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Driving with a cracked windscreen: Rules and risks explained

Driving with a cracked windscreen: Rules and risks explained



  • 02 December, 2019
  • 5 min read

Cracked windscreens can happen very easily – and if you’re in the car every day, the consequences can be severe!

If you’re wondering whether or not the chip in your windscreen is worth worrying about, or if you can get away with this week’s commute after a windscreen mishap, make sure you read on to find out the rules and risks of driving with a cracked windscreen.

How does a windscreen crack?

A windscreen cracking can be a frustrating issue to deal with – but it’s a common occurrence. Sometimes windscreens crack for an obvious reason, but sometimes it can seem like they crack out of nowhere. So, first of all, how do windscreens actually crack?


One of the most common causes of windscreen cracks is ignored or unnoticed chips. While not all chips turn it to cracks, a lot of them do. When your windscreen sustains any kind of damage, it’s weakened, so anything from the pressure of driving at high speeds or the slight contraction and expansion of glass in different temperatures can cause a chip to become a full-blown crack.

If you spot a noticeable chip, it’s a good idea to get it fixed – this is usually done by injecting resin into the chip and smoothing it over. It’s relatively cheap and will save you the hassle of a full windscreen replacement if it did get worse.

Road debris

Another common cause of cracked windscreens is road debris. This is the little stones, rocks, pebbles and other debris that are flicked into your windscreen by other moving vehicles.

Even though the majority of road debris that hits your windscreen is small in size, it collides with a massive force which is sometimes enough to shatter your whole windscreen, especially if it hits a weakened spot – like a chip.

Of course, stones and pebbles aren’t the only things that can hit your windscreen while out on the roads. Windscreens are also often damaged by poorly loaded items falling from other vehicles – which is another good reason to avoid tailgating lorries and any other vehicles!

Weather and temperature

The weather can impact your windscreen in a whole host of different ways. For example, stormy, windy weather can lead to pressure on any weak points or chips and can cause things like branches and debris to hit your windscreen, which may lead to cracks.

On the flip side, leaving your car in direct sunlight on a very hot day can also be damaging. This is similar to using boiling water to defrost your windscreen or suddenly blasting the air con when your car is at a really high temperature – it’s not good for the windscreen. All of these actions cause a sudden change in temperature, making one side of glass expand whilst the other side contracts. This results in stress and tension – ultimately causing a crack, or even shattering.

Low quality repairs or installations

While this point speaks for itself, it’s surprising how common this issue is. When having a windscreen replaced, you must visit a regulated and experienced technician – if a windscreen isn’t fitted properly, it’s extremely likely to crack, or even shatter.

This is because the pressure placed on the windscreen by the frame isn’t where it’s intended to be – which is causing stress points. Windscreens are big, fairly weighty pieces of glass, so it’s important for them supported properly so they can withstand minor road debris impacts and the pressure of driving at speed.

Similarly, if your windscreen is replaced with another, lower quality one, it won’t be able to withstand the same conditions as the one before. This can also be very dangerous, as both situations can cause not only a crack, but potentially a serious shatter which could be without warning – which would be especially catastrophic with passengers or if you are driving at a high speed.

Can you drive with a cracked windscreen?

You shouldn’t drive with a cracked windscreen. In fact, if you’re caught driving with a crack in the drivers view, you’re looking at a fixed penalty of three points on your licence and a fine.

It can also cause you to fail your MOT.

In fact, if there is damage anywhere on the windscreen which is 40mm in size or bigger, your car will fail its MOT.

If you look at the diagram below, in zone A – the 290mm portion directly in front of the steering wheel – if you have damage more than 10mm in size, you’ll also fail your MOT.  

It’s not just the legalities that should put you off though – you can’t see properly in the area of a crack in your windscreen, so you’re putting yourself and others at risk if you're driving. Not only that, but the force put onto your windscreen when you’re driving could easily shatter it, which is very dangerous.


Your windscreen also plays a special role in an accident. When the passenger airbag is deployed, it goes upward and bounces off of the windscreen to make sure your passenger’s head doesn’t go into it. If it’s cracked, it won’t be able to withstand the impact of the airbag – they’re deployed at around 150mph – and will likely shatter your windscreen, offering no protection (and probably more danger) to your passenger in a crash.

If your car rolled over due to a collision or accident, over half of the strength provided to stop the roof caving in is provided by your windscreen. Again, if you were to have an accident with a cracked windscreen, it’s possible that it would just shatter, and create a more dangerous situation with the roof likely to cave.

All in all, you shouldn’t drive at all with a crack in your windscreen – even if you think the crack is small, it can make your whole car unroadworthy. As a general rule, if you have any cracks – or even chips – in your windscreen that are bigger than 10mm and are in the A-Zone (which is a 290mm area directly in front of the driver), your car will fail its MOT. A crack or chip bigger than 40mm anywhere else on your windscreen will also result in a fail.

What to do if your windscreen cracks

Bear in mind that it’s best to get any chips sorted as soon as you spot them, to prevent any further costs or the possibility of it becoming a crack. It’s a quick job and won’t cost a lot – certainly less than a windscreen replacement.

Some cracks can be repaired in the same way but can cost a little bit more. Again, they’ll be filled with resin – but usually, you can only go for this fix if the crack is smaller than 10mm in the A-Zone and smaller than 40mm anywhere else. Anything bigger, you’re usually looking at a replacement.

Check your insurance to see if your windscreen is covered – if it is, you’ll pay a smaller excess, if not, you’ll have to pay for the repair or replacement yourself. If you are covered, your insurance will usually pass you on to someone for repairs – if not, it’s up to you to find someone to carry them out for you.

Your windscreen is an important component of your car – it gives you a clear view of the road and your surroundings, protects you from the elements, and even acts as a safety barrier in an accident. It’s important you check it regularly for any chips or cracks, and get them looked at as soon as possible.