Measure instructions request that the width and the height be measured in three places – end the ends and in the middle – and the smallest be used in providing the “rough opening” dimension to a window manufacturer to product the properly sized replace- ment window. While it is desirable to have a window that fits – especially with any irregularities in plumb and square - making the glass area smaller by shrinking the new window, or restricting the egress of the replaced window are not desirable so the largest window that fits is the goal. The most common replacement is “blind-stop or “pocket” install where the old operating sash, weights and parting stops are removed to make a space for the new replacement window to fit. Alternatively, windows with plastic or aluminum tracks have the old sash and the track assemblies removed leaving the same opening for the new window. Common industry traditional cut-back has been ¼” on the width and ¾” on the height when a head expander is used and when the new window is to be fit against the exterior stop. Cut-back for an “outside-in” install, where the interi- or stops are left intact but the exterior blindstop is cut or fractured off to allow the new window to be inserted into the opening without a head expander, has traditionally been ¼ to 3/8”. These cut back dimensions have been the most common because stop surfaces (interior stop or blind stop) are 3/8”. Cutting back any more than 1/4” (which leaves 2/8” on each side) risks having too little contact surface for the new window impeding proper sealing and mounting and risking air and water intrusion. However, many openings are not square and when the three measures are taken, while they may even be equal, they are not square and the new window would need to be racked out of square to fit with these minimum tolerances. This has led many replacement cut backs to become larger – to nominal 1/2” cutback on width and 3/4” on height when using a head expander, and 1/2” on height when not. AWDI recommends three circumstances to determine the appropriate cutback, because as a window becomes taller (and wider), the out-of-square condition of the opening becomes a more progressive problem. For example, a 1/16” deviance over 2 feet tall will become 1/8” out at 4 feet tall. An un-level sill (more common) might require shimming the new window off the old sill, and that distance will lessen the room at the top of the new window. Variance in Measurements Different procedures are also being recommended depending on the space between the new window ordered and sized by the factory, and the rough opening created by the removal of the old window. 1/4” GAP 1/8” GAP Foam Insulation Sealant Batte Insulation Sealant & Backer Rod Sealant Capping Sealant 3/4” Sill Drop 3/8” Blindstop Surface 3/8” TrimStop Surface Window Fitters Guide to Window Installation 25 ©2022 AWDI, LLC Do not reproduce without permission 1. Windows under 60 U.I. should be cut back 1/4”. Windows between 61 and 94 U.I. be cutback 1/4” to 3/8”. Windows above 95 U.I. cut back 1/2”. 2. If the cutback is 1/2”, then backer rod is mandatory and probably exterior capping when outside in. Additionally new interior stops will be needed to create proper seal if it is installed inside out. 3. Windows cutback 1/4 to 3/8, require no backer rod and can be conventionally installed using existing blindstop or interior stop. Capping would be optional. * PU Foam or other appropriate insulation SAMPLE MANUAL
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