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ROOFING INSPECTION & REPAIR
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CHIMNEY FLASHING Mistakes & Leaks
COLD WEATHER ROOF TROUBLE
DECKS, ROOFTOP CONSTRUCTION
EPDM, RUBBER, PVC ROOFING
EXTRACTIVE BLEEDING on SHINGLES
FIRE RETARDANT PLYWOOD
FLASHING on BUILDINGS
FLAT ROOF MOISTURE & CONDENSATION
Green House or Solarium Roof Leaks
HEAT TAPES & CABLES on Roofs for Ice Dams
ROOF ICE DAM LEAKS
MASONITE WOODRUF FIBERBOARD ROOFING
NOISE CONTROL for ROOFS
PLASTIC ROOFING TYPES
PVC, EPDM, RUBBER ROOFING
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF CLEANING RECOMMENDATIONS
ROOF COLOR RECOMMENDATIONS
ROOF DORMER TYPES - PHOTO GUIDE
ROOF INSPECTION SAFETY & LIMITS
ROOF JOB PROBLEMS, RESOLVING
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
ROOFING TILE SHAPES & PROFILES
ROOFING UNDERLAYMENT BEST PRACTICES
SADDLE CONSTRUCTION at CHIMNEYS
SNOW GUARDS & SNOW BRAKES
STANDARDS for ROOFING
STRESS SKIN INSULATED PANELS
TEST LABS - ROOF SHINGLE
TREES & SHRUBS, TRIM OFF BUILDING
TRUSSES, Floor & Roof
UNDERLAYMENT REQUIREMENTS on ROOFS
VENTILATION in BUILDINGS
WALK-ON ROOF SURFACES
WARRANTIES for ROOF SHINGLES
WORKMANSHIP & ROOF DAMAGE
Hail damage to asphalt roofing: This article tells readers how to identify roof shingle hail damage and how to distinguish a hail damaged roof from blister rash damage, foot traffic damage, wind, and other conditions such as shingle rash blistering, cracking, and granule loss associated with normal shingle aging.
We include photographs and text that help identify different types of roof damage and wear, and we discuss up-close examination of wear areas to improve the accuracy of roof damage or wear diagnosis. We explain the role of hailstone size, direction, and velocity in roof damage from hailstorms, and we discuss the role of roof slope or pitch, orientation and other factors in roof wear.
We also include a set of codes that can be used in marking areas of wear or damage on roofs to assist in photo documentation of roof condition. The article includes a list of manufacturers of hail-damage resistant roofing products and invites questions or comments from building owners, roofing experts, insurance company adjusters and roofing manufacturers.
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How to Distinguish Wear Types on Asphalt Shingle Roofs: hail storm damage vs. blister rash vs. foot traffic wear on asphalt shingles
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.
Readers are also invited contribute roof failure information to the web author for research purposes. web author for research purposes.
Notice to insurance adjusters, building owners, roofing contractors:
We invite questions, comments, polite debate, and importantly, sharp photographs of damaged and un-damaged roof shingles to permit us to develop a free public reference library of roof conditions, damage types, and thus to assist in damage assessment.
Start by looking in your roof gutters and on the ground for large amounts of mineral granules lost from the roof. And if you are on-site during the hail storm, record the storm details: storm duration, hail size, wind direction, if you can. The photos below of mineral granules in a previously clean roof gutter, and of actual hailstones were provided by reader L.B. whose roof damage case is discussed below.
Areas of lost mineral granules will be apparent, with more severe granule loss on roof slopes facing the direction from which the hailstones fell (or blew) during the storm. Inspecting an asphalt shingle roof shortly after a hail storm, if the roof has been damaged, should show that the areas of mineral granule loss have exposed "fresh" looking shingle substrate - the asphalt impregnated shingle substrate will not yet have been weathered by sun exposure.
Hail-damaged roof shingles we've seen or which have been sent along to us as in photo form, show more of a "scouring" effect in which larger, more irregularly-shaped areas of shingle surface have lost granules (and thus have produced a shingle nearer the end of its product life than before the storm.
