crack in winshield - will it eventually shower me with glass debris?

I have a five-year-old car (Honda accord) and there is a crack in the windshield that is slowly spreading from the right horizontally across to the left side. In the interest of science I am thinking of letting it spread all the way across the windshield to see what would happen. My girlfriend says that one day, going over a speed bump, the windshield will explode and we will both be covered in dangerous glass shards. I say that the crack can spread all the way across without the windshield breaking. Which of us is right?

      • In the fall about two years ago, I bought a SUV that started out with a small 1/2-inch crack/pit in the windshield near the horizontal center, up about four inches from the lower edge. During the winter the windshield defroster would blow hot air right on it, and during quiet times every now and then I would hear the window cracking. After a few days of very cold weather, the windshield had cracked all the way across both directions, reaching to the left and right edge. The window still held together and was useable, I only had it replaced that soon because I thought it looked trashy, but it didn’t leak or anything. The windshield replacement guy said that if I had wanted it patched instead of replaced, they would not have warranted it, because they won’t warranty patch jobs if they are near where the heater blows hot air on the window… because the heating and cooling of the defroster tends to make cracks spread. Go figure.

Windshield glass is laminated with plastic, which is to keeping it from breaking into dangerous glass shards.

Even without the plastic lamination, windshield glass for several decades now has been tempered in such a way that it shatters into smooth-edged pieces, not sharp edged shards.

Well, since you’re going to eventually get your windshield replaced anyway, here’s what you do. You sit in the front seat of your car, and have your girlfriend hit the windshield as hard as she can with a baseball bat (wooden or aluminum, but I might go with wooden). The glass will probably hold and you will have taught your girlfriend a valuable lesson. :slight_smile:

Seriously though, you’re in no danger. You can beat a windshield senseless and while you might get a few pieces of glass hitting you, you’re not going to get covered in it.

I read you can score a line & crack at the end of the crack to keep it from going any further. I don’t know if that creates another one going the other direction. I wonder if you can get cited for having long cracks?

I know that in CA at least you can get sited for windshield cracks if they affect your ability to see the road, like it was going straight across the driver side.

Is that true for windshield glass? I thought windshields were just laminated, and that the tempered glass was elsewhere like the doors.

I ask because doesn’t tempered glass shatter into these hundreds of little pieces almost instantly when it breaks? That doesn’t happen in a windshield. If it did you wouldn’t be able to see out of it, and there would be a bunch of accidents caused by rock chips.

handy I tried that several times and it didn’t work at all. Either I was doing it wrong or it’s an old wives tale.

You’re right, Dag Otto, tempered glass is used only for the side and rear windows of a car. When shattered, tempered glass breaks into pebbles instead of shards. The problem with tempered glass is that it can’t be repaired if damaged, only replaced.

No, the side windows are tempered. Tempering makes them very stiff and strong for their thickness (much thinner than a windshield). They are so strong that, if one were supported on all edges like a windshield, you could break your head smacking into one before breaking the glass. Tempering also causes them to shatter into many small, squarish pieces rather than large pointy shards. The small pieces are still quite sharp and can cut, but they won’t impale you.
Head-breaking strength is not a desirable feature in a windshield, and having a windshield that will shatter into a bazillion pieces if chipped at all is not good either. Windshields are made of two layers of more “normal” glass bonded to a layer of flexible plastic in the middle. The crack you see is in only one of the layers, which is why the windshield will not leak. It also will not easily break into bits and come back at you. If you took a baseball bat and whacked away at it, it would break into lots and lots of jagged shards… all still firmly glued to the flexible plastic, which is still pretty much in an unbroken sheet and attached to the frame on all the edges. If you did the same to the side window, you might find it hard to break. But, once broken, it disintegrates and falls to bits.

Darn, you guys are fast! I guess I type too slow!

As in CA, same true in TX, as long as the crack doesn’t interfere with you seeing out of it; it will still pass the state inspections for probably most states, so they don’t seem to think it much of a danger.


In a recent edition of Car and Driver magazine, one of the writers told how he tested those little ‘emergency escape tools’. You know, the kind that are supposed to break the tempered windows and cut through seatbelt webbing in the event you are trapped in a burning or submerged car (or worse, a burning AND submerged car) all for $19.95 + S&H. He took the tool to a junkyard and tried to break a window. Couldn’t do it. Eventually, the big burly tow truck driver who ran the place was able to break a window, but he had to use a full swing (not likely in the confines of a car). The seat belt cutter turned out to be useless also.

I have made a crack go where I wanted it to, more or less by, by gently tracing it with my index finger from inside.

I don’t know about the windshields, but yes side windows are amazingly hard to break unless you know how to do it.

Years ago I was stripping a car down at a friend’s body shop to run in a demolition derby. I wanted to break the side windows out (as required by the demo derby rules.) I’m a pretty big guy, and I took a sledge hammer and hit the a side window repeatedly as hard as I could. Nothing happened. I was amazed.

My friend who ran the body shop took a small hammer with a chisel shaped head on it and gently hit the top edge of the window. It shattered into thousands of pieces.

I’ve got a crack in my windshield literally from side-to-side right now on my car. The windscreen is still nicely in place.

I’m waiting for spring to replace it since Colorado puts “sand” the size of large gravel on the roads in the winter and cracked windscreens are very common.

      • In the US at least, all window glass is tempered–the difference between the front/rear windows and the side windows is that the front/rears are laminated with plastic, but if you manage to break any of them, they still shatter into lots of tiny little almost-cube-shaped pieces.
  • Also: I was told y a cop pal that to break tempered glass, you need to hit it with something that is harder than the glass. Inner-city disadvantaged youths have a tendency to carry around old spark plugs for this purpose–sometimes it has to be thrown two or three times, but the ceramic part striking a car window will shatter the window rather easily.

A couple of things.

  1. The windshields are not laminates in the traditional sense. They are real glass, with a very thin plastic shield sandwiched in between the two layers of glass. Windshields very rarely shatter out fully unless something penetrates them and rips them out suddenly. This is by design.

  2. The edges are not smooth by any stretch. I can swear by multiple personal experience, that the chunks of front AND side AND back window glass that spray when the glass is shattered, are extremely sharp. I’ve come home from M.V.A.'s ( motor vehicle accidents ) with cuts on legs and arms, and a shoulder once.

  3. As far as old spark plugs being used, we carry spring-loaded centerpunches. They are made available in all EMS and Firefighting supply catalogues. They will pierce the windshield glass, without having to either swing a brick or use a sparkplug.

As for the O.P., if the window is at all a modern one, it will not blow out if sitting still when the crack severs the sheet of glass in half. The plastic mid-layer will stop that. If you’re doing 65 mph, I’d watch out if I were you.


Belrix, no shit! And if you replace the damn thing in the Springtime, the hail will smash it out. And if you replace it in the Summertime, it’ll get stolen along with the rest of the car (Denver is a car-thief-friendly place). And if you replace it in the Fall, well, you’ll have a nice windshield for those 3 weeks!

Yeah, you don’t have to worry abouth the windshield coming in on you unless something massive like a deer hits it and comes in along with it. The only time I can recall seeing a windshield detatch along the edge or breach in the middle is in the event of a VERY heavy hit. Airbags will shatter the windshield as well, but as has been noted, you don’t have to worry about life-threatening debris.

Many windshields are considered structural elements of the car’s design. A crack can compromise crash safety to some degree in that the unit is designed to keep you IN the car (should your seatbelt somehow not be fastened, for instance). Cartooniverse can tell you if remaining inside the car during a rollover is important to any degree.