Measuring a Window Opening

A window opening looks square, and it almost never is, which is why the breakdown sitting on window installation carlsbad madame and me lists the checks in order before any unit is ordered. A window installation in Carlsbad, CA starts with a tape measure and a level, and the numbers taken from the rough opening decide whether the new frame drops in or fights the wall.

Three widths and three heights

Width is measured at the top, the middle and the bottom of the opening, and height at the left, the center and the right. The smallest of each set is the number that counts, because a frame has to pass through the tightest point. Stucco and old plaster return edges often bow in a few places, and a single measurement at one spot hides that.

Checking for square

Two diagonals tell the story. Measure corner to corner both ways, and if the two numbers differ by more than a quarter inch, the opening is out of square and the installer plans shim space accordingly. A picture window is the least forgiving, since fixed glass cannot flex to meet a leaning frame the way a casement sash sometimes can.

Level and plumb of the sill

The sill is checked with a four foot level, not a short one. A sill that drops toward the interior holds water against the framing, and one that tilts sideways shows up later as a sash that drifts open. Houses near the Agua Hedionda Lagoon and along Interstate 5 often settle slightly, so a sill that was level in 1985 may not be now.

Frame depth and wall thickness

Wall depth decides whether a double hung or a slider fits without extension jambs. Wood-frame walls with exterior stucco and interior drywall give a finished depth that varies from house to house, and the measurement is taken from the exterior face to the interior face at several points. Vinyl window frames come in standard depths, so a gap is bridged with a jamb extension rather than by forcing a thinner frame.

Recording what was found

Every number goes onto a sheet with the opening labeled by room and wall. A second person rechecks the long dimensions before the order is placed, since a window made a quarter inch too large cannot be trimmed and a window made too small has to be filled with more foam and sealant than the manufacturer allows.