Step 5: Check the window for square

installing a window
Photo: David Carmack
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Double-check that the window is square by measuring the frame diagonally from corner to corner. Measurements should be within 1/16 inch. If not, recheck the frame's side for plumb and the sill for level. You may have to pull out the last two temporary nails and adjust the frame.

When the sill is level and the frame square, drive home the nails at each corner. If the windowsill is level and corner-to-corner diagonal measurements are identical, the sides of the window are plumb.

Holding a tape measure horizontally, measure the width of the window at several places to make sure the side jambs aren't bowed. If they are, push the frame in or out at the center of the bow and nail the fin at that point. Then nail the sides, top, and bottom of the nailing fin or casing, driving one nail through every other prepunched hole.

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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