What Would Cause Glass to Crack in Straight Lines in Windows?


Glass windows, particularly those in older homes, may develop small cracks over time. This is actually quite common, and while it cannot always be prevented, it helps to understand why these cracks occur. Direct damage, such as from an impact, cause spidery cracks to appear. Straight cracks however, are caused by temperature fluctuations and stress on the glass.

Temperature Fluctuations

  • A window is just like a glass bowl -- if you take it out of the microwave and place it under cold water, it's going to break. Temperature fluctuations are the most common cause for small window cracks. If you have your heat turned way up and the outside temperature drops suddenly, your window may crack. This is less common with newer windows, but in older homes with original glass, cracking is a possibility. These straight line cracks will usually appear in areas of the glass that have already been weakened or chipped, even if the chip is small.

Stress Cracks

  • Stress is next on the list for causing straight line window cracks. If you force a window down sharply, the concussion may break the window, or at the very least, cause a small stress crack to occur. Another common cause of stress cracks is the movement in your home. As a house settles into its foundation, it moves slightly. In addition, the wood frame around the window may expand or contract as the weather changes. This puts stress on the glass pane and if it the stress is high enough, a crack will appear.

Age of the Glass

  • Old glass is much thinner than the glass that is manufactured today. If you've ever been in an older home that was built prior to 1950, this is very evident. The glass will be thin and very clear, but it is also very brittle. Slamming a door and rattling this kind of glass is enough to make it crack. You must treat older windows gently. Consider replacing them with newer glass if you are overly concerned about straight cracks appearing.

Why Replacement is Necessary

  • While a crack on a window may look small and if it's not directly in the center, it won't disturb your view, that doesn't mean that the glass should not be replaced. Straight line cracks not only cause an energy loss by letting in hot or cold air, but the crack can worsen over time and the entire window may eventually break. If you have a small crack, it is best to replace the entire glass pane. This process is called reglazing and can easily be completed by just about anyone.

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