Mastering Roof Inspections: Asphalt Composition Shingles, Part 35

by Kenton Shepard and Nick Gromicko, CMI®

Sometimes, problems occur during the manufacturing process that affect the way shingles look or perform. Defects that can cause poor performance or shorten the shingles' lifespan may not be apparent right after installation. When defects in the manufacturing process create areas of weakness, weathering will cause these areas of the shingles to deteriorate more quickly.

Blister Pops

In the past, shingle ingredients were added manually, and mistakes were sometimes made that caused gasses from moisture, poor asphalt mix, or incomplete curing of the fiberglass mat to migrate toward the surface of the shingle where a bubble-like blister was created. Eventually, the bubble cap eroded away, leaving an exposed pockmark.

Nowadays, ingredients are metered and mixed automatically using computers and automated equipment, so, while incorrect mixtures are not impossible, they are rare.

Blisters are now usually formed when volatiles flash out of the asphalt in newly installed shingles. This is caused by excessive heat from poorly ventilated and over-heated roof systems.

Blisters may shorten the service life of shingles. If the problem is widespread, the entire slope may need to be replaced sooner than expected. If blisters are limited to a few shingles, it may be possible to ignore the damage.

Since each course consists of two layers of shingles, blisters will not evolve into leaks, but they may cause premature deterioration of the appearance of the roof, if they’re large and widespread.

Can you tell why this is not a blister?

The shingle has been fractured, but the granules are still in place.  Blisters lose their granules when the cap erodes away. This has been caused by an impact.

Looking at these two photos, you can easily see the difference between…

…the blister damage…

…and the hail damage.

The mat is visible in the blister, and granules remain in the hailstrike. You will encounter situations in which a hailstrike has removed most of the granules in a damaged area.  But if you look closely, a blister does not resemble hail damage.

Blisters look like pockmarks and are characterized by a loss of asphalt, sometimes to the extent that you can see the mat, as in this photo. The mat is visible in these close-ups.

You don’t need to identify the cause of blistering.  Just be able to identify it reliably so that you can comment on it accurately in your report.

Spliced Shingles