How to Install a Flanged Window

Whisking water away from your home's openings is important and basic care. Do just that with a flanged window.

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Metal and vinyl windows have an integral flange (also known as a nailing fin) that attaches to the house. It's great for keeping water and moisture outside of your home. Once the flange is firmly fastened, there is no need to anchor the window jamb. It's a great option for your home, especially if you're an avid DIYer. Below, we show how to measure and cut siding, then install and paint the flange. After reading our steps, you'll feel confident to tackle this weekend project yourself.

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Before You Begin: Choose the Right Material

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 this page 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.

In this arrangement, felt or building paper is first stapled to the sheathing, with the top pieces overlapping the lower pieces. A bead of caulk is applied to the felt and the window is set in the caulk. Next comes another layer of building paper, installed like the first layer. At the top, metal drip cap flashing is tucked up under the siding and on top of the second layer of felt, and the exterior casing is installed just under the drip cap.

What You Need

  • Tape measure
  • Drill
  • Hammer
  • Level
  • Stapler
  • Caulk gun
  • 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: Screw in Temporarily

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: Mark and Cut Siding

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: Install Roofing Felt

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: Caulk and Place Window

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: Secure Window in Place

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: Add More Roofing Felt

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: Install Casing

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: Install Drip Cap

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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