People usually ask this with a coin in their hand. The coin test is a decent start, but the honest answer is that whether a crack can be repaired depends on three things, and size is only one of them. Below are the limits in the industry standard, the limits we actually work to, and the part most articles skip: what makes a repairable chip turn into a replacement while you wait.
The quick answer
The auto glass industry works to a published standard, the Repair of Laminated Automotive Glass Standard (ROLAGS), developed by the National Windshield Repair Association and recognized by the Auto Glass Safety Council. It sets maximum sizes by damage type. These are the standard's limits next to what we will take on:
| Damage type | ROLAGS limit | What we repair |
|---|---|---|
| Bullseye or half-moon (round, dark centre) | 25 mm (1 in) diameter | Up to toonie size, about 28 mm |
| Star break (short legs from an impact point) | 75 mm (3 in) across the legs | Same, if the legs have not started to run |
| Combination break (bullseye with legs) | 50 mm (2 in) body, legs excluded | Same |
| Single crack | 35 cm (14 in), with conditions | About 15 cm, roughly a business card |
| Surface pit (no legs, outer layer only) | Repairable | Yes |
| Anything through the inner layer | Not repairable | Replacement |
Our crack limit is tighter than the standard on purpose. A 30 cm crack can technically be filled, but the repair line stays visible and the strength recovery drops the longer the crack gets. We would rather tell you that up front than hand back a windshield you are not happy looking through.
1. Size
Chips: anything up to about the size of a toonie repairs well. Bullseyes and half-moons repair best because the break is a clean cone the resin fills completely. Star breaks repair well while the legs are short. Combination breaks are fine if they have not started to run.
Cracks: a single crack up to roughly 15 centimetres can usually be repaired if it meets the location rules below. Some shops will attempt longer ones. We generally will not. Past about 15 cm you are better off with a replacement, and your insurer will usually agree.
2. Location
This is the one that overrides size.
- Edge of the glass: a crack that starts at or reaches within about 5 cm of the edge cannot be reliably repaired. The edge is where the glass is bonded to the body and carries the most stress, and a repaired edge crack tends to run again. The standard excludes edge cracks and stress cracks outright.
- Driver's primary viewing area: the standard defines this as a strip about 30 cm (12 in) wide, centred on the steering wheel, within the wiper path. Damage here is repairable in principle, but a faint mark remains, and in a spot you look through all day some drivers find it distracting. We will tell you what it will look like before we start.
- In front of the forward camera or rain sensor: not repairable in practice. Even a good repair changes how light passes through that patch of glass, and the camera looks through it. Replace and recalibrate.
- Inner layer damage: a windshield is two layers of glass bonded to a plastic interlayer. Repair only works on the outer layer. If the crack has gone through to the inside (you can feel it with a fingernail from inside the car), it is a replacement.
- Near another repair: the standard will not repair damage within about 10 cm of a previous repair. Two chips close together usually means replacement.
3. Age and contamination
A fresh chip repairs almost invisibly. A chip that has been open for a month has dirt, road salt and moisture in the break, and resin cannot bond to dirt. It will still stop the crack from spreading, but the mark will be more visible. A chip that has been through several freeze-thaw cycles has usually already grown legs.
Why cracks spread: temperature
Glass expands when it warms and contracts when it cools. On a flat sheet that is harmless. On a curved windshield glued to a steel body, with one layer facing the weather and the other facing the cabin, it is not, because the two layers rarely sit at the same temperature. The outer skin at -25°C has contracted; the inner skin with the defroster on it is pushing toward +40°C and expanding. The glass is already in tension across the break, and the tip of a crack is where all that stress concentrates. That is the moment a 5 mm chip grows a 10 cm leg.
The same thing happens in reverse in summer. A windshield sitting in direct sunlight in a Barrhaven parking lot can reach 60°C on the surface. Start the car and put the air conditioning on the glass and the inner layer contracts against a hot outer layer. Cracks run on the first hot day in June for the same reason they run on the first cold morning in November.
This is why the same chip is a 30-minute repair in October and a full replacement in January in Ottawa. It is not that the chip got older. It is that every temperature swing between those two dates was a chance for the break to run, and we get a lot of them: overnight lows below -20, a defroster blast every morning, an afternoon thaw, and a refreeze that night. Four stress cycles a day, five months of the year.
How to keep a chip from spreading until the repair
You usually cannot get the repair done in the next hour. Here is how to protect the break for the day or two until you can:
- Cover it with clear tape. A piece of clear packing tape over the chip keeps dirt, debris and water out of the break. That is the single most useful thing you can do, because contamination is what makes a repair visible. Do not use duct tape or anything with a heavy adhesive.
