Blistered asphalt roof shingles may be mistaken for storm damage and vice-versa. Asphalt Shingle Blister Rash
Cause & Effects of Asphalt Roof Shingle Blistering & Blister Rash vs. Storm or Hail Damage

  • BLISTERS on ASPHALT SHINGLES - CONTENTS: Asphalt roof shingle blisters and splices, photographs. Are asphalt shingle rash blisters cosmetic, aesthetic, or a defect? Does rash blistering affect shingle life or shingle warranty? How to distinguish asphalt roof shingle rash blisters from hail or other storm damage
  • POST a QUESTION or READ FAQs about blistering & hail damage on roof shingles.

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Asphalt shingle blister rash: definition, causes, impact on roof life: this article explains how to identify & explain the causes of asphalt roof shingle blistering or "blister rash" on asphalt roofing. Storm damage from wind and hail occur and need to be and can be distinguished from defective (or cosmetically-defective) asphalt shingle product or asphalt shingle installation errors.

By listing common causes of asphalt roof shingle failures and how to recognize them, building owners and roofing contractors may also be able to reduce the occurrence of asphalt roof shingle storage, handling, and installation errors that affect roof life.

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Asphalt Shingle Blisters - Are Shingle Rash Blisters Cosmetic?

Blisters in shingles or "rash blisters" are a cosmetic defect in the opinion of some roofing manufacturers and an indicator of reduced shingle life in the opinion of some building professionals. In the photo shown here, some blister tops have lost granules and are beginning to expose the shingle interior substrate. These Atlas™ roof shingles are less than one year old.

OPINION: Few roof defects, cosmetic or more serious, have been subject to more arm-waving and speculation than "blister rash". Yet the science underpinning the understanding of blisters in asphalt shingles, similar to the explanation of solvent blisters in some paint failures, is quite clear, as we explain in this article.

Building professionals, contractors, and homeowners enmeshed in debate and speculation about shingle blister rash can help sort through the claims and blames by resorting to ... well heck: the experts, science, and physical evidence - sources of data that underpin all good building science and investigation.

Rash blistering considered cosmetic: Atlas roofing has offered reassurance to their customers by indicating that rash blistering is an aesthetic characteristic only.

Atlas roofing does not classify blistering as a manufacturing defect. The company has said that rash blistering will not affect the intended performance or life of the shingles and that the shingle warranty will not be affected.

Shingle blistering as a roof defect: although the Atlas view is reassuring, based on field observation and experience (and visible in beginning stages in the photo of an Atlas™ roof shingle above), we've seen early granule loss at the blister site. So at least on some roof shingles, rash blisters may indeed be an indication of a reduced-life expectancy.

Some roof inspectors, home inspectors and roofing contractors have observed a shingle wear pattern characteristic of granule loss at the shingle blister sites. Mineral granules at the raised portion of each rash blister can wear off from weather or foot traffic, becoming pits which expose the underlying shingle substrate.

Exposed granule-loss pits on roof shingles increase moisture absorption into the shingle body and in cold climates increase in wear rate from frost. This becomes more apparent when inspecting an older pitted asphalt roof shingle.

Causes of Shingle Blisters & Blister Rash on Asphalt Roof Products

Uniform asphalt sihngle blister rash (C) Daniel FriedmanDetails about the cause of asphalt shingle blister rash or roll roofing blisters along with authoritative citations are found

Rash blisters on asphalt shingles result from the manufacturing process, (and may be cosmetic or possibly a more serious defect) which are sometimes mistaken for hail damage or other types of asphalt shingle roof wear or damage indicators.

The most likely causes of asphalt shingle blisters or "blister rash" visible in product right from the factory is the expansion in gas form of either moisture trapped in the shingle substrate at the time of manufacture or gases from volatile organics or resins used in the shingle construction. Either of these can produce trapped gas bubbles as the shingles are exposed to high temperatures during production.

Shingle blisters might be caused as well by excessive use of roofing mastic or additional adhesives that are applied during or after roof installation. A warning to this effect issued by GAF Materials Corporation is found

In such a case of adhesive-caused blister rash, the blisters ought to appear in a pattern that matches the blobs of adhesive, not in the regular pattern such as shown in our photo at above left (click to enlarge).

