Key takeaways
- Sidewall damage — cuts, bulges, cracks or impact bubbles — cannot be safely repaired and means the tyre must be replaced.
- A bulge means the internal cords have already broken; the tyre can fail without warning at speed, so do not keep driving on it.
- British Standard BS AU 159 allows puncture repairs only within the central three-quarters (the tread), never in the sidewall or shoulder.
- Kerbs, potholes and under-inflation are the most common causes of sidewall damage in UK driving.
- If a sidewall is damaged, stop and arrange a replacement rather than risk a blowout.
The sidewall is the smooth area of a tyre between the tread and the wheel rim, and it does a huge amount of work flexing thousands of times a minute while carrying the weight of your car. Damage here is a different problem from a nail in the tread. This guide explains what sidewall damage looks like, why it cannot be repaired, and what to do safely.
Can sidewall damage be repaired?
No. Sidewall damage cannot be safely or legally repaired, and a tyre with a cut, bulge or crack in the sidewall must be replaced. The British Standard for tyre repairs, BS AU 159, only permits repairs within the central three-quarters of the tread. The sidewall flexes constantly and any patch or plug there will not hold.
This is the single most important fact to take away: a reputable fitter will refuse to repair a sidewall, not to sell you a tyre, but because the repair would be unsafe and would not pass the standard the DVSA recognises.
What does sidewall damage look like?
Sidewall damage shows up as a visible bulge, a cut, a deep crack, or a scuff that exposes the cords underneath. The most serious is a bulge — a bubble in the rubber — which means the internal structure has already been compromised. Run your hand gently around the sidewall and look in good light.
- Bulge or bubble — air has pushed through broken internal cords; replace immediately.
- Cuts or gashes — anything deep enough to reach the fabric layers is unsafe.
- Cracking or crazing — fine cracks from age or perishing weaken the rubber.
- Kerb scuffs — light cosmetic scuffing is usually fine; deep gouges are not.
- Exposed cords — any visible fabric or steel means the tyre is finished.
What causes sidewall damage?
Most sidewall damage comes from impacts and from running tyres under-inflated. The sidewall is the thinnest, most flexible part of the tyre, so a sharp blow concentrates force exactly where the tyre is weakest. In UK driving, kerbs and potholes are the usual culprits, and tired roads make it worse.
| Cause | How it damages the sidewall |
|---|---|
| Hitting a pothole | Pinches the sidewall against the rim, snapping internal cords |
| Kerbing the wheel | Scrapes and gouges the rubber, sometimes to the cords |
| Under-inflation | Excess flexing overheats and fatigues the sidewall |
| Age and perishing | UV and time cause cracking, especially on little-used cars |
| Overloading | Carrying more than the rated load stresses the sidewall |
Is it safe to drive with sidewall damage?
No. Driving on a tyre with a bulge or deep cut is dangerous because the damaged area can fail suddenly, especially at motorway speed when the sidewall is hottest and flexing most. A blowout at speed can cause loss of control. If you spot a bulge, treat the tyre as unroadworthy and stop using it.
There is also a legal angle. A tyre with a cut deep enough to reach the ply or cord, or with a lump or bulge from separation of its structure, is a specific MOT failure and an offence to use on the road, according to the DVSA MOT testing manual. So it is both a safety risk and illegal.
Sidewall damage versus a repairable puncture
The key difference is location. A puncture in the central tread area, from a typical nail or screw, is often repairable to the proper standard. The same object in the sidewall or shoulder is not. Knowing which you have saves a wasted trip and tells you whether to expect a repair or a replacement.
If you are not sure where the damage is, our guide to whether a puncture can be repaired walks through the rules in more detail, and a nail in the tyre is a common repairable case worth checking first.
How to avoid sidewall damage
You cannot avoid every pothole, but most sidewall damage is preventable with a few habits. The biggest single factor is keeping your tyres at the right pressure, because an under-inflated tyre flexes far more and is much more vulnerable when it does meet a kerb or a pothole edge.
- Check pressures monthly and before long trips — see our guide on how to check and set tyre pressure.
- Slow down for potholes you cannot avoid; braking hard over them makes the impact worse.
- Park clear of kerbs and turn the wheel gently when manoeuvring, so you do not scrub the sidewall.
- Do not overload the car beyond its rated capacity, which over-stresses the sidewalls.
- Inspect after a big impact — a hard pothole or kerb strike deserves a look at both wheels on that side.
Do you need to replace one tyre or two?
If a sidewall is damaged you must replace that tyre, but you may want to think about its partner on the same axle. If the other tyre on that axle is significantly more worn, replacing in pairs keeps grip balanced left to right, which matters most in the wet and under braking. A fitter will measure and advise honestly.
There is no legal requirement to replace in pairs, and if the other tyre is fairly new and in good shape, replacing just the damaged one is usually fine. The deciding factor is the difference in tread depth across the axle, not a blanket rule.
What to do if you have sidewall damage
If you find a bulge or cut, stop driving on that tyre as soon as it is safe to do so. Fit your spare or space-saver if you carry one, or call for help. Because the tyre needs replacing rather than repairing, the quickest route back to the road is to have a new one fitted where you are.
Our mobile tyre fitting service brings the replacement to your home, work or the roadside across London and central England, usually within 30–60 minutes, and balances and fits it on the spot. If you are stranded with a sudden failure, our blowout guidance covers staying safe until help arrives.
Frequently asked questions
The sidewall flexes constantly as the tyre rotates and carries the car. A plug or patch there cannot bond reliably and would work loose. The British Standard BS AU 159 only allows repairs in the central tread, so any sidewall repair is unsafe and not permitted.
Yes. A bulge means the internal cords have already broken and air is pushing through the remaining rubber. There is no warning before it fails, and it can blow out at speed. Replace the tyre rather than risk driving on it, even for a short distance.
Probably not. Light cosmetic scuffing of the rubber that does not reach the underlying cords is generally fine. The tyre only needs replacing if the scuff is a deep gouge, exposes any fabric, or there is a bulge or cut. If unsure, have it checked.
Yes. A cut deep enough to reach the ply or cords, or a lump or bulge caused by structural separation, is a specific MOT failure under the DVSA testing manual. It is also an offence to use such a tyre on the road, so it should be replaced before driving.
It is not advisable. A damaged sidewall can fail without warning, and even a short trip at low speed carries real risk. It is safer and often quicker to have a replacement brought to you so the damaged tyre never goes back on the road.