Worn or weathered asphalt roof shingles which are losing their mineral granules in the course of normal aging, if inspected early in the wear cycle, already show small areas of granule loss, beginning with bald areas on the shingles which may be just the diameter of a few mineral granules. As the sun and weather wear will accelerate in these "bald" spots or micro-spots, when the inspector sees a larger bald spot it will be also weathered, having developed over time as opposed to having developed suddenly during a storm.
Confounding this distinction between hail damage and shingle wear as a source of granule loss, is the wear on an older asphalt shingle roof when exposed to a hailstorm. If the roof were worn (and its mineral granules less securely attached to the shingle surface), we posit that roof will lose more granules more quickly in the hail storm than a newer surface.
The impact of hailstones on a roof depends on at least these factors:
The hailstones in your photo look as if they averaged roughly 1 cm or larger (you reported 0.88 inches or 2.2 cm).
A 1 cm hailstone has a theoretical terminal velocity of about 20 mph (9 meters/second). Larger hailstones have a much higher terminal velocity. And a ten-gram hailstone falling from 10 km (this distance is probably more than enough) has a terminal velocity of about 15 meters per second. 
What the experts cite as hailstone terminal velocity is, in my opinion, misleading because that figure does not consider the role of winds that typically occur in a storm, nor the relationship between the angle of the roof and the angle of movement of the hailstone. For example, wind-driven hail may be accelerated beyond simple terminal velocity (gravity).
In LB's "old shingle" shingle photos taken on the roof itself, most of the open pits or craters on shingles where mineral granules have been lost appear to be in relatively flat areas shingles. Mineral granules that have been lost to expose shingle substrate in the center of raised blisters will in at least some instances leave a crater around the pit edges.
Examination of the shingle pits or craters in a stereo microscope such as we use in our lab would be helpful in making this examination.
Asphalt shingle blistering or rash blisters or other visual anomalies on a roof surface versus visual evidence of asphalt shingle hail damage can be tricky to distinguish.
Some owners and some roof inspectors who have not seen various types of roof damage may have difficulty distinguishing between blistering, thermal splitting, age cracking, general product wear and granule loss, and other markings on asphalt roof shingles due to specifically hail, ice, or other storm damage.
Shingle rash blisters [BLISTERS on ASPHALT SHINGLES] 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. - [Photo above /left courtesy of reader Jim Todd.]
Hailstones can be quite large, even golf-ball sized in some cases. Hail might produce a "dent" or a damage point in an asphalt shingle roof surface, resulting in granule loss and reduced remaining roof life. But I'm highly doubtful that hail ever produces raised "blisters" on the shingles such as shown in our description of shingle rash blistering.
Hail damage can dislodge the protective mineral granules of an asphalt shingle, producing areas of exposed asphalt shingle substrate. If inspecting an asphalt shingle (or mineral-granule-covered roof roofing) roof shortly after a hailstorm the exposed shingle substrate should be expected to show freshly-exposed asphalt coated or asphalt impregnated shingle base material.
If the same area is examined much later the exposed shingle areas of granule loss may have weathered or even cracked and this distinction (hail versus wear or other sources of granule loss) will be more difficult to distinguish.
Asphalt shingle blisters, are raised bumps or protrusions in shingle surface, either closed blisters or open ones showing a small black pit or crater when the protective mineral granules have been lost from the peak of the blister.
Photos of Asphalt Shingle Blisters Converting to Pitting
Asphalt roof shingle manufacturers emphasize that blisters on shingles are simply a cosmetic matter and that they do not impact the wear life of the roof nor do they impact the roof shingle warranty.
See BLISTERS on ASPHALT SHINGLES and our shingle blister photo at left.
We are not sure that everyone would agrees about the impact of blistering on shingle life nor the view that blistering cannot ever affect the roof life.
While on some roofs blistered shingles may last at least as long as the shingle warranty period., in other cases, especially if the blistered roof is exposed to mechanical damage (being walked-on, for example) or other challenging exposure conditions, the blisters may play a role in roof wear and roof life, as we elaborate below.
In at least some cases, blistered asphalt shingles are more easily converted to pitted shingles by foot traffic, hail, and in some cases during the course of aging but before the end of the shingle warranty period.