Is Asphalt Shingle Blister Rash More than a Cosmetic Concern? Is Roof Life Affected?

Blistered IKO roof shingles (C) Daniel FriedmanThis asphalt roof shingle blister pattern is classic "cosmetic shingle blistering" by usual manufacturer description.

Reader Marcia Reid provided these photographs of blisters on an 8 1/2 year old IKO Asphalt shingle roof in Saskatoon, Saskatchewan, Canada. The shingle manufacturer's local representative opined that the blisters were due to tree overhang debris or bird droppings.

What the rep told the owner sounded silly to everyone else: the blistering pattern is just too uniform to be due to a tree or droppings.

Besides, as this shingle blister rash photo shows, we don't have much in the way of overhanging tree branches at this home.

It is true that we do indeed look at differences in shingle exposure (sun vs shade for example) to explain differences in roof wear. The owner reports that the blistering appears only on the rear slope of the home. We speculated that the blistering shingles were from a common pallet of shingle bundles, perhaps a different one from those used on the front slope.

Unfortunately, on some roofing shingles asphalt shingle bumps or blisters that may appear early in the life of the product or may even be present when the bundle is unwrapped sometimes convert into wear pits when the tops of each blister give up their mineral granules to the weathering process before the remaining area of the shingle. Our photo above shows the beginning of this process.

Each asphalt shingle blister that becomes an asphalt shingle "pit" has exposed the asphalt substrate or mat of the shingle. Where the protective mineral granules are lost from roof shingles wear accelerates and the remaining life of the product shortens at an increasing rate as the shingle begins to absorb water and suffer more in freeze-thaw cycles than before.

This IKO® shingle (photo, above left), (in April 2009) at about 8 1/2 years old, was about 1/3 through the rated life of a 25-year shingle. But if pits and exposure of the shingle substrate appear soon, then the remaining life of this asphalt shingle roof may be less than the rated period. Otherwise, indeed this will have proven to be only a cosmetic concern. We will include future roof condition reports here.

The two photographs of shingle blister rash shown below were provided by reader Jim Todd, a homeowner who commented that both blisters and pitting (where the blister top has ruptured) on these four year old Atlas Chalet™asphalt roof shingles.

Asphalt shingle blisters on Atlas Chalet shingles 4 years old (C) Daniel Friedman Jim todd Asphalt shingle blisters on Atlas Chalet shingles 4 years old (C) Daniel Friedman Jim todd

At left, oblique sunlight shows blister rash on these asphalt Atlas brand roof shingles. The photo at above right provides a closeup of ruptured shingle blisters, exposing the asphalt shingle mat substrate. Mr. Todd described this blistering pattern as common on several hundred roofs that he has observed, located in Statham Georgia. Close ups of the photo at right are also found

Atlas roofing is to be commended for attention client satisfaction: While the reader above reported dissatisfaction with Atlas, in an earlier client letter from Atlas to a concerned homeowner the company and provided to us by that homeowner, Atlas also stated that "in the unlikely event that the rash blistering should negatively affect the intended shingle performance regarding weather protection of the roof, please contact Atlas and we will further evaluate the roof."

So is it aesthetic or not? Perhaps the company is saying "time will tell, But we will stand behind our product warranty."
See ROOFING WARRANTIES for general comments on that topic.

Just how much shingle life reduction can be attributed to rash blistering will doubtless remain a debated topic since many factors enter into asphalt shingle life (sun orientation, weather exposure, shingle color, roof slope, nailing, venting, and material quality). OPINION-DF: we figure that the rated life of the shingle is going to be reduced by 5% to 15% based on field observations.

How to Distinguish Hail Damage from Asphalt Shingle Blister Damage - relationship of hail to blister damage on roofs

Reader Question: could hail damage to roof shingles lead to later shingle rash or shingle blisters?

Thank you for your web site and all the information you provide!!! I have done some research on hail damage and blistering.

I was wondering if it would be reasonable to suggest that hail damagecould be a cause of blistering? Seems blistering requires some sort of initiating cause such as manufacture defect or moisture. And, appears some hail damage is quite small, only the removal of few granules and possible underlying asphalt leaving behind a small void. Could moisture then get absorbed into the shingle? And later cause blistering?


