Convert between unit size, rough opening (RO), and masonry opening (MO) for windows and doors. Uses generic industry-standard shim gaps and clearances — always cross-check against the specific product's installation manual before framing or ordering.
Generic tolerances only. These use industry-standard values (3/8" shim gap around perimeter, 1/2" head clearance for windows, 3/4" for doors). Individual manufacturers (PGT, CGI, WinDoor, etc.) publish their own required RO dimensions in the installation manual — always defer to the manufacturer's spec on any real job.
Calculation Mode
Field tip: Always measure the opening in three places (top, middle, bottom for width; left, middle, right for height) and use the smallest dimension when ordering. Openings are almost never square — the widest window that will fit is the one that fits at the tightest measurement.
Standard Tolerances Used
Windows in frame wall: RO = unit width + 3/4" (3/8" shim each side), unit height + 1/2" (3/8" bottom + 1/8" head + shim room).
Windows in masonry: MO = unit width + 1" (1/2" gap each side for buck + shim), unit height + 3/4" (buck head + shim).
Doors in frame wall: RO = unit width + 2-1/2" (jamb + shim), unit height + 2" (jamb + threshold).
Sliding glass doors in masonry: MO = unit width + 1", unit height + 3/4".
Why This Matters
Too tight: Frame will bind, sash won't operate correctly, sealant joint will fail under thermal cycling — Florida sun expands vinyl and aluminum measurably.
Too loose: Can't be sealed properly, air/water infiltration ratings from the NOA won't hold, and the opening may fail final inspection or a blower-door test.
Impact glazing note: HVHZ impact products have stricter RO tolerances than non-impact — the anchor spacing in the NOA assumes the frame is properly aligned. Always verify against the NOA's installation detail sheet.