Our photo (left) shows blisters on an Atlas brand asphalt shingle roof in Georgia. Some, circled in red [click to enlarge] show remaining blisters that are either not opened to its or are just beginning to open.
The blister-damaged Atlas brand roof shingle shown at left is not the hail-damaged Owens Corning brand asphalt shingle roof described by reader B.L. in this article.
In those cases, in our OPINION and also our EXPERIENCE, because mineral granules have been lost from the shingle surface sufficient to expose the shingle asphalt-impregnated substrate to the sun and weather, the shingle wear rate may accelerate and the shingle life may be reduced.
CONTACT us to suggest technical corrections, additions, or references on hail damage and blister damage to roofing.
Question: How Can You Distinguish Between Blister Damage and Hail Damage on an Asphalt Shingle Roof?
I notice your website attributes blistering most frequently to organic (rag based) asphalt shingles. So, I have a question for you:
My roof [on a Pennsylvania home] has 13 year old Owens Corning three tab 25 year warranty Supreme fiberglass shingles. After a severe hail storm on 29 March 2009, with wind gusts of 60+mph and .88" diameter hail, my roof sustained severe damage to the southern roof slopes.
The southern slopes were perpendicular to the incoming hail. After the storm, I had over 1/2" of granules in a brand new gutter and the southern slopes were strewn with thousands of 1/8" to 3/16" pits. The underside of the shingles did not show any cracking or indentations (as would be expected from .88" diameter hail).
I was on my roof 5 months prior to the storm and replaced 3 pipe boots on the southern roof slopes. The shingles NEVER exhibited any raised bubbles; they were always smooth and flat. My weight did not disturb anything more than a few stones from the roof surface. Prior to the storm, there was nothing on my roof that looked like the attached photo. (Please see the photo [above left, sharpened by InspectAPedia.com] - sorry it is a little blurry).
An inspector from the insurance company is saying my 13 year old roof is blistered. He showed me a photo of burst blisters, which I have to admit, looked like my damage. All the research I can find (by the way your web-site is awesome!), indicates that 13 year old fiberglass shingles don't suddenly develop a manufacturing or installation issue known as blistering. I say this is hail damage!
My question is this: have you ever seen this scenario before and can you draw any conclusions from my information? - B.L., Pennsylvania
Reply: Blisters on Asphalt Shingles,Should Be Visible In and Out of the Hail Damage Area & on All Slopes Regardless of Roof Orientation & Direction of Hailstones if They Are or Were Present Before the Storm
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem. That said, here are some things to consider:
Here is my reasoning on the question of distinguishing among simple hail damage, hail damage to a blistered roof, and shingle blister wear on a roof:
B.L. continues our hail damage and roof blister discussion with a do-it-yourself home test of roof shingles and more photographs of shingle granule loss.
Are you saying that the photos look like blistering?
I did some testing over the weekend. I took brand new shingles and some from my unaffected front roof. I mounted them to a piece of roof sheathing, added felt paper, and overlapped the shingles as they would be installed in the field.
I then tried to simulate the weather they were exposed to before the hailstorm. First I put the assemblies in the oven and heated them to 100F (checked with a calibrated laser sensor) to simulate the roof temperature with an outdoor ambient air temperature of 70F.
[Photo below left: closeup of shingle condition on the roof following a hail storm. Photo below right, closeup of shingle mineral granule loss after B.L.'s home oven test.]
Watch out: readers should keep in mind that cooking potentially combustible materials in a home oven is a potential fire hazard.
I next set the assemblies outside for 45 minutes in the 50F air temperature to simulate the temperature drop before the hailstorm. The temperature of the shingles dropped to around 65F. I next sprayed the assemblies with cold water from the hose to simulate the ice cold rain. The shingle temperatures dropped to around 45F. I next threw 1" diameter ice cubes at the assemblies and then washed them down with another spray of water from the hose.
Interestingly, the ice impacts did not imbed granules into the asphalt. Some granules broke away from the surface of the shingles taking the asphalt with them. These small spots revealed the fiberglass mat. My assumption is that the coldness of the rain and ambient air temperature made the asphalt hard and brittle versus soft and pliable. When the hail hit the roof it separated the hard asphalt from the fiberglass. Then the stones just washed away with the asphalt still intact.
