Cracked Window Glass

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

Who fixes this

• Broken window repair
• Glass unit replacement

Questions people ask

Sources & References

1. GOV.UK, Protection from falling, collision and impact: Approved Document K
2. Glass and Glazing Federation
3. Glass and Glazing Federation: Technical library