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Breaking Point: When a Damaged Windscreen Can & Cannot Be Repaired

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A chip or crack in your car's windscreen can be inconvenient or even dangerous, but nowadays they are pretty easily repaired. Using a specialised resin injection kit, windscreens can be repaired within a few minutes, either by yourself using a home kit, or by professionals. Many car insurance schemes now offer free glass repair as part of their coverage.

However, there are a number of factors than can make conventional windscreen repair difficult or even impossible. Obviously you'll want to avoid this wherever possible, since the price of a new windscreen is considerably higher than that or a simple resin treatment. With that in mind, here are some of the reasons a damaged windscreen may be irreparable, and how you can mitigate them.

The age of the crack

This is probably the most important factor dictate how easy damage will be to repair. As a general rule you should have windscreen damage attended to as soon as possible, for two reasons:

  • Physical stresses caused by driving and varying temperatures can cause the glass to vibrate or expand, causing the crack to widen
  • Dirt and grit accumulating in the crack will discolour the resin when it is applied, and weaken the glass/resin bond, making the repair more fragile and visible.

If seeking immediate repairs isn't practical or feasible, you can still protect the glass from further damage. Keeping your car in the shade, particularly in summer, will leave the glass cooler and more easily repaired. You should also keep the car's windows open for this reason, if you leave your car in a secure place. You should avoid driving if possible, but if you absolutely need the use of your car, tape a piece of clear cellophane over the damage to prevent dirt infiltration.

The location of the crack

If a crack or chip is within the immediacy of the driver's eye line, have it repaired at your own risk—not only does the repaired damage present a dangerous distraction to the driver's vision, but many insurance companies will consider this an unacceptable risk factor and refuse you coverage. As a general rule, this area is 30 centimetres across and centred on the driver's position, extending from the top of the windscreen wiper's sweep to the bottom. However, some insurance companies adhere to stricter or more lenient guidelines, so check with them first. 

The size and shape of the crack

First things first—if the crack has fractured the inner pane of glass in your windshield, get a replacement, as it will be impossible to repair properly. The length or width of a crack or chip is also an issue, as insurance companies do not look kindly on repairing particularly large cracks. Once again, you should check with the insurance company on what size of crack or chip is considered unacceptable by them.

The shape of the crack can also play a large role in how effective a repair might be. The vast majority of windscreen cracks can be put into one of five categories:

  • Round chip—sometimes referred to as a crater or bullseye. These chips, usually caused by loose stones or gravel, are the easiest shape to repair as long as they are relatively shallow and do not penetrate the inner layer of glass. When repaired well, the mark left behind will be almost invisible.
  • Semi-circle chip—sometimes known as a horseshoe or half-moon. These are essentially round chips that are inconsistent in depth as a result of a stone or other projectile striking the windscreen at an angle. These are slightly more difficult to repair than round chips, but leave results almost as invisible if repaired well.
  • Straight crack—a simple crack, extending in a single line. You should call the professionals in to repair one of these, as the thinness of the crack does not allow resin to bond with the glass easily, and specialist equipment (such as pressure-controlled resin guns) may be required. Marks left by repair will be visible but thin and translucent. 
  • Star crack—several cracks spreading out from a single point, often resembling a spider web. These are the most difficult cracks to repair, and will leave noticeable marks behind even if expertly repaired.
  • Combination break—these involve two or more of the above types of damage, and no two are the same. Get professional help immediately, and prepare to shell out for a new windscreen, as they are often irreparable.