A flat tire is an inconvenience, but it doesn't necessarily mean it is time for a costly tire replacement. In many cases, you can simply have the tire patched and repaired.
1. Damage Location
One of the most important factors that immediately impact whether a tire can be repaired is where the damage is located. Damage located anywhere in the tread, which is the part of the tire that comes in direct contact with the ground, can be repaired if no other factors are impacting the tire. Any punctures in the sidewalls of the tire, whether it's the outer or inner facing sidewall, cannot be repaired and you will need to get a new tire.
2. Puncture Size
Even a clean puncture right through the tread isn't always repairable. Generally, a puncture needs to be small enough to be patched properly, which means smaller than about a quarter of an inch. In other words, a puncture from a screw or nail is repairable, but one caused by a railroad spike will mean it is time to replace the tire.
3. Tire Condition
Just because a puncture can be repaired doesn't mean that it should be repaired. Replacement is a better option if the tire is old and worn with tread depth well below the safe level. Your repair service may also decline to make a repair if there is damage unrelated to the puncture, such as bubbles on the sidewall. This is because the tires are no longer safe to drive on even if the puncture is repaired, and no shop wants that liability.
4. Post-Flat Driving
The quickest way to make a tire with a simple puncture unrepairable is to drive on the flat tire. It's okay to drive onto the shoulder a few yards to change the tire, but attempting to drive home or to a repair shop will crease the sidewalls and cause permanent damage to the tire structure — issues that can't be repaired. Always carry a spare and a jack so you can change out a tire as soon as it goes flat.
5. Tire Type
There are certain types of tires that can't be repaired. Generally, these are advertised as "run-flat" tires. Run-flat tires are designed so that they can be safely driven on when flat, which allows you to travel to a shop or a safe area to make a tire change. Unfortunately, the same design that makes them safe to drive on when flat also makes it impossible to install an effective repair patch.
Contact a tire repair service if you have a flat tire that needs to be assessed for repairs.