Blistered roof shingles are a cosmetic defect and an indicator of reduced shingle life. Blistering shingles are a common problem with asphalt shingles. There are two main reasons that asphalt shingles blister. If the shingles on a roof have trapped moisture, they are highly likely to start blistering. As the air heats up the shingles the trapped moisture heats and can cause blisters to rise up on the shingle. Roofs that do not have an adequate ventilation system is another cause of blistering. When the heat builds up in an attic it then transfers through the plywood decking and literally bakes the shingles from the inside out.
Blister damage from Hail damage
Distinguishing blister damage from hail damage can be an issue when looking to file an insurance claim. A properly trained and factory certified roofing contractor can often tell the difference. When going this route, it is always recommended to look for someone who holds a HAAG certification that is trained to inspect roofs from a forensic view point. Many times fly by night companies will try to pass blistering off as hail damage as a way to force an insurance claim because they have no other way of generating business. Buyer be ware as they say.
One of the ways to distinguish the differences is by paying close attention to the damage pattern on the roof. Is the damage scattered? Is the damage only on one slope? Take close observation to the area, the slope, and overall direction of the damaged shingles.
Wind direction during the hail storm will damage sides facing that direction. If you see consistent pitting on your shingles on all roof faces, chances are it is not hail damage. Remember, storms are directional and do not drive from winds blowing in all directions at the same time.
Again, when facing these issues, always consult with a local and certified roofing contractor. Reputation and longevity are always worth the investment.