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In this video, This Old House general contractor Tom Silva explains how to install a waterproof window.

Steps on How to Waterproof Your Window:

  1. Staple 15-pound builder's felt along bottom of window opening.
  2. Apply adhesive-backed waterproof membrane over the felt; cut the corners and fold the excess membrane over the rough sill.
  3. Cut small pieces of waterproof membrane and cover voids in corners of opening.
  4. Apply a wide strip of waterproof membrane to each side of the opening; run the flashing past the header and overlap the bottom flashing.
  5. Set the window in the opening.
  6. From inside, slip shims under the window.
  7. Center window in the opening, then check the bottom of the window for level.
  8. Drive a single nail through the nailing flange on the window's high side.
  9. Raise the low side of the window to level, then nail through the flange.
  10. Check vertical edge of the window for plumb; shim if necessary.
  11. Measure the opposing diagonals of the window to ensure that it's square.
  12. Finish nailing the window to the wall.
  13. Apply a wide strip of waterproof membrane across the top of the window.