- Park out of the sun and the cold. A garage is best. Shade is next. If the car has to sit outside overnight in winter, a windshield cover slows the temperature swing.
- Warm the glass gradually. Start the defroster on low with the fan on low and let the cabin warm up for a few minutes before you aim heat at the glass. Never pour hot water on an iced windshield with a chip in it. That is the fastest way we know to turn a chip into a crack.
- Close the doors gently. Slamming a door with the windows up sends a pressure pulse through the cabin that flexes the windshield. Crack a window if you have to shut a door hard.
- Drive like there is a cake on the seat. Speed bumps, potholes, railway crossings and rough gravel all put vibration through the body, and the body flexes the glass. Slow down over them. Skip the car wash, especially the high-pressure wand.
- Do not press on it. People poke the chip to see if it moves. Leave it alone.
None of this fixes anything. It buys time. The repair still needs to happen within days, not weeks, and we can usually come to you the same day.
Nail polish, superglue and repair kits
You will find all three recommended online. Here is what each actually does to the repair.
Clear nail polish does not bond glass under stress, so it does not stop a crack. What it does do is seal the break with a lacquer that resin cannot displace. The chip is now contaminated, the professional repair cannot reach the bottom of the break, and a repair your insurer would have paid for in full turns into a replacement with a deductible.
Superglue is the same story with a worse ending. Cyanoacrylate cures hard and brittle, shrinks slightly as it sets, and yellows in UV. It does not flex with the glass, so the crack runs anyway, and the glue in the break makes the chip permanently unrepairable.
DIY windshield repair kits are the honest option of the three, and on a small, fresh, simple bullseye they can work. The limits are real, though. The kit's dropper or syringe pushes resin in by hand pressure; it has no vacuum stage, so on a star break the air in the legs has nowhere to go and the legs stay unfilled. Any dirt or moisture already in the break stays there. And you get one attempt: once resin has cured in the break, a professional cannot pull it out and redo it. If the chip is covered by your comprehensive insurance, which in Ontario it usually is with the deductible waived, a kit saves you nothing and risks the free repair. Tape it and book the professional repair.
What the technician actually does
A professional repair follows the same sequence whether it is done at the shop or in your driveway. The technician inspects the break with a probe and a light to confirm the type and depth, picks out loose glass from the impact point, and mounts a bridge tool over the chip. The bridge pulls a vacuum on the break to draw out air and moisture, then injects an optically matched resin under pressure so it fills the whole cone and every leg. The resin cures under UV light, the surface is scraped flush and polished, and you drive away. The whole thing takes about 30 minutes.
A good repair restores most of the glass's original strength and leaves a faint mark you have to look for. It also passes Ontario's safety inspection standard, which judges the glass as it is on the day. If you are choosing a shop, ask whether the technician works to ROLAGS and what they will not repair. A shop that says it can fix anything is telling you something. More on how rock chip repair works.
Why bother with repair at all?
Cost and the original glass. A repair costs a fraction of a replacement, most Ontario comprehensive policies waive the deductible for it, and you keep the factory glass and the factory seal. A replacement, done well, is as good as new, but on a vehicle with a windshield camera it also means a recalibration. The best windshield is the one you already have, repaired early.
Common questions
Can a 6-inch crack be repaired?
Sometimes, if it starts away from the edge, is clean, and is not in the driver's primary viewing area or in front of a camera. The industry standard allows cracks to 35 cm, but most shops, including ours, stop around 15 cm because the repair becomes visible and the strength recovery drops off.
Can you repair a crack that reaches the edge of the windshield?
No, not reliably. Edge cracks are under the most stress and tend to run again after repair. The repair standard excludes them.
Will a repaired chip be invisible?
Nearly, if it is fresh and clean. Older or dirty chips leave a more visible mark but the repair still stops the spread.
Does nail polish or superglue stop a windshield crack from spreading?
No. Neither bonds glass under stress, and both contaminate the break so repair resin cannot reach it. A chip that would have been a free insurance repair often becomes a replacement. Cover it with clear tape instead and book the repair.
How long can I wait to repair a chip?
Days, not weeks. Every freeze-thaw cycle and every blast of the defroster is a chance for the break to run, and dirt and moisture work in from day one. In Ottawa winters the same chip is a 30-minute repair in October and a replacement by January.
Do DIY windshield repair kits work?
On a small, fresh, simple bullseye, sometimes. Kits have no vacuum stage, so air stays trapped in the legs of a star break, and you get one attempt: once resin is in the break a professional cannot redo it. For a chip covered by insurance, a kit saves nothing and risks the free repair.
Related
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