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How to Caulk and Cap Windows

Emily Patterson

Window capping and caulk seal the crack around a window frame to prevent penetration of moisture and air. Capping, also called “flashing,” is a solid piece of metal that is installed above the window to drain rainwater away from the area where the window meets the exterior wall. In addition to creating a drainage area, when you properly caulk and cap windows, air flow is eliminated providing a more energy-efficient home.

A window with capping.
  1. Remove the existing capping, if the caulk and capping are being replaced. Place a pry bar between the capping and the wall and gently pull the capping away from the wall. If the installation is behind siding, remove the course of siding above the window with the pry bar.

  2. Measure the depth of the thickness of the window that is protruding from the house. Add ½ inch. Use this measurement to draw a line across the edge of the sheet of metal capping.

  3. Align the corner of a straight board on the fold mark drawn in the previous step. Fold the capping to create a 90-degree angle using the edge of the board as a guide. Measure the width of the window and trim the capping to the width of the window with an additional ½ inch.

  4. Fold ¼ inch down on each side and ½ inch down on the front using the corner of the board to keep the fold straight.

  5. Apply a bead of exterior caulk around the perimeter of the window.

  6. Position the capping on top of the window. Trim the remaining portion above the window if needed to slide the capping under at least one full course of siding.

  7. Attach the window cap to the wall with roofing nails. Replace the siding if the cap was a replacement.