Common Reasons Why Windows Break & Crack

Spotting a crack in one of your windows can be alarming. Windows perform an important role in the way your home functions so any defect can be concerning. It isn’t just their appearance that is compromised, but their ability to insulate as well as their safety and security. 

Beyond the dread of having to source replacements, a broken window can often inspire curiosity, confusion and frustration. It’s hard not to wonder how it came to be broken and how you can stop it from happening again?

Sometimes the cause is obvious, a football that misses its goal, or a heavy-handed close. Other times, a crack can seem to appear spontaneously which is even more concerning.

In this article, we’ll look at the common causes of broken windows. We’ll explore why they happen and how to identify the exact cause of each crack.

These tips might be a bit late for a window that is already broken, but they’ll help you identify the cause and hopefully stop it from happening again.


A stress crack is usually simple to spot, as it is unlikely to completely shatter the window. 

It will usually start off small, somewhere near the edge of the window (often near a corner). If ignored, stress cracks will continue to grow across the surface of the windowpane. 

The most likely cause of a stress crack is extreme temperature changes. This could be between the inside and outside of the window, but can also happen across two portions of a window surface. 

Stress cracks can often happen during winter. When it’s frosty outside and you crank the central heating up high, a sudden difference between indoor and outdoor temperatures on the glass can cause the panes to crack.

This difference in heat could also happen across the same window surface. For example, if part of your window is in shade due to an overhanging roof or a large tree, while the other part is exposed to full sun. If this happens over prolonged periods (or if it suddenly changes) this will weaken the glass until it shatters.

Another cause for these window breaks is physical stress. If you slam your window shut too hard for example, this can cause the window to crack. It can also happen with badly fitted sash windows – if the mechanism fails, gravity can slam them shut and crack the pane.


Unfortunately, this is one of the most common causes of a broken window. This cause is completely unpredictable (and usually unavoidable except in hindsight) so it’s only getting a brief mention on this list.

If the pane is still intact, you can usually tell this type of breakage from the way the glass has broken. All the cracks will radiate from a central point where the impact happened. 

If you are unlucky enough to have a cracked window caused by an impact, don’t assume it’s caused by a human and try to place blame. As well as large projectiles like balls, this type of break can be caused by chips flying up from passing cars and even unsuspecting birds.


Sometimes a window may seem to spontaneously break when it is actually the result of a long period of gradual weakening. This is more common among low quality or poorly installed windows. It may seem unexpected, but this type of window breakage is a long time in the making. 

There are many different reasons that the glass could weaken. The first is age. However strong modern glass is, it will still weaken over decades – and single pane glass that is not made to modern standards will already be at a strength disadvantage. Age-weakened windows will usually have served a long life already though and replacing them is necessary to protect the energy efficiency of your home.

The shape of a windowpane will also affect its strength. A square window will have a consistent spread of weight across the pane whereas tall or narrow windows will have more intense stress on the shorter edges. Buying high-quality windows with toughened glass should address this issue though, and you can still expect a decent lifespan from windows of this shape.

Similarly, large windows will weaken quicker than standard-sized windows. This is something to bear in mind anytime you install an exceptionally big window.

Another reason for weakened windows is if they are stored, handled or transported incorrectly. Even if they look fine when installed, any scratches or chips at the edges of the window will affect the integrity of the whole window in the long term. 


As well as improper storage and transportation, improper installation can seriously damage the integrity of your windows and cause window breaks later down the line.

This is because incorrect installation can affect the pressure of the glass. Installing double or triple glazed windows at high or low elevations can also create pressure cracks.

Pressure cracks are caused by an inconsistency between the pressure outside the window and the gas that sits between the panes to provide insulation. A similar pressure problem can be caused if there are dramatic pressure changes in the weather system, though this is completely out of your control. 

You can usually spot a pressure crack because it will follow a curved shape, similar to the outline of an hourglass. 

While there are a number of reasons why windows break, there is usually one outcome – your windows will need replacing. Whether you’re choosing to replace timber windows or aluminium ones, one way to make sure you don’t have to repeat the process again a few months later is to make sure get high-quality replacements.
Our replacement windows come with extensive guarantees including a 5-year guarantee on the glass used in JELD-WEN windows. This means you won’t have to worry about cracks in your panes for years to come. Contact us today to get a quote and find out more.