I have lived in both AZ and N. TX. Hail occurs more frequently in TX. And, roof in AZ undoubtedly get hotter than in N. TX. But I didn't find that blistering (of asphalt shingles) was much of a problem in AZ. The examples found seem to be in areas which are more prone to hail. So, this is pushing me to believe that hail could be causing the blistering in many cases. What do you think? - B.S.


I have not thought about hail damage as causing shingle blistering since that is a different phenomenon. But you raise an interesting question about the the relationship between blistered asphalt roof shingles and hail damage.

My OPINION supported by technical research cited
at ASPHALT SHINGLE BLISTER RASH CAUSE has been that blistering or shingle rash starts as a manufacturing artifact, a bumpy shingle surface that is caused by the manufacturing process itself - perhaps the shingle granules are clustered or stuck together, or perhaps there are temperature variations that cause bubbles or just lumps in the asphalt on the shingle substrate to which the granules are applied. But I do not know - and I bet the manufacturers know exactly how this is happening.

There is no doubt that we see blistered or rashed shingles that come out of the bundle, new, from the factory in that condition. And manufacturers generally opine that it's a cosmetic only condition - a viewpoint with which I do not always agree.

What's the Difference Between Hail Damage and Asphalt Shingle Blister Damage?

But you raise the interesting question of whether or not hail impact could cause a more subtle damage to shingles that leads to a second type or source of shingle blistering. We discuss hail damage to roof shingles in detail at HAIL DAMAGED SHINGLES.

That explanation doesn't match with the close-up examination I've made of some hail damaged roofs on which I saw that granules are dislodged or scoured off of the shingle surface. But I grant that an impact that leaves granules in place could have the more subtle effect you suggest: a loosening or opening of the granule surface to allow water and perhaps freezing impact on the shingle surface.

To investigate the question in a more credible and scholarly way would involve at least dissecting some hail impacted asphalt roof shingles. We'd cut the shingle in cross section and make microscopic examination of the cross section for visual evidence of changes in the material - as a start. Even more subtle effects of hail impact on the adhesive properties of the shingle surface, adhering granules to the asphalt substrate, could be present and would require a more technical, perhaps chemical analysis to observe.

Frankly I don't think the hail as previously un-recognized source of shingle blisters explanation sounds at all likely. Blisters are formed in an asphalt shingle by trapped gas bubbles in a very hot substrate. Conversely, there is sufficient (size, mass, velocity, angle of impact) hail damage to an asphalt shingle roof that was already blistered, I would expect the blisters to play a role in the subsequent hail damage or roof wear that would be observed. But it's worth taking a closer look at your hypothesis by examining some representative shingles microscopically and by asking shingle manufacturers for their opinion.

Significantly, on an already blistered roof shingle, it is plausible that subsequent exposure to hail might indeed cause the blisters on the asphalt shingle to open. So in addition to the roof shingle granule loss caused by hail storms we might find open pits where mineral granules were lost more at the raised blister tops on a shingle than on the flatter and better-adhered shingle surface areas.

How to Extend the Life of an Asphalt Roof with Shingle Blisters or Shingle Rash

OPINION: you may be able to extend the life of any asphalt roof, including one that has shingle blisters or rash by

  • Keep foot traffic on the roof to a minimum. Walking on shingles in hot weather when they are soft easily damages them, especially if you "turn" with weight on your shoe, twisting the shingle surface. Walking on shingles that are blistered, cupped, or curled, or simply brittle with age is likely to damage them. We have tracked the footsteps of some roof inspectors by observing where roof shingle blisters have become wear pits and craters.
  • Make sure that your roof is properly vented - a cooler roof lasts longer. If your under-roof ventilation is inadequate, correcting those conditions will extend roof life.

  • Proper roof maintenance: in addition to inadequate roof ventilation, repair any roof leaks promptly, keep debris
    (Debris Staining on Roofs), moss, and lichens off of roofs
    (MOSS & LICHENS on SHINGLES), and inspect your roof annually for damage that may need additional repair.
  • Proper roof installation: once your roof has been installed it's a bit late to "make sure that the roof was installed according to the manufacturer's specifications as well as industry best practices such as nailing pattern and location, proper fasteners, sound roof decking, and shingle layout pattern. Next time.


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