Identifying your crack.
Thermal stress cracks are at a perpendicular angle compared to the glass edge. The crack then continues over straight away from the glass edge.
Impact cracks form a pattern that radiates away from a central point. These types of cracks can easily be identified as there is a starburst pattern that goes away from the impact point. Hitting your glass with a rock or golf ball can create an impact crack.
Pressure cracks can be identified with the shape of the break in the glass. The crack will follow a curve in the shape of an hourglass.
Thermal stress cracks in glass typically occur when thermal gradients cause different parts of the glass to expand by different amounts. This expansion can sometimes overcome the strength of the glass and create a crack that doesn’t look like anyone hit it. These thermal shocks are more likely to occur over large glass surfaces or beneath overhangs. The reason being that in the presence of an overhang, stress factors quickly change between sunny parts of the glass compared to the parts in the shaded area.
Seasonal factors like dramatic temperature changes that can sometimes occur overnight are a common cause of thermal shock.
How to identify a thermal stress?
When the edge of the glass is intact, it is generally a stress crack. Thermal stress cracks can occur in any type of glass but the size of the glass definitely plays a role. This being said, they are more likely to appear in larger windows especially when there are overhangs above.
How to prevent thermal stress?
Thermal stress is a natural result of expansion and contracting glass in response to temperature changes. Thicker glass based on climate and sun exposure may decrease the risk of thermal stress but there is no way to fully prevent thermal stress.