Why Did My Window Crack on Its Own?
8 July 2025 · 4 min read
A crack appears with no impact, no cause. Usually it's 'thermal stress' — here's what that means.
Thermal stress cracks
When part of a pane heats up much faster than the rest — for example, strong sun on one area while the edges stay cool in shade — the glass expands unevenly and can crack. These 'thermal stress' cracks usually start at the edge and run in a smooth, often wavy line, with no point of impact.
South-facing windows, partially shaded glass, and dark blinds close to the glass make them more likely.
How to tell it apart
An impact crack radiates from a central point, like a star. A thermal crack typically begins at the edge of the pane and curves across it. Knowing the difference helps a glazier confirm the cause — and it is useful for insurance, as thermal cracking is a glass fault rather than accidental damage.
Either way, the cracked pane or unit will need replacing.
Preventing it next time
Keeping blinds a little off the glass, avoiding stickers or dark coverings directly on panes, and choosing toughened glass in high-heat locations all reduce the risk. If a window has cracked on its own, we can replace the glass and advise on the best specification to avoid a repeat.
Contact us for a free quote to replace a thermally cracked pane.
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