Bumps on shingles

[Tim H]

This is a new one to me. Asphalt shingles, 13 yr old roof. There are small (1/4"-3/8")bumps on some of the shingles. They are primarily on the darker colored shingles (which has me thinking manufacturers defect), and occur on all sides of the roof. I do not see any damage or break through on the back side of the shingle. There are micro cracks on the surface of the bumps. Obviously not a good condition, but I was wondering if anyone has seen this before and could comment on the cause. Thanks - Tim

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[Bill Kibbel]

...I was wondering if anyone has seen this before and could comment on the cause.

They're asphalt blisters (don't scratch, it'll spread and get infected).

Air bubbles were present in the asphalt that was applied to the mat. When exposed to the sun on a roof, the tiny bubbles expand into the blisters. My personal experience is that it doesn't significantly reduce the life of the roof, unless traffic or other abrasion has punctured the blisters.

[Terence McCann]

Originally posted by inspecthistoric

They're asphalt blisters (don't scratch, it'll spread and get infected).

For mild to sever cases of roof itch.

[Tim H]

Originally posted by Terence McCann

Originally posted by inspecthistoric

They're asphalt blisters (don't scratch, it'll spread and get infected).

For mild to sever cases of roof itch.

LOL. A little too much time on your hands on a Saturday, Terry?

[Jim Katen]

Originally posted by inspecthistoric

...I was wondering if anyone has seen this before and could comment on the cause.

They're asphalt blisters (don't scratch, it'll spread and get infected).

Air bubbles were present in the asphalt that was applied to the mat. When exposed to the sun on a roof, the tiny bubbles expand into the blisters. My personal experience is that it doesn't significantly reduce the life of the roof, unless traffic or other abrasion has punctured the blisters.

I agree with Bill. They're known as rash blisters and they're generally harmless. Even if they shear off from foot traffic, I haven't ever seen them cause a problem. My sense is that the shingles will fail from something else long before the blisters come into play.

- Jim Katen, Oregon

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[Rob Amaral]

Ditto to the above responses. I usually see this coupled with a general lack of ventilation as well.