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You lurch through a pothole, slam the car door, or bump another car’s fender, and suddenly your windshield cracks or even shatters right before your eyes. It may seem like your car’s window just imploded for no reason, but there are actually several reasons why this happened. Keep reading to discover the other factors involved in this seemingly spontaneous act of window self-destruction in cars and the maintenance measures to avoid it.

What causes a windshield to spontaneously break?

A cold car windshield covered in ice
A car windshield covered in ice | Philipp Schulze/picture alliance via Getty Images

According to MS Glass Outlet, most ‘spontaneous’ breakage occurs because of previously unnoticed (or ignored) damage to the car glass. It might be a small, nearly invisible crack or chip near the edge. Whatever it is has weakened the glass to the point where something as insignificant as driving over a bump is enough to cause your windshield to shatter — seemingly for no reason.

Thermal damage to auto glass 

Once the initial blemish happens, your auto glass is susceptible to further damage. Not only that, but anything that compromises the integrity of your windshield (like a crack) also compromises your safety. Yes, it really is dangerous to drive with a damaged windshield! That’s why a cracked windshield probably won’t pass a vehicle safety inspection.

Thermal stress is one of the major causes of windshield breakage. Auto glass is designed to take a lot of stress. However, extreme and/or uneven temperature changes on already damaged glass are sometimes too much stress for it to handle. 

Extremely cold or hot temperatures can cause the glass in your car’s windows to contract or expand. This is more likely to happen with the curved glass of your windshield. When your windshield contracts in bitterly cold temperatures, it puts extra pressure on any small cracks in the glass. That added stress is often enough to cause any cracks to spread. Although uncommon, if those cracks spread far enough, your windshield could end up shattering. 

Auto Glass Express explains that there’s almost no chance of an undamaged windshield shattering from extreme heat. If the glass is damaged, however…

Under extreme heat, your windshield glass will expand. As the glass expands, you can watch existing cracks grow and widen. The glass will expand unevenly if the sun is shining more on one side than the other. That uneven expansion is sure to cause even more damage to the glass.

Thermal damage often occurs around the perimeter of the windshield first because that’s where the temperature of the glass changes the quickest. The temperature differential between the edges and the center of the windshield is sometimes enough to cause a crack to form. A seriously cracked windshield might even shatter from a sudden shock, blow, or shake.

Other causes of auto glass and windshield breakage

One type of non-thermal windshield damage is caused by careless or improper repairs or installation. Abrasions or nicks caused by the haphazard use of tools might not show immediately but could eventually result in a crack that suddenly appears out of nowhere.

Another cause of breakage lies in the glass itself. Not every glass manufacturer makes windshields the same way. Glass that’s the wrong thickness won’t fit properly and is subject to early damage. If the glass contains hard impurities or has developed imperfections during the manufacturing process, it’s also likely to fail sooner rather than later. Your windshield might even be the subject of a recall.

For these reasons, you’ll want to ensure a replacement meets or exceeds the vehicle manufacturer’s glass specifications and hasn’t been recalled. 

Once auto glass shows signs of damage, things will only get worse. Windshield damage can take many forms, depending on what caused it. You may find pitting, bullseyes, radiating starbursts, cracks, or some combination. Even the smallest of these can result in shattered glass if you ignore them long enough. That’s why it’s important to have an auto glass professional take care of damage as soon as you notice it. Windshield replacement is definitely not a DIY car repair!


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