Ideally, windshield damage is minor so you can opt to have it repaired. Unfortunately, not all types of damage meet this standard, so it's important to know when replacement is the better option.
1. Large Cracks
Small chips and cracks are usually repairable, which means you won't need a full replacement. Large cracks are defined as those more than 3 inches or so in length or if there are multiple cracks such as those that spiderweb out from a chip in the glass. Large cracks usually can't be patched and repaired, so you will need to get a whole new windshield.
2. Leaking Water
If your window is leaking water around the edges, then the seal is failing. A failed seal around the windshield isn't just an inconvenience, it is also a safety risk. Without a proper seal, a windshield is more likely to shatter or even fall out in the event of an accident — even a small accident. Cracks will also become larger much more quickly if damage does occur to an improperly sealed window, so replacement is necessary.
3. Sight Line Issues
Cracks or large chips that occur directly in front of the driver may not be repairable. The issue is that even a full repair is not completely invisible. Light may refract differently through the repaired glass, which can affect visibility and cause distortion. If any type of windshield damage occurs directly in the driver's sightline, the best option is to completely replace the glass.
4. Rattling Glass
Rattling glass is caused by an issue similar to a water leak — the seal is no longer secure around the perimeter of the windshield. If you have cracks in the glass, the problem is even more severe since the window is more likely to shatter due to rattling when the glass is already damaged. If your glass is rattling at all, it's time to take it in to see if it can be repaired or if replacement is the only safe option.
5. Edge Encroachment
Cracks or chips that occur within a couple of inches of the windshield edge often can't be safely repaired. There are a couple of reasons for this. First and foremost, chances are the seal around the windshield has been compromised by the proximity of the glass damage. Further, damages near the edge of the windshield are more likely to grow and worsen over time, as repairs don't hold well in this area.
Contact a windshield replacement service for more assistance.