A 10mm Windscreen Chip in the Wrong Spot Will Fail Your MOT

A stone flicks up off the motorway, taps the glass and leaves a chip the size of a five pence piece. Most drivers shrug and forget about it. Yet depending on exactly where that chip sits, it can be the difference between sailing through your next MOT and being handed a straight fail. The rule that decides it is precise, it is measured in millimetres, and a lot of drivers have no idea it exists until the tester points at the glass.

The area directly in front of the driver is treated far more strictly than the rest of the windscreen. Damage there only has to be 10mm across to fail, while elsewhere on the swept area the limit is four times larger. With summer bringing more motorway miles, more loose grit on resurfaced roads and more stone chips, it is worth knowing exactly where the line falls before a small mark turns into a failed test or, worse, a fine.

The rule: Zone A and the 10mm test

Under the DVSA MOT inspection standards, the windscreen is split into zones. The critical one is Zone A, a band 290mm wide centred on the steering wheel and running up the driver’s side of the glass. This is the area in the driver’s direct line of sight, and the tolerance here is tight. Any damage larger than 10mm within Zone A is an automatic MOT failure. That is roughly the width of a fingertip, so it does not take much.

Outside Zone A, across the rest of the area swept by the wipers, the threshold is more forgiving. Damage there has to be larger than 40mm to fail, about the size of a two pound coin and a bit more. The same thresholds apply to both chips and cracks. So the same small chip that would pass without comment on the passenger side becomes a fail if it happens to sit in the narrow strip in front of the driver. Position, not just size, is what counts. Testers use a clear template to measure damage against the zones rather than judging by eye, so there is little room for argument on the day, and a chip an owner has lived with for months can still come as a surprise at test time, regardless of how well the car otherwise drives.

What counts as a fail across the windscreen

It is not only impact damage that a tester is looking at. Anything that obstructs the driver’s view of the road can cause a failure, including stickers, dashcam mounts, air fresheners or parking permits placed within the swept area in front of the driver. A satnav or phone holder fixed in the wrong spot can fail an otherwise sound car. Wiper blades in poor condition that cannot clear the screen properly are also a failure point, because a smeared screen is as much of a hazard as a cracked one.

Chips and cracks are assessed on their longest dimension and against the zone they fall in. A crack that starts small on the passenger side and creeps toward the driver’s side can change a car’s MOT status overnight, because once it enters Zone A the 10mm limit applies to it. Windscreen damage sits alongside the bigger mechanical failure points that catch drivers out each year, and our coverage of the most common reasons cars fail the MOT shows how often avoidable faults turn into a fail.

Why a small chip is not just an MOT problem

An MOT failure does not by itself create a criminal offence. The MOT is a snapshot of roadworthiness on the day of the test. The bigger risk is the separate offence of using a vehicle in a dangerous condition, which applies regardless of when your MOT is due. Under section 40A of the Road Traffic Act 1988, driving a vehicle in a condition that involves a danger of injury is an offence, and a windscreen so damaged that it obstructs the driver’s view can fall under it.

If the police judge that a damaged windscreen makes a car dangerous to drive, the usual outcome is a fixed penalty of 100 pounds and three penalty points, recorded under endorsement code CU20, which covers using a vehicle in a dangerous condition. If a case goes to court and the vehicle is deemed genuinely dangerous, the maximum penalty rises to a fine of up to 2,500 pounds, three points or even a driving ban. There is an insurance dimension too, because driving with damage the insurer considers dangerous could complicate or invalidate a claim if you are involved in an accident. The same broad principle covers anything that blocks the driver’s view, which is why the law also limits heavily tinted front windscreens and side windows, and why a row of stickers or a deep sun strip across the top of the glass can attract attention too.

How to stop a chip becoming a crack

The cheapest moment to deal with windscreen damage is the day it happens. A small chip can usually be repaired with resin for a modest sum, and many comprehensive insurance policies cover chip repair with no excess and without affecting your no claims discount, because a repair is far cheaper for the insurer than a full replacement. Leave the chip and temperature swings, vibration and water getting into the damage can turn it into a spreading crack that needs the whole screen replaced.

Summer makes that progression more likely, not less. A windscreen sitting in direct sun expands, and a sudden cold blast from the air conditioning or a rain shower causes it to contract, stressing the glass around any existing chip. The practical steps are simple: get chips repaired quickly rather than waiting, check your policy to see whether glass cover is included and whether repairs are excess free, avoid blasting cold air straight onto a hot screen, and keep wiper blades in good order so grit is not dragged across the glass. If a crack has already entered Zone A or is larger than the limits, book a replacement rather than risk a fail or a roadside stop.

What it costs and what happens next

A resin chip repair typically costs in the region of 30 to 80 pounds if you pay for it yourself, and nothing at all on many policies that include glass cover. A full windscreen replacement is a different order of expense, often several hundred pounds and considerably more on a modern car, because screens increasingly carry cameras and sensors for driver assistance systems that have to be recalibrated after fitting. That recalibration is one reason replacement costs have climbed, and it is another argument for catching damage early while a cheap repair is still possible.

The rules themselves are stable, and there is no sign of the zone thresholds changing. What is changing is the cost of getting it wrong, as more cars rely on windscreen mounted technology and replacement bills rise. With the government having stepped back from earlier plans to stretch MOT intervals, as we reported when the four year first MOT idea was scrapped, the annual test remains the main backstop that catches a creeping crack before it becomes a danger. Knowing where Zone A sits, and acting on a chip the day it appears, is the easiest way to keep a small piece of grit from turning into a failed MOT or a fine. It is also worth a quick monthly glance at the glass directly in front of you, because catching a chip early, before heat and vibration spread it into Zone A, is the single cheapest piece of car maintenance most drivers will ever do.

Sources:

• https://www.rac.co.uk/drive/advice/car-maintenance/how-to-deal-with-damaged-windscreens/
• https://www.lawble.co.uk/can-you-drive-with-a-cracked-windscreen/
• https://www.gov.uk/government/publications/mot-inspection-manual-for-private-passenger-and-light-commercial-vehicles
• https://www.legislation.gov.uk/ukpga/1988/52/section/40A