A crack with nothing behind it
It is more common than people expect and it causes a lot of unnecessary arguing about who did it. A crack that starts at the edge of the pane, runs roughly at right angles to it for an inch or two and then wanders across the glass in a smooth curve is a thermal crack. Nothing hit it. It happens when one part of a pane gets a good deal hotter than another — strong sun on the exposed part while the edge stays cold in the rebate, or a dark blind or heavy furniture trapping heat against the inside. The glass expands unevenly and lets go. An impact crack looks quite different: it starts at a point, usually with a small chip or bullseye you can feel with a fingernail, and radiates outward in straight lines.
Why it matters which one you have
Because if it was thermal, the replacement will do exactly the same thing unless something changes. That might mean toughened glass in that opening, which tolerates the stress far better, or it might be as simple as not putting the blind back hard against the glass. It is worth two minutes of looking rather than fitting the same pane back into the same conditions.
Made safe, then replaced
A crack that is holding will normally wait for the right glass to be ordered. A pane that has gone through gets cleared out of the frame and the opening boarded so the house is secure until the replacement arrives.
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Common questions
My window cracked and nothing hit it. How?
Almost certainly thermal stress — one part of the pane much hotter than another, so the glass expands unevenly. The tell is a crack that starts at the edge and curves. An impact crack starts at a point you can usually feel and radiates in straight lines.
Will the new glass just crack again?
Not if the cause is dealt with. That may mean toughened glass in that opening, or something as simple as keeping a blind off the glass. We will tell you what we think caused it.
Which parts of Dudley do you cover?
Netherton, Woodside, Kates Hill, Russells Hall, Priory, Sedgley, Coseley, Upper and Lower Gornal and Quarry Bank.
Everything else we do in Dudley
Repairs
Something's broken or stuck
Emergencies
Broken glass, made safe
Glass cut to size
Supplied, and fitted
Windows & doors
When repair isn't the answer