When does a windscreen chip spread? Repair it before it cracks

A small stone thrown up by a truck’s tyre on the motorway leaves a dot-sized mark on your windscreen. Most drivers ignore it at first, because it does not block the view and is hard to notice. But a chip is a stress point that has already started inside the glass, and under the right conditions it can turn into a crack running across the windscreen within a few hours. In this article we explain why chips spread, when they can be repaired and when windscreen replacement becomes unavoidable.

Why does a chip spread?

Windscreens are laminated: two layers of glass with a flexible interlayer called PVB between them. A stone usually damages only the outer layer, but micro-cracks form at the point of impact. The main triggers that make those cracks grow are:

• Temperature differences: Pouring cold water on sun-heated glass, blasting hot air onto an icy windscreen in winter, or aiming the air conditioning at the glass in summer creates different expansion across the glass. The stress concentrates at the weakest point: the chip.
• Vibration and road impacts: Potholes, speed bumps, manhole covers and the twisting of the body when you drive onto a kerb are transferred to the glass through the body. Every impact opens the micro-cracks a little more.
• Slamming doors and pressure changes: Shutting doors hard briefly changes the air pressure inside the car and flexes the glass.
• Dirt and moisture: Water, dust and washer fluid getting into the chip make it harder for resin to fill it completely during repair. In winter, water inside can freeze, expand and enlarge the chip.

Why does the position matter?

Of two chips of the same size, one may be repairable while the other needs replacement. Position is what decides:

• Chips near the edge: The areas of glass close to the frame carry most of the body stress. Chips within a few centimetres of the edge tend to turn into cracks quickly.
• The driver’s line of sight: A repair directly in front of the steering wheel can leave a slight mark because of light refraction. For larger damage in this area, replacement is recommended for safety.
• The camera and sensor area: Damage in the camera area behind the rear-view mirror can affect the view of driver assistance systems such as lane keeping and emergency braking.

Which chips can be repaired?

There is a practical rule of thumb used widely in the industry: chips no bigger than a coin, roughly under 2 cm in diameter, and short star cracks can usually be repaired with resin. This is a general rule; the final decision depends on the glass thickness, the depth of the damage and its position.

During repair the chip is cleaned with a vacuum, a special resin is injected under pressure and cured with UV light. The process usually takes 20 to 30 minutes. The aim is to stop the chip spreading and to preserve the strength of the glass; the mark may not disappear completely, but it becomes much less visible. See our windscreen chip repair page for details.

Replacement is needed instead of repair when:

• The crack is longer than a few centimetres and still growing
• The damage has reached the inner layer
• The chip is very close to the edge of the glass
• There is large damage in the driver’s line of sight

In these cases windscreen replacement is the only safe solution.

Will a cracked windscreen pass the vehicle inspection?

The windscreen is one of the parts checked at the vehicle inspection because it directly affects visibility. Cracks or breaks that obstruct the driver’s view or compromise the integrity of the glass may be recorded as defects. Which damage falls into which defect category depends on the inspection station’s assessment. That is why repairing small chips and replacing cracked glass before the inspection is the most trouble-free approach.

What should you do as soon as a chip appears?

1. Put a piece of clear tape over the chip to keep dust and water out.
2. Postpone washing the car until it has been repaired.
3. Do not aim the air conditioning or heater directly at the glass and avoid sudden temperature changes.
4. Close the doors gently and slow down on rough roads.
5. Book a repair on the same day or within a few days if you can.

Does comprehensive insurance cover chip repair?

Many comprehensive policies include glass cover, and because chip repair costs much less than replacement, insurers generally prefer it. Simply check the excess and glass cover terms of your policy. To learn the process step by step, read our insurance glass replacement guide or visit our insurance-covered glass replacement service page.

Conclusion

Repairing a chip while it is small is both quicker and cheaper. Temperature differences, vibration and closeness to the edge all increase the risk of waiting. Send us a photo of the chip or fill in the quote form to find out whether it can be repaired. If replacement is needed, we have collected what to watch for afterwards in our after windscreen replacement guide.