Measuring a rough opening for a replacement window

One opening, six readings, and the smallest one in each direction wins. That is how a tape measure gets used on a replacement window job. The order is measure, check square, then subtract clearance, as the notes at replacement windows austin gagewood photo describe. A window ordered a quarter inch too wide for a house in Austin, TX cannot be trimmed on site, and it goes back as a delay of weeks.

Three readings each way

Width is taken at the top, the middle and the bottom of the opening, and height at the left, the centre and the right. The shortest reading in each direction is the working dimension. Old openings are rarely uniform, because plaster, paint layers and years of settling pull a jamb in at one point and out at another. A single reading in the middle of a double hung opening is the most common cause of a unit that will not seat.

Square and level

After the tape comes the level and a pair of diagonals. Measure corner to corner both ways. If the two diagonals differ by more than a small fraction of an inch, the opening is racked. In Austin the clay soils east of the Balcones Fault swell and shrink with the seasons, and houses on slabs and piers lean slightly out of plumb as they move. A sill that falls away to one side by an eighth of an inch is normal. A bigger slope needs a shimmed, levelled seat, or the sash will drift open or bind in its track.

Insert measurements and frame measurements

For an insert, the measurement is taken between the stops, inside the existing jambs, and the new unit is ordered a little under size so it slides in with room for shims and foam. For a full-frame job the measurement is the rough opening itself, stud to stud and header to subsill, and the unit is ordered smaller than that by the clearance the manufacturer specifies. The clearance covers shims, flashing and a bead of sealant at each side.

Style changes the numbers

A picture window has no moving parts, so a tight fit matters less than a level seat. A casement or awning unit has operating hardware that needs the frame dead square, or the crank binds. A slider rides on a track that will not tolerate a twisted sill. A bay window or bow window involves seat boards, head boards and a roof, and the angle measurements matter as much as the width. A vinyl window expands in heat, so in an Austin summer a fiberglass window or aluminum window sized with the same clearance behaves differently from vinyl, and the manufacturer's tolerance sheet governs.

Recording what was found

Each opening gets labelled and written down, with all six numbers, the diagonals and the sill slope. The sheet goes with the order and it settles any argument later about whether the unit or the opening was off.