Punctures are the result of driving a car over sharp objects like screws, nails or glass which damage the tyre.
A tyre puncture causes a loss of air pressure and, depending on the extent of the damage, will either deflate slowly or rapidly. If there is complete deflation, the wheel's rim may end up riding on the ground or the tyre tread. This is dangerous.
In addition to sharp objects, several other factors could lead to a tyre puncture. A faulty or damaged valve stem may be responsible. Or the possibility that the tyre was not fitted correctly to the wheel. Alternatively, a tread that's excessively worn could result in tyre failure if debris from the road tears through the tyre.
Should one or more of your tyres be continually losing pressure, or if you discover a nail or screw embedded in the tread, visit your local mechanic immediately to have your tyres inspected. If it's not repaired soon enough, a puncture may lead to permanent damage to the tyre.
In the UK, tyre puncture repairs are subject to a British Standard called BSAUl59, which sets out limits to the repair materials which can be used and the total number of repairs that may be carried out on a single tyre. This regulation is to ensure safety for all road users.
In short, repair work is forbidden on a tyre if:
Factors like the puncture location also play a part. The same regulation stipulates that tyre repairs must stay within the central three-quarters of the tread, called the "minor repair area". And if the size of the puncture is 6 mm diameter or larger, a repair isn't permitted at all. There's no alternative but to buy a new tyre.
A puncture that falls outside the minor repair area is not eligible for mending because of its proximity to the sidewall.
The sidewall is off-limits because, when the tyre is in use, it's the part which carries the most significant load. Attempts at repair in this area have the potential to weaken the structural integrity of the tyre.
Moreover, the sidewall experiences the most flex in the tyre. Any repair patch applied here is unlikely to stay in place for long, and the tyre will fail.
While driving on a flat tyre is never a good idea, it's even more important to avoid doing so if you hope to salvage the tyre. Without the air pressure to maintain its shape while driving, the tyre sidewall will be pinched between the wheel rim and the road, extending the damage beyond the minor repair area.
In other words, if you drive on a deflated tyre while the puncture is inside the minor repair area (and repairable), the tyre sidewall may sustain additional damage that renders it completely irreparable. It would be wiser to replace it with the spare tyre until you reach a garage.