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This article explains how to identify & explain 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 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. Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman. Asphalt Shingle Blisters - Are Shingle Rash Blisters Cosmetic?
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 ProductsRash 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. Solvent-caused shingle blistering? 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 at WIND DAMAGE to ROOFS. Cosmetic Asphalt Shingle Blister Rash?
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 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.
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 at HAIL DAMAGED SHINGLES. 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 Extend the Life of an Asphalt Roof with Shingle Blisters or Shingle RashOPINION: you may be able to extend the life of any asphalt roof, including one that has shingle blisters or rash by
Also see GRANULE LOSS from SHINGLES (all causes of granule loss) and see HAIL DAMAGED SHINGLES where we describe how we distinguish between blister damage and hail damage to asphalt roof shingles. Readers are also invited contribute roof failure information to the web author for research purposes. web author for research purposes. Questions & Answers about blistering & hail damage on roof shinglesQuestion: 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 damage [see HAIL DAMAGED SHINGLES] could 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. Reply:I have not thought about hail damage as causing blistering but you raise an interesting question about the possible causes of that shingle anomaly. My OPINION 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. 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. 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 very likely. 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. Let's pursue it. - Editor Frequently Asked Questions (FAQs) about the cause, cure, or prevention of asphalt shingle roof blisters & the effects of shingle blisters on roof life.... Ask a Question or Search InspectApediaQuestions & answers or comments about the cause, cure, or prevention of asphalt shingle roof blisters & the effects of shingle blisters on roof life. . Ask a Question or Enter Search Terms in the InspectApedia search box just below. Technical Reviewers & ReferencesRelated Topics, found near the top of this page suggest articles closely related to this one.
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