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Will a windscreen crack cause your car to fail its MOT?

Damaged windscreen? What you can fix and what you can’t

The noise a stone makes when it hits your windscreen is horrible. Windscreens don’t shatter easily, but they do chip and crack, and when you hear that sound you should inspect your windscreen for damage.

The good news is that small chips can be permanently repaired. The bad news is that larger chips and cracks may mean the windscreen will need to be replaced - and if they’re left alone, even small chips and cracks can quickly spread.

Here are four reasons not to delay getting your windscreen repaired:

  • The windscreen is designed as an integral part of the car’s body shell and contributes towards its overall strength and stiffness. Any major structural failure in the windscreen affects the car’s rigidity and structural integrity.
  • The windscreen is subject to mechanical stresses. Driving over uneven or bumpy surfaces can turn a small crack into a big one very quickly.
  • Thermal shock – sudden and sharp temperature changes in cold weather. The inside of the car quickly becomes warm when you put the heater on, while outside it could be well below zero.
  • It is possible to repair stone chips, but only if the surfaces are clean. Old chips fill with dirt and grime which can’t be removed and make a repair impossible. Putting it off could mean the difference between a relatively cheap fix now and an expensive new windscreen later.

Even if your windscreen damage is below the statutory MOT failure threshold, there may be complications in the event of an accident.

The UK’s Highway Code states that the driver should have a full view of the road and traffic ahead and all glass fitted to a motor vehicle should be maintained in such a condition that it does not obscure the vision of the driver whilst the vehicle is being driven. If a stone chip is considered to be a contributory factor, you could be in trouble.

5 stupid MOT failures you could have fixed yourself

So what will it cost to fix a windscreen?

Check your motor insurance policy. Many policies cover windscreen repair and replacement automatically, sometimes with a reduced excess or none at all. It may not cost you anything to get your windscreen fixed, so that’s even more reason not to delay!

Damaged windscreen? What you can fix and what you can’t

Windscreen damage you can fix

Whether or not the windscreen can be fixed depends on the size of the damage and where it is. The key area is right in the driver’s line of vision and is often called the ‘A-zone’.

In the UK it’s defined as a strip 290mm wide and in line with the steering wheel – roughly the width of the steering wheel, in fact, if you need a quick guide.

Within this area, it’s possible to repair stone chips up to 1cm (10mm) in width. Outside of this area, the limit is 4cm, but although these are the basic requirements of the UK MOT regulations, there are other factors.

For example, cracks which extend right to the edge of the windscreen will affect its structural integrity and it will have to be replaced.

UK windscreen specialist Autoglass has posted a video with more information, defining an additional area (Zone B) swept by the driver’s windscreen wipers, where repairs up to 1.5cm can be carried out, a Zone C (passenger’s wipers) for repairs up to 2.5cm and a Zone D up to the edges of the screen for repairs up to 4cm in size – but excluding a 4cm margin at the very edge. Damage here will need a replacement windscreen.

Your wipers should be renewed once a year. Find out how to do it in minutes here.

Windscreen repairs are carried out using a special resin which bonds with the glass to form a near-invisible repair that’s flush with the surface of the glass.

Typically the job takes from 30-60 minutes and can be carried out at your home or workplace.

Windscreen damage comes in different types, for example ‘star breaks’, ‘bulls-eye’ impacts, ‘half-moons’, ‘crack-chips’ and ‘pits’.

Generally, if you can cover the damage with a small coin, such as a five pence piece, it can be repaired. If the damage is worse, you’ll have to take the advice of the screen repair technician.

Windscreen damage you can’t fix

‘Cracks’ are a grey area. ‘Star cracks’ caused by an impact are likely to be OK, but the structural role of the windscreen means that larger cracks are likely to be unrepairable, especially if they are ‘stress cracks’ caused by temperature changes or body deformations rather than stone impacts.

Scratches, surprisingly, are another problem area. They can have a profound effect on the driver’s vision, especially when driving into a low sun, but they tend to be shallow and extend over a large distance.

It’s possible they could be polished out, but they are too shallow to be filled with resin in the usual way, and the screen may have to be replaced even though the ‘damage’ appears slight.

What the law says about your windscreens and the MOT test

  • In the UK, a car will fail its MOT test if the maximum damage size is 10mm in the drivers’ line of vision (the ‘A’ zone – that vertical strip 290mm wide centred on the steering wheel), and if it exceeds 40mm elsewhere.
  • In the US, the same general guidelines about windscreen damage, repair and replacement are the same, but different states may have different statutory requirements, and in any event you should take the advice of a good screen repair technician.