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Cracked Window Glass
A crack runs across the pane and nobody hit it — or a small chip has grown into a line since last week.
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What's happening?
The shape of a crack tells a glazier what caused it. An impact crack radiates outward from a single point, often with a small cone of missing glass. A thermal crack starts at the edge, usually at right angles to it, and curves away — it happens when the middle of a pane in direct sun expands while the edge stays cool under the frame, and the difference exceeds what annealed glass will take. An edge crack from bad support starts at a corner where the glass is bearing on the frame with no setting block between them. None of these can be repaired, because glass does not knit; but knowing which one it is decides whether the replacement pane needs a different specification or the frame needs attention first.
What usually causes it?
- Thermal stress — strong sun on part of the pane with the edge shaded by the frame or a close blind
- An existing edge chip or nibble concentrating stress until the crack runs from it
- Glass bearing directly on the frame with missing or perished setting blocks
- Frame movement or a distorted sash putting the pane into bending
- A minor impact that started a crack which then grew with temperature cycling
- Solar-control film applied to the inside of annealed glass, raising the temperature difference across the pane
- A radiator, spotlight or heater directed at one part of the glass while the rest stays cold
Is it dangerous?
A single crack in an unstressed pane is usually stable in the short term, but it is no longer full-strength glass, and it can fail without warning under a knock or a wind gust. Two situations should not be left: a cracked pane in a door, side panel or low-level window, where the position is a critical location and a break happens where people are; and any crack that has split the pane into pieces which can move relative to each other. Both are same-day calls.
Can I fix it myself?
Nothing that repairs it — the epoxy and resin kits sold for windscreen chips are not a fix for a cracked flat pane in a frame. What is worth doing is taping across the crack on both faces to hold the pieces if the pane has to survive a night or two, keeping the room's heat off that pane, and looking at what may have caused it: a blind closed hard against the glass, or a heater aimed at it, will crack the replacement too.
When should you call a glazier?
Call a glazier promptly for a cracked pane in any door or low-level position, or where the crack has separated the pane. For a stable crack in an upper window, call to have it measured and replaced before it fails in weather. Say what you observed — where the crack starts, and whether anything struck it — because that is what tells the glazier whether the replacement needs a different glass.
What will the glazier actually do?
- Read the crack's origin and path to establish whether it was impact, thermal, or poor edge support
- Measure the pane or sealed unit and record the existing make-up
- Specify a replacement that suits the position, including safety glass where the location is critical, and heat-soaked or toughened glass where thermal stress caused the failure
- De-glaze the frame and clear the rebate completely
- Fit new setting and location blocks so the replacement is not bearing on the frame
- Glaze in, re-bead or re-gasket, and confirm the sash closes cleanly on its seal
Frequently asked questions
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