Step 1: Measuring for Replacements

measuring window replacements
Photo: Russell Kaye
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The most important step in the window-replacement process happens long before installation day. It's when you measure the dimensions of the existing window frame to make sure you order a replacement unit that’s the right size . Here's how to do it.

Start by measuring the inside width of the old window frame, jamb to jamb, in three places: across the top, middle, and bottom. Write down the smallest of the three measurements.

Next, measure the frame's height from the top of the sill to the underside of the head jamb in three places: at the left jamb, in the middle, and at the right. Again, record the smallest measurement.

Check the squareness of the frame by measuring the diagonals from corner to corner. The two dimensions should be the same. If the frame is out of square by 1/4 inch or so, don't worry; the replacement can be shimmed to fit. Anything more may require adjustments to the frame. If the frame is so out of whack that a square replacement wouldn’t look right, you'll need a full-frame replacement.

Finally, use an angle-measuring tool to determine the slope of the sill; some replacements come with a choice of sill angles.
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    Tools List

    • 100-foot tape measure
      Tape measure
    • hammer
      Hammer
    • flat prybar
      Flat pry bar, used to remove old window stops and jamb liners
    • putty knife
      Putty knife, for applying wood putty
    • caulk gun
      Caulking gun with exterior-grade elastomeric caulk
    • drill
      Cordless drill/driver, used to drill holes and drive screws
    • spade bit
      3/8-inch spade bit, for boring foam-insulation access holes in window frame
    • paint scraper
      Paint scraper, used to scrape loose, blistered paint from window frame
    • utility knife
      Utility knife, used to trim shims flush

    Shopping List

    1. Insert replacement window
    sized slightly smaller than existing window frame

    2. Exterior-grade wood putty
    used to patch holes in window frame

    3. Minimally expanding polyurethane foam
    used to insulate wall around window

    4. Wood shims
    for adjusting the window to sit level and plumb in opening

    5. 100-grit sandpaper
    for smoothing window frame prior to priming and painting 6. 2-inch screws
    used to fasten replacement window to frame

    7. Foam-rubber backer rod
    used to fill 1/4-inch-or-wider gaps prior to caulking

    8. Interior window stops
    needed if the original stops broke during removal

    9. Paintbrush, primer and paint
    for applying fresh finish to window frame