Why Has My uPVC Window Pane Cracked or Shattered?
For a glass pane in a uPVC window that has developed a crack, star break or sudden shattering, whether or not there was an obvious impact.
What this usually means
Window glass can crack from impact, edge damage, frame stress or thermal stress, and toughened safety glass can sometimes shatter into many small pieces after damage. Do not keep operating a sash with unstable glass. Keep people away from the pane, avoid pressing or taping directly across loose fragments, and arrange a glazier to make the area safe and identify the correct replacement glass specification. If the crack appeared without an obvious impact, the frame, glazing support and edge condition should also be checked.
What may be causing this
Direct impact damaged the glass
Clues: There is a visible strike point, chip or star pattern.
What to do: Keep clear and have the pane replaced with the correct glass type.
Thermal stress caused a crack
Clues: The crack begins at an edge and may follow a strong temperature difference across the pane without an impact point.
What to do: Have the glass specification and installation checked when it is replaced.
Edge damage or glazing stress weakened the pane
Clues: The crack starts at the perimeter, or failure followed sash movement or previous glazing work.
What to do: Have the frame, packers, beads and glass edge condition inspected.
Toughened glass suffered delayed failure after damage
Clues: The pane suddenly breaks into many small pieces rather than a few long cracks.
What to do: Treat the opening as unsafe and arrange prompt glazing replacement.
How to check what is happening
Mechanical damage is the likely trigger.
Thermal or edge stress may have contributed.
Frame or sash stress should be investigated during reglazing.
Can you fix this yourself?
Keep the area safe and arrange glazing replacement. Handling and removing cracked insulated or safety glass should be left to a glazier, especially for large or high-level panes.
- Phone for photographs
- Barrier or closed room to keep people away
What you can safely try
- 1
Keep people away from the damaged glass
A cracked pane can lose stability without warning.
- 2
Avoid operating the sash unnecessarily
Opening and closing can flex already damaged glass.
- 3
Photograph the pattern from a safe position
The crack origin can help the glazier assess possible impact, edge or thermal stress.
- 4
Arrange glazing replacement
The replacement must match the required thickness, safety classification and insulating-glass specification.
Call a glazier if…
- Glass is loose, falling or actively breaking.
- The damaged pane is above a public or occupied area.
- The opening cannot be secured against weather or intrusion.
- Safe handling would require touching shards or working at height.
Get the right glazier.
We already know this problem maps primarily to a glazier. Choose the closest type of work, add your postcode and continue in the form.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- When to repair or replace your windowsGlass and Glazing Federation
- Window Stiff to Open — Maintenance AdviceEverest
Last reviewed: 6 September 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.