Some types of windshield damage are quite easily repaired by a professional repair shop, but other types are trickier and some are even considered unsafe even after repair, which means that it's a better idea to just discard the windshield if those types of damage appear. Here are three things that may make your windshield a better bet for replacement than for repair.
1. Cracks have reached the edge of the windshield
The structural integrity of the windshield is most likely gone for good once cracks have reached the edge area. Consequently, it's recommended that you have your windshield replaced as soon as any cracks appear there. Of course, if the crack started somewhere else in the windshield, it can still spread to a vital area such as the edge if you don't get it repaired at once. The vibrations and jolting movements of a car in motion can cause cracks to gradually worsen (or more suddenly, depending on the crack).
2. Cracks or chips are in the driver's direct line of sight
This is another safety issue. Having a crack in the driver's line of vision doesn't damage the structural integrity of the windshield beyond repair (although it does damage it for as long as the crack remains unrepaired. Cracks can actually make your windshield less likely to hold together in the event of a collision, which prevents the airbags from working properly since they're designed to fill the space between the windshield and the people in the seats). However, it can decrease the effectiveness of the driver's field of vision. This is because, after a glass repair, there may be a slight smudge or cast to the glass in that area, depending on the situation, and if this happens right in the driver's line of sight, it could mean that the driver is fractionally slower to detect critical information about the road ahead, which could mean the difference between life or death in a high-speed collision incident. So if your windshield has a crack or chip right in the driver's line of sight, it's recommended that you simply replace the windshield.
3. You tried a DIY repair and it didn't work
DIY repair kits, although their packaging may seem to indicate otherwise, should really only be tried at home for repairing slight scratches, rather than cracks or chips that involve a substantial portion of the glass's thickness. Cracks and chips should always be repaired by professionals. If you don't know this and try a repair at home, it's easy to mess up (especially if you have no experience). Unfortunately, the resins used for glass repair allow no do-overs. If you take your windshield to a repair shop after a DIY job gone wrong, they may try to drill out the dried resin and replace it, but the chances of success are lower than they'd be if you'd simply brought the chip to them in the first place. If your repair attempt has gone very badly wrong, you may end up needing to replace the whole windshield.
These are three reasons why you may need windshield replacement instead of repair. Be sure to take your repair jobs to auto glass repair professionals every time, and you can easily avoid at least one of these situations.