Step 2: Protect against water infiltration

Tom Silva installing a window
Photo: David Carmack
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Cut a 6-inch-wide strip of self-adhering waterproof membrane (or a 9- to 12-inch-wide strip of 15-pound builder's felt) 18 to 24 inches longer than the window's width. Center the membrane under the rough opening and adhere it to the existing builder's felt or house wrap. Make sure its top edge doesn't extend above the edge of the opening.

Cut two more strips of membrane (or felt) 1 foot longer than the height of the opening. Center and attach them along each side of the opening, overlapping the strip under the window.

Cut another strip of membrane (or felt) 1 foot longer than the window is wide; center and attach it across the top of the rough opening so that it overlaps the two side strips.

TIP: When applying flashing, layer the material so that any water running down the wall is directed out: Seams should never face up.

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    Tools List

    • hammer
      20-ounce hammer
    • two-foot level
      2-foot and 4-foot levels
    • tacker hammer
      Hammer-tacker,
      for stapling builder's felt
    • utility knife
      Utility knife
    • 16-foot tape measure
      Measuring tape
    • caulk gun
      Caulk gun
    • drill
      Optional: Drill-driver and 1 5/8-inch stainless screws,
      for attaching sill extender to bottom of narrow aluminum sill

    Shopping List

    1. WINDOW

    2. CAULK

    3. MINIMIALLY-EXPANDING FOAM INSULATION

    4. 15-POUND BUILDER'S FELT

    an alternative to membrane for flashing the rough opening



    5. WATERPROOF MEMBRANE

    6. METAL FLASHING

    7. FOAM INSULATION

    8. 1 1/2-INCH ROOFING NAILS

    for fastening nailing fins to the sheathing