Why Did My Windscreen Crack on Its Own?

Why Did My Windscreen Crack on Its Own?

If you have found a crack in your windscreen and cannot remember anything hitting it, you are not imagining things. Windscreens can crack without an obvious impact. In most cases it is a stress crack, and it behaves differently from the chip-and-crack damage most drivers know.

How to Tell a Stress Crack From Impact Damage

Impact damage almost always has a starting point you can see or feel: a small pit, bullseye or star where a stone struck the glass, with the crack running out from it. A stress crack usually has none of that.

• No impact point — run a fingernail along the crack and you will not find a pit or crater.
• It often starts at the edge — many stress cracks begin within a few centimetres of the frame.
• It tends to be a clean line — often fairly straight or gently curved rather than branching like a star.
• It can appear overnight — or during a hot day, or straight after using the demister.

If you are not sure what kind of damage you have, our guide on how to assess vehicle glass damage walks through the common types.

What Causes a Windscreen to Crack by Itself

Temperature differences across the glass

Glass expands as it warms and contracts as it cools. When one part of the windscreen is much hotter or colder than another, the difference creates stress. A car parked in the Sydney sun with the air conditioning then blasted at the glass, or a cold morning demister aimed at one spot, can be enough to start a crack — especially if the glass already has a weak point. We cover this in more detail in why Sydney weather is tough on windscreens.

A tiny chip you never noticed

Sometimes there is an impact point, but it is so small it was never spotted. A pinhead-sized chip near the edge can sit unnoticed for months until heat or body flex turns it into a long crack.

Body flex and the windscreen frame

The windscreen is bonded to the car body and helps make it rigid. Driving over kerbs, potholes or steep driveways twists the body slightly, and that movement is shared with the glass. If the frame (the pinch weld) is uneven, damaged or corroded, the glass can be held under constant stress.

Installation stress

A windscreen that was fitted with uneven adhesive, or that sits against a hard spot on the frame, can be under strain from day one. Stress cracks from this cause often appear in the weeks or months after a replacement, which is why it is worth checking what your windscreen warranty covers.

Can a Stress Crack Be Repaired?

Usually not. Resin repair works best on small chips and short cracks that start from an impact point away from the edge. Stress cracks tend to start at the edge, run long, and keep growing as the glass heats and cools, so a full replacement is normally the safe option. A technician can confirm this on the spot.

Windscreen replacement with Aussie AutoGlass starts from $330, depending on your vehicle. See our windscreen cost guide for what affects the price.

What to Do Until It Is Replaced

• Avoid aiming hot or cold air directly at the crack.
• Park in the shade or under cover where you can.
• Avoid slamming doors and take driveways and speed bumps slowly.
• Keep an eye on whether it is spreading — mark the end with a whiteboard marker on the inside of the glass.
• If the crack is in your line of sight, read whether it is legal to drive with a cracked windscreen in NSW.

Frequently Asked Questions

Can a windscreen crack from heat alone?

Heat by itself rarely cracks sound glass, but a big temperature difference across the glass can turn a small weak point — an unnoticed chip or an edge flaw — into a crack.

Why did my new windscreen crack?

A crack soon after replacement can come from an impact you did not notice, or from stress in the installation or the frame. Contact whoever fitted it and ask them to inspect it.

Will a stress crack keep spreading?

Usually, yes. Temperature changes and body movement keep working on it, so it is best not to leave it. Our article on whether a crack can spread overnight explains why.

Is a stress crack covered by insurance?

It depends on your policy. Any insurance excess depends on your individual policy, so it’s worth checking your policy documents or asking your insurer.

Found a crack you cannot explain? Aussie AutoGlass is a mobile service across greater Sydney — we come to your home or workplace. Book an appointment or call 0405 412 951.