Installing a Flanged Window

This story shows you how to install a flanged window.


Metal and vinyl windows have an integral flange (also known as a nailing fin) that attaches to the house. Once the flange is firmly fastened, there is no need to anchor the window jamb.

Carefully seal the house's sheathing correctly so water flows away from the house and cannot be trapped next to the sheathing. The sealing instructions on these pages will suit most situations, but your window may come with different instructions or your local building department may have different requirements.

Choose a metal or vinyl window that feels solid when you try to twist it and has substantial weatherstripping. Buy from a well-established manufacturer so you can easily buy replacement weatherstripping and other parts if needed in the future.

Prestart Checklist

Several hours (working with a helper if the window is large) to install the window and trim

Tape measure, drill, hammer, level, stapler, caulk gun

Basic carpentry skills

Cut the opening and check for square. Buy a flanged window to fit.

Window, exterior casing, shims, caulk, roofing felt or building paper, drip cap flashing, other flashing (such as self-stick) if required by codes, finishing nails, staples, galvanized roofing nails

Step 1

Set the window in the opening, pressed against the siding, and drive a screw at the center of the top flange. Shim as needed so the window is square and the sashes work smoothly and are aligned with the frame. Drive three or four more screws through the flanges to hold the window in place temporarily.

Step 2

To mark for cutting back the siding, hold pieces of exterior trim against the window and scribe along the siding with a pencil. Cut the siding. Keep the blade on the outside of the cutline, so the opening will be about 1/8 inch wider than the casing.

Step 3

Cut 10- to 12-inch-wide strips of roofing felt or building paper. Slip them behind the siding, wrap them around the framing, and staple in place. First install the piece at the sill, then the sides, then the top piece. Finally cut four pieces about 4 inches square and slip them into the corners to cover the V-shape gaps.

Step 4

Apply a bead of caulk all around the opening, where the flange will be. Tilt the window into place. Drive a screw near the center of the top flange and shim. Check for level and operation as in step 1. Make sure the casing will fit all around the window.

Step 5

Once the window is correctly aligned, drive roofing nails into the nail holes and remove the screw. (Nails are preferred because screw heads are bulky, making it difficult to install the casing.)

Step 6

Cut a strip of roofing felt or building paper 8 inches wide, slip it under the siding above the window, and staple it in place.

Step 7

Cut pieces of exterior casing to fit and nail them in place with 10d casing (exterior finishing) nails. Caulk around the casing and on the joint between the casing and the window. Prime and paint.

Step 8

Cut drip cap to cover the casing. Apply a bead of caulk just below the siding and slip the drip cap up under the siding. Nail 4d galvanized nails down into the casing.

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