with "real", meaning not aluminum replacement, windows. Luckily I have wood siding so the only adjustment I needed to perform was cutting it back about 1" on each side. If you survive the first window, the next 20 will seem like a walk in the park. Thanks for your interest.