Published on Dec 31, 2025
Welcome to another Q&A session where I answer questions from y'all on the internet.
Today's question is:
Did I break the windshield by turning my defrosters on the car? So this is another common thing that we see. So uh a customer will come to me and say, "Hey, I don't know what happened. I was defrosting my car in the morning. All of a sudden, I see this line coming across my windshield of a crack." So what happened was that car received a ship in it. Whether it was from a rock the day before, a night before while it was parked, a rock hit that windshield, usually in the black part around, we call that the frit band. So a rock chip hit on a frit band, you can't see that from the inside of the vehicle. And if you're not looking for it on the outside of the vehicle, it could be hidden by the wipers, you know, in the black part.
It's really hard to see. You don't know until you go to defrost that windshield. And you've got hot air heating up the windshield on the inside and a really cold rock chip on the outside. That volatility just cracks it right across.
Magnatech Autoglass indicated that poking a rock chip may cause it to crack further, though the reaction of the glass can be unpredictable (00:08:58). They confirmed that blasting a defroster can cause a crack to spread if there was a pre-existing, often hard-to-see chip in the frit band, due to the extreme temperature difference between the hot interior air and the cold outside rock chip (00:10:02). Furthermore, extreme heat in summer can also cause a chip to crack out, especially when combined with sudden cooling like hitting the wiper squirters or going through a car wash (00:23:55).
