Glass · Ordering
How to measure an opening for glass
Measure the hole, not the glass that is in it. Three widths, three heights, take the smallest of each, then check both diagonals. The deduction comes after that, and it is the part people get wrong. All of this matters more than it sounds, for one reason: toughened glass cannot be trimmed. If it comes 4 mm too big, it is a new pane and another week.
The five measurements
Ten minutes with a tape, and it is the whole job. Everything downstream — the price, the lead time, whether it fits — is decided here.
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The aperture, not the pane
Measure between the rebates: the actual hole the glass has to sit in. Not the pane that is in there now, and not the outside of the frame.
The pane in the opening is not evidence. It may have been cut short and packed out, it may have been ground down on site to make it go, and the frame may have been painted three times or re-beaded since it went in. Every one of those moves the number.
The exception: if the pane is out and the frame is not reachable — a unit already removed, a door off its hinges — measure the old glass and say so on the order, so the merchant knows it is a copy and not a survey.
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Three widths, three heights, smallest wins
Width at the top, the middle and the bottom. Height at the left, the middle and the right. Take the smallest of each pair of three.
Openings are almost never parallel. Timber swells, render is thicker at one end, a frame racks when the building settles. A pane cut to the widest measurement is a pane that goes in at one end and jams at the other, and by then it is your pane.
Write all six down, not just the smallest. If it later does not fit, the six numbers say why; the one number says nothing.
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Both diagonals
Corner to corner, both ways. Equal diagonals mean a square opening. If they differ by more than a few millimetres, the hole is a parallelogram and a rectangle cut to the tight size will catch on two corners however carefully it was measured.
Out of square is not a reason to stop. It is a reason to say so on the order and to take a little more off, or, past about 10 mm of difference, to template the opening instead of dimensioning it.
This is the measurement nobody takes, and it is the one that explains most of the panes that come back.
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The deduction, and the thing it fights with
The glass has to be smaller than the hole so that the frame can move, the building can settle and the glass can expand without being squeezed. On an ordinary domestic window that is usually 3 to 5 mm off each side; on a large pane, on anything in a metal frame, and on anything in full sun, more.
But the deduction cannot simply be increased until it certainly fits, because it is pulling against edge cover: the frame still has to hold enough of the glass edge to keep it there and to keep the sealed-unit edge out of the light. Take too much off and you gain clearance and lose cover, and a unit whose edge seal sees daylight fails early.
If you are not sure of the figure for that frame, ask the merchant and give them the aperture. A supplier who cuts for that system all week knows its deduction; guessing it is how a 3 mm clearance becomes a 1 mm one.
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Thickness, make-up, and which face is out
Measure the thickness of what is there. Hold a pen tip against the glass at an angle and look at the two reflections, one off each surface: the gap between them grows with thickness, and it separates 4, 6 and 10 mm with a little practice. A thickness gauge does it properly and, on a sealed unit, also gives you the cavity.
Then note what it is and which way round it goes. Coated glass has the coating on one specific face, laminated has a defined build-up, patterned glass has a texture that must face the same way as before, and a fire or acoustic unit is not symmetrical. Photograph the opening from inside and label the photograph.
While you are there: if the pane is broken and you want to know why before you order the same thing again, read the break. A thermal break replaced like for like will happen again.
Why the measurement is the whole job
One sentence that changes how carefully anybody measures.
Toughened glass cannot be cut, drilled or ground after toughening. The strength comes from the surfaces being locked in compression against a core in tension; nick that surface and the balance goes, and the pane dices on the bench. So a toughened pane that comes 4 mm oversize is not adjusted, it is replaced — with the lead time and the cost of the first one, again.
Laminated is the same in practice: it can be cut in a shop with the right kit, but not by anybody standing at an opening with a wheel. Only plain annealed glass takes a trim on site, and that is exactly the glass that is least often specified for the places where measuring is hardest — doors, low windows, balustrades, anything in a critical location.
What to send with the order
Everything a merchant needs to cut without ringing you back.
| What | Why it is on the list |
|---|---|
| Aperture, W × H | the smallest of your three widths and three heights, said to be the aperture |
| Both diagonals | tells the cutter whether the opening is square before anything is cut |
| Make-up | 4 mm clear, 6 mm toughened, 33.1 laminated, 4-16-4 with a low-e — in your own words is fine |
| Which face is out | coated, patterned and laminated glass is not symmetrical |
| Frame and system | timber, uPVC, aluminium; the merchant's deduction depends on it |
| Two photographs | the opening square on, and a close-up of the rebate with a tape in shot |