Rough opening: measuring for aluminium windows

Three sizes travel together on every opening and confusing them is the most common cause of a frame that does not fit. The structural opening is the hole the builder forms. The frame size is the outside dimension of the window unit, sash and all. The rough opening is the structural opening plus the installation clearance on each face and head. A buyer who sends a frame size to the builder gets a hole the frame cannot enter; a buyer who sends the rough opening to the factory gets a frame that fits the hole.

The clearance itself is not a fixed number, it is a consequence of the fixing method. A screw-fixed aluminium frame needs enough room to shim and pack it plumb without crushing the profile. A frame set into a plastered reveal needs room for the render. A frame bedded into a pre-finished opening needs almost nothing. The schedule states the method, and the clearance follows from the method.

• Levelling and packing: enough space at the jambs and head to bring the frame true with a packer, without loading the profile past its shape.
• Sealant and membrane: the space the backer rod, flashing tape and perimeter sealant occupy. This is not air — it is a designed drainage and weather line.
• Reveal closure: room for the internal trim, architrave or plaster return, where the opening is finished internally.
• Tolerance for the structure: masonry openings are built to a tolerance, and the frame absorbs what the builder built, and does not fight it.
• The number goes on the schedule against the method, not carried as a general site rule.

Before a frame is ordered, the opening is checked in four directions. Width at three points — top, middle and bottom — because an opening that tapers will not square a frame. Height from finished floor to head, taken on the reveal, because floor levels decide the sill line. Both diagonals, corner to corner, because equal diagonals are the quick test for a square hole. And the sill height above the finished external grade, because that number decides whether water runs away from the opening or toward it.

Diagonals are the check most often skipped and the one that explains most of the fittings that arrive loose. A frame squared into an opening whose diagonals differ by more than the packing tolerance has to be twisted to fit, and a twisted frame carries that torsion into the sash, the lock and the gasket for the life of the unit. No hardware adjustment recovers it.

The height of the opening above the outside ground is not a cosmetic setting. Water that lands on a sill too low cannot drain away from the opening, and no amount of gasket quality changes where the water goes. The sill is set high enough that the drainage path runs to the outside face, and low enough that the internal floor level still meets the reveal cleanly.

This is where the rough opening and the finished levels have to be decided together, and where a unit specified from a frame size alone goes wrong most often. The external grade is not finished until paving or landscaping is in, which means the sill line on the schedule is provisional until the site confirms it. Where the external works are scheduled after the windows are ordered, the confirmation step belongs on the delivery programme, not in an email.

• The structural opening, measured at the narrowest point, not at the widest.
• The installation clearance, stated per face and head against the fixing method.
• The diagonal tolerance the packing absorbs before the frame is twisted.
• The sill height above external grade, provisional until the outside works are finished.
• The reveal finish and the structural reaction, both decided before the frame is made.
• The weather line — flashing, membrane and sealant — scheduled as one system.

• EN 14351-1:2006+A1:2010 — external pedestrian doorsets and windows, the European product standard behind the performance figures.
• EN 12211:2010 — wind load, the test the frame is subjected to once the calculation is set.
• EN 1027:2013 — water tightness under static pressure.
• AAMA 2604 and ASTM E330 — the North American wind and water path.
• AS 2047:2015 and AS 1288 — the Australian system, where the opening is sized to the wind class the region states.
• SANS 10400-N:2012 — the South African glazing rulebook, which governs the pane, not the hole.

• Ordering to the frame size and getting a hole the frame will not enter — the clearance was never decided.
• Setting the frame into a tapered opening — diagonals were not measured, and the frame carries torsion.
• Sill too close to external grade — water reaches the opening instead of running from it.
• Reveal finished to a level that conflicts with the frame line — the trim gets cut to suit a gap that never got filled to suit the frame.
• Fixing assumed instead of scheduled — the anchor spacing is decided by the wall build-up, not by habit.