I have attached photographs of the test results [see above]. I think the damage looks the same on both old and new shingles (but it is more severe on the old shingles that were about half way through their service life). Unfortunately I could not simulate hundreds of hits in the same area for about a half an hour.
My test had very few impacts because it was limited to the amount of ice I had available. I think if I could have hit some of the same areas that lost the stones already, I think I would have been able to replicate the hail damage sustained to my roof on March 29, 2009.
Reply: The Account Supports Hail Damage, But Asphalt Shingle Blisters, Even Hard-to-See Blisters may Increase Shingle Hail Damage Vulnerability
No, I'm not saying that the primary cause of your roof shingle granule loss is due to blistering alone. Your account of the before and after roof inspection and description of the damage (no visible granule loss before storm, lots of granules lost after storm) very strongly supports your claim of hail damage.
Here we are discussing the role that pre-existing and not easily-visible roof blisters might play in hail damage on a roof.
However I reported that one of your photos shows what appears to be slight blistering and one or more blisters that have only slight opening or top punctures.
Therefore I pose that there may be some blisters on shingles that were not obvious to you - you can check best using oblique lighting as I said earlier.
Nevertheless, your account points to hail damage. The relationship of hail damage to blisters is, in my opinion, derived from the premise that if an asphalt roof has shingle blistering, just as I've said elsewhere that foot traffic can damage those areas, so might hail damage cause additional damage by puncturing and opening the tops of blisters earlier than would have otherwise happened due to normal aging.
The theory of relationship between preexisting blisters on a roof (possibly not showing as open craters or pits) and hail damage is based on an important observation: that the raised "bumps" are true blisters - that is that they are true raised blisters, with some hollow core and a top blister shell (to be burst by foot traffic, hail, or other mechanical damage). In contrast, apparent blistering or blister rash on shingles is comprised of similar looking bumps, but each raised area is comprised of solid material (asphalt and mineral granules, for example), those bumps may be less vulnerable to converting to open craters or pits due to mechanical wear.
About the Do-it-Yourself Roof Shingle Test for Hail Damage
I am not sure that a professional shingle test lab would necessarily like your home oven test, as there are ASTM and other roof shingle testing standards, but I agree that your photographs of your oven-tested roof shingles and the on-roof shingle damage look similar.
Certainly your home test was likely to be less stressful on the shingle granule coating than an actual hail storm that would impact the shingles repeatedly over a longer period and more uniformly over roof surfaces (depending on storm duration, wind direction, hailstone size, and the hailstone terminal velocity etc.) than your test.
You did not report the role that aging and repeated thermal heating cooling cycles have on roof shingles, nor the size nor terminal velocity of the ice you shot at your roof shingles.
CONTACT us to suggest technical corrections, additions, or references on hail damage and blister damage to roofing.
To return to the roof blister damage versus hail damage question and summarize:
Storm damage is likely to affect different roof slopes differently as their weather exposure varies.
Look closely at the above photo of an asphalt shingle roof after a hailstorm. In the photo sent to me by a reader) we see cracks in some of the shingles. We would be surprised to learn that the impact of ice pellets on a roof would produce cracking and we pose that the cracks were a preexisting condition on an older shingle roof. (I have seen cracking appear suddenly on asphalt shingles in response to cold weather, in the form of thermal splitting however, a failure for which we have a very different explanation and a different cracking pattern than shown in the photo here--DF.)
Look at the uniformity of roof defects over the field of a given slope to help understand the probable cause. Blistering of asphalt shingles caused by the product itself might appear uniform over all of the roof on all slopes independent of weather exposure.
In other cases, if only a few bundles of shingles were defective, say from improper manufacture or storage, asphalt shingle blistering may appear in shingles in a specific pattern on a roof following the application pattern of the shingles themselves as they were nailed to the roof. Since roof shingles from a single bundle are usually applied over a single area of a roof, this pattern and cause may be self-evident on close inspection of the whole roof.
