Follow our step-by-step instructions, plus tips and tricks, for successfully installing an unflanged wood window.
traditional white bathroom

Installing a window isn't difficult, just make sure you understand what type of window you're working with. These instructions are for a window with no nailing flange. Most unflanged windows are made of wood, and most have brick molding.

When ordering a window be sure to specify a jamb of the correct width. If your walls have 2x4 studs and 1/2-inch exterior sheathing, the jambs should be 3 5/8 inches wide (the extra 1/8 inch allows for imperfections). For a wall with 2x6 studs, a jamb width of 5 5/8 inches is correct.

Unpack the window and inspect it before installing. Make sure all the weatherstripping is in good shape and the mechanisms and sashes operate smoothly.

Check with your building department to find which method of wrapping the opening is preferred; some departments have stringent requirements.

Once the opening is framed, expect to spend about 3 hours sealing the opening and installing a window and its exterior trim. Also allow time for finishing the interior wall and installing interior trim.

  • Working time 3 hrs
  • Start to finish 8 hrs
  • Difficulty Kind of Easy
  • Involves Driving Nails, Installing Insulation

What you need


How to do it

Part 1

Step 1

Cover Rough Opening

Cut strips of roofing felt or building wrap and cover the bottom of the rough opening. Later you will install additional wrapping and/or flashing.

Editor's Tip
Editor's Tip

If you have a brick wall, hire a professional mason to cut the opening for you. The window attaches to the wood framing behind the brick veneer with metal masonry clips. You'll have to purchase the clips separately and attach them to your window jamb with deck screws.

Step 2

Temporarily Set Window

Set the window temporarily in place, check for level, and shim the bottom as needed. If your window calls for installing a thick flashing with a piece of plywood at the bottom install it first or raise the window by the same thickness.

Step 3

Tap in Shims

Tap in shims at the sides, checking for plumb as you go. Don't wedge the shims too tightly, or the jambs will warp. Use the level as a straightedge to confirm that the jamb has not warped.

Step 4

Adjust Shims

Make sure the window operates smoothly with the shims in place. Watch the alignment of the sash against the jambs as you move it. Adjust the shims as needed.

Step 5

Tack Nails or Screws

Tack (partially drive) nails or screws near the shims to hold the window temporarily in place.

Step 6

Mark and Cut Siding

On the outside, trace around the brick molding to mark the siding for cutting. Cut the siding.

Step 7

Install Flashing

Pry back the siding and install the felt, building wrap, or self-adhesive flashing. Cut pieces of felt or self-adhesive flashing to fit along the sides. Slip them in behind the siding, fold them over the studs, and staple. Cut a piece 6 inches longer than the width of the opening and install it the same way. Cut slits at the corners.

Editor's Tip
Editor's Tip

Newer building techniques have specific requirements for flashing a window. In general the upper pieces should overlap the lower pieces so water can flow downward without seeping in. When installing a window in new construction (left), the building wrap overlaps the self-stick flashing. In a remodel situation (right), a simpler arrangement is often used. Self-stick flashing is installed to the bottom, then the sides, then the top. Finally small pieces of flashing are applied over the V-shaped gaps at the corners.

Step 8

Cover Slits with Felt

The slits cut on all four corners create V-shaped openings in the felt or flashing. Cover these with small pieces of felt or self-stick flashing.

Step 9

Install Drip Cap Flashing

If your window calls for it, use tin snips to cut a piece of metal drip cap flashing and slip it under the felt at the header.

Step 10

Set Window in Place

Set the window back in place, with the brick molding tight against the felt or flashing. Inside, the front of the jambs should be flush with the finished wall. (Where you have not yet installed new drywall, the jambs should be 1/2 inch proud of the framing.)

Step 11

Shim and Check for Level

Again shim the bottom and sides, check for level and square, and check that the window operates smoothly. Following manufacturer's directions (you may need to remove pieces of trim first), partially drive 6d finishing nails through the jambs to attach the window.

Step 12

Drive Casing Nails

Outside, drive galvanized casing nails to attach the brick molding. Caulk between the trim and the window and between the trim and the siding.

Step 13

Drive Finishing Nails

Use a nail set to drive the finishing nails slightly below the surface of the wood. Where these nails are not covered with trim, apply wood putty to the resulting holes.

Step 14

Cut Shims Flush

Use a handsaw to cut the shims flush with the studs, so you can install the drywall up against the jambs.

Step 15

Insulate Window

Gently stuff fiberglass insulation into the gaps around the jambs or fill the gaps with nonexpanding spray foam insulation. Expanding foam could warp the window frame.


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