Repairable windscreen cracks: chip vs crack repair explained
Most windscreen damage starts as a chip. Whether it can be repaired, or whether you end up paying for a new screen, usually depends on four things: the size of the damage, the type of break, where it sits on the glass and how long it has been left.
Every laminated windscreen is a sandwich: two layers of glass bonded to a tough plastic interlayer. When a stone hits the screen, it normally breaks only the outer layer. A repair works by injecting resin into that break so that it bonds the glass back together, restores much of its strength and stops it spreading. Once the damage has turned into a long crack, or reached the inner layer, a repair can no longer do that job.
Chips and cracks: what is the difference?
A chip is a small impact point, sometimes with short cracks radiating from it. A crack is a line running through the glass. Cracks often start from a chip that has been left alone, or at the edge of the screen where the glass is under the most stress from the body of the car twisting over bumps.
Repairers recognise several kinds of chip, and some take resin better than others:
| Type | What it looks like | Usually repairable? |
|---|---|---|
| Bullseye | A round, cone-shaped crater, often with a dark ring | Yes, if small and clean |
| Star break | An impact point with short cracks or "legs" running out from it | Often, if the legs are short |
| Combination (bullseye and star) | A crater with legs | Often, depending on overall size |
| Half-moon | A partial bullseye | Usually |
| Edge crack | A crack that starts at or runs to the edge of the screen | Rarely |
| Long or stress crack | A line across the glass, sometimes with no visible impact point | Generally no |
The four checks a repairer makes
1. Size
There is no single legal size limit for a repair in Ireland, but most repairers work to industry practice. A common rule of thumb is that a chip that fits under a €2 coin, which is about 26mm across, is a good candidate for repair, while cracks much longer than a few centimetres are usually beyond it. Some specialists will repair longer cracks, but the result is less predictable, and many insurers' repair networks will only approve smaller damage.
2. Position
Position matters as much as size. Two areas are treated with particular caution:
- The driver's direct line of sight. Even a good repair leaves a slight mark that can cause glare or distortion. Many repairers will not repair in the area directly in front of the driver, roughly a band centred on the steering wheel, and the NCT pays close attention to damage there. See will a cracked windscreen fail the NCT?
- Near the edge of the glass. The edges carry the stresses from the bonded joint. Damage close to the edge tends to spread quickly and is often not repairable.
Damage over a camera or sensor area at the top of the screen also needs care, because even a small blemish can affect how a camera sees the road. See windscreen calibration and ADAS.
3. Depth
If the damage has gone through the outer glass and reached the plastic interlayer, or if the inner layer of glass is also damaged, the screen needs replacing. You can sometimes feel this from inside the car: if the inside surface is rough or chipped, a repair is not possible.
4. Age and contamination
A fresh chip is the easiest to repair. Over days and weeks, water, road film, washer fluid and dirt work their way into the break. That makes it harder for resin to fill the damage cleanly and leaves a more visible mark. A chip that has been through a few frosts may also have grown legs that were not there at first.
Why chips turn into cracks
Glass expands and contracts with temperature. A chip creates a weak point where those stresses concentrate, so anything that changes the temperature of the glass quickly can make the damage run. In Irish conditions the usual culprits are:
- Turning the heater to full on a frosty screen, especially with hot air aimed at the glass.
- Pouring hot or even warm water over ice.
- Bright sunshine on a cold screen, or a cold shower after a warm day.
- Potholes, speed ramps and slammed doors, which flex the body and the glass with it.
Until the chip is repaired, cover it with clear tape, warm the car gradually on cold mornings and use a scraper and de-icer rather than water on the glass.
What a repair can and cannot do
A resin repair is designed to stop the damage spreading and to restore strength to the area. It will usually improve the appearance considerably, but it is not always invisible. Expect a small, faint mark where the impact point was. If the chip had legs, those may still be faintly visible in certain light.
Occasionally a chip cracks during or soon after a repair. Reputable repairers explain this risk beforehand. If the damage is borderline, ask whether the repair cost would be credited towards a replacement if it fails.
Repair or replace: a quick checklist
- Small bullseye or star chip, away from the edges, not in front of the driver: likely repairable.
- Chip with long legs or several chips close together: ask a repairer to assess.
- Crack running to the edge, crack longer than a few centimetres, damage in the driver's central view or damage on the inside layer: likely replacement. See windscreen replacement.
Does a DIY repair kit work?
Kits sold in motor factors and online use the same basic idea: a resin injected into the chip and cured in sunlight or under a UV lamp. They can work on small, clean bullseye chips. The difficulty is getting a vacuum and pressure strong enough to fill the fine legs of a star break, and getting it right first time, because a poor DIY fill makes a later professional repair much harder. If your insurance includes windscreen cover, a professional repair may cost you nothing, so it is worth checking first.
Frequently asked questions
How big a chip can be repaired on a windscreen?
There is no single legal limit in Ireland. Many repairers use a chip that fits under a €2 coin, about 26mm across, as a guide. Position, depth and how long the damage has been left matter as much as size.
Can a crack in a windscreen be repaired?
Short cracks can sometimes be repaired by specialists, but cracks longer than a few centimetres, cracks that reach the edge of the glass and cracks in the driver's direct view are normally replaced rather than repaired.
Will a repaired chip be invisible?
Usually not completely. A good repair makes the damage much less noticeable and stops it spreading, but a small faint mark normally remains where the stone hit.
How long can I leave a windscreen chip?
As short a time as possible. Dirt and water get into the break and temperature changes can make it spread, so a chip repaired within days has the best chance of a clean result.