While many roof inspectors and home inspectors are expected to be un-biased neutral professionals, we cannot assume that this is always the case. The building owner should consider possible sources of conflicting interests on the part of the person performing the inspection. These conflicting interests can occur in both expected and less obvious ways:
In sum, ask the roof inspector for specific details that support his or her conclusions about the condition of the roof and about what caused its damage or wear. An inspector who offers only the "bottom line conclusion" with no supporting evidence, does not deserve the confidence of the building owner.
For a professional investigation of hail damage to asphalt shingles, see "Hail Damage to Asphalt Roof Shingles", Timothy P. Marshall, Richard F. Hertzog, Scott J. Morrison, Haag Engineering, Dallas TX.
In our roof damage photo shown at left, the chalk marks made on the roof by a local inspector were intended to aid in photo-documentation of roof conditions at the time of the inspection, and the inspector opinion about the cause of wear or damage.
To permit close examination and to support those conclusions, sharp closeup macro photographs of the roof surface can often make quite clear the type of roof wear or damage at that site.
A roof inspector might also use that mark to indicate where they were going to recommend removing a shingle for testing or submission to a manufacturer.
It's SOP for inspectors to use chalk marks to indicate specific observations on a roof that are also to be photographed. Inspectors use chalk usually to mark or indicate the following
I am not aware of a national standard on roof inspection markings that specifies particular codes or symbols, but we'll continue to research this topic. CONTACT us to report other roof shingle chalk marking schemes used during roof inspections.
Other roof condition details to be photographed and recorded during a roof inspection: rulers & scale indicators
Incidentally, in my own roof inspections I will often place a ruler or other indication of scale in some photographs, and unlike the page top photo repeated just above, I also make closeup macro-photographs that indicate clearly instances of pitting, thermal splitting, mechanical damage etc. I also photograph the entire field of the roof and I prepare comparative photographs of both damaged and un-damaged roof areas and photographs that allow comparison of the condition of roof slopes facing different directions, at different pitches etc.
And a roof inspection is incomplete without a site and building inspection that identify other conditions that might effect the roof. Even a wet or flooding basement or crawl area can have a significant impact on the condition of a building roof, as that moisture moves up through the building and into the attic or roof cavity.
Those details help remove ambiguity or avoid arguments about just what was found in an up-close inspection of roof surface conditions.
Size of Roof Test Square When Reporting Hail Damage
Some inspectors suggest marking off a 10' x 10' test square area and counting the number of hits or marks ascribed to hail damage as a step in assessing the extent of hail damage to a roof. This approach has some sense to it in that it attempts to get past arm-waving speculation about the extent of damage to a roof.
But the roof damage test square approach can have some shortcomings too, depending on how it is applied. Just counting marks indicating hailstone strikes does not adequately consider the depth of damage or the degree to which the roof shingle substrate has been exposed.
And choosing an area for a test square is vulnerable to subjectivity or deliberate skewing of the report results. For example, choosing as a test square a roof slope that was by its orientation less damaged or not damaged at all compared with other roof slopes on the same building will give an inaccurate assessment of the building's roof system.
See PLASTIC ROOFING TYPES for examples of impact resistant roof materials.
Some insurance companies offer insurance premium credits for building owners who have installed damage resistant roof shingles or roofing. The Texas Department of Insurance provides information about hail-damage resistant roofing in their article "Products Qualifying for Impact Resistant Roofing Credits" where that source describes two classes of hail damage resistant roofing products:
Continue reading at GRANULE LOSS from SHINGLES
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Frequently Asked Questions (FAQs) about Roof Shingle Hail Damage & how Hail Damage is Distinguished from Blistering, Foot Traffic, or Other Roof Wear, & about Roof Inspection Procedures
Question: Roof inspection procedures: decoding common chalk marks used by roof inspectors to indicate areas of damage or wear
The photo at page top [and shown again at left] in this web article on roof damage inspections shows a circle with a line drawn thru and FT which I figure is meant to indicate "front". What does the circle with the line indicate? Appreciate your help. - R.B.
Reply: Convention for roof damage chalk marking codes
The chalk marks were made on the roof by a local inspector. In our roof damage photo shown at left and above the circle/slash was an area where the roof inspector opined there was no damage, and the "FