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Installing a window yourself will only work when the replacement window is the same size and shape as the old one and when the existing window opening is square. To determine whether your window opening is square, measure it diagonally one way (from the upper left corner to the lower right corner) and then the other (from the upper right corner to the lower left corner). If the opening is square, the two measurements will be the same, give or take 1/8 inch. The slightest deviation from square can prevent the sash from closing and the weather stripping from sealing properly. If that happens, you'll be stuck with drafts, condensation, and high utility bills.
Successful replacements depend on good measurements of the existing opening to find out how big your new window needs to be. Start by measuring between the left and right jambs at the top, middle, and bottom of the window; the shortest of these three measurements is the window width. After that, measure the distance between the head jamb and sill at the far left side, middle, and far right side of the window; the shortest of these three measurements is the window height.
Slide a flat pry bar behind the inside stops and remove the pieces.
After scoring the head stop, gently pull it loose with pliers. Set aside for later use.
Raise the lower sash 6 to 8 inches, then pry back the old jamb liner. Repeat with the upper sash.
Fold the jamb liner at a 90-degree angle underneath the sash. Repeat with the upper sash, folding the jamb liner across the top of the sash.
Remove the old sashes and jamb liner as a single unit. Ask a helper for assistance with larger windows. Use proper lifting techniques to avoid back injuries.
Install a liner bracket every 4 to 5 inches along each jamb. Make sure each bracket is level before you nail it on.
Attach the jamb liners to the liner brackets. Each liner should pop into place.
Insert the upper sash in the exterior track of the jamb liner. Open and close the sash to make sure it slides smoothly. Insert the lower sash in the interior track and check that it operates.
Reattach the head stop. Set the nails below the wood surface. Fill the holes with wood putty and touch up with paint.
After removing a stop or other trim pieces, use pliers to pull the nails through the back of the piece. Doing so reduces the odds that the piece will split or fracture.
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