How to Install the Flashing on a Replacement Window in a Rough Opening

Installing a replacement window is a little more involved than just mounting the window into the opening and screwing it in place. The rough opening needs to be prepared, before window installation, to ensure there is no water intrusion around the edges of the window later on. Flashing the rough opening will ensure that any water is stopped before it runs down the inside of your wall and causes serious damage.

Utility knife
15-pound felt paper
Tape measure
Staple gun
Self-sealing flashing membrane

Instructions

- 1
  Use a utility knife to cut a piece of 15-pound felt paper to a width of 12 inches and a length that is equal to the width of the window opening, plus 16 inches.
- 2
  Position the cut piece of felt paper on the exterior wall of the building so that its top edge is flush with the bottom of the rough window opening. Ensure that the felt paper extends past the rough opening 8 inches on each side. Secure the paper in place with a staple gun.
- 3
  Cut a length of self-sealing flashing membrane -- available at lumberyards and building centers -- to a length that is equal to the width of the rough window opening, plus 16 inches. Peel a few inches of the protective backing from the membrane. Apply the membrane over the rough opening so that the bottom edge overlaps the top edge of the felt paper a few inches. Note that the ends of the membrane should be even with the ends of the felt paper. Slowly peel the remaining backing from the membrane. Press the membrane into place around the edges of the rough opening by hand. The membrane should partially cover the bottom of the rough opening.
- 4
  Start at one of the bottom corners of the rough opening and cut through the membrane that is exposed in the opening with the utility knife at a 45-degree angle. Do the same for the other bottom corner. Fold the membrane over the sides and the bottom of the rough opening and press it down by hand.
- 5
  Cut a 12-inch length of membrane. Remove the backing and press the membrane into one of the bottom corners of the rough opening so that the overhang protrudes outward from the building. Cut the membrane with the utility knife so that the edges can be folded over and pressed into place on the outside surface of the wall around the opening. Do the same for the remaining bottom corner.
- 6
  Cut a length of membrane that is equal to the height of the window opening, plus 16 inches. Remove the backing from the membrane. Position the membrane vertically on the edge of the rough opening so that it is flush with the edge of the opening. Ensure the membrane extends past the top and bottom of the opening by 8 inches. Press the membrane into place by hand. Do that same for the remaining vertical side of the opening.
- 7
  Install the window into the rough opening as per the window manufacturer's instructions.

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