Blisters on flat roofs often look like a small low bubble (similar to a blister on a body) and if stepped on may burst. Roofs may have blisters that are not visible. Blisters often develop when the moisture or gases trapped between the different layers of the roofing materials get heated up from long hot days or hot weather.
Roofers may or may not recommend doing anything to roofs that have blisters. They generally do recommend that if a blister has burst, that the area be patched.
Composition shingles and flat roofs experience blistering. Blistering does not necessarily mean that the roof will leak, but these area should be maintained. The blistering may cause some granular loss to the shingles in small circular areas on the shingle, which affects the longevity of the roof.
If an attic has poor ventilation, at times it can cause a higher temperature level. This heat can contribute to blistering. Improving attic ventilation may reduce the blistering and improving the attic ventilation usually improves the health of the roof. It will usually last longer.
Who to consult? Roofing contractor.
Generally blistering is a maintenance item for a roof and roofers do the maintenance on these roofs. In some circumstances a home owner may do limited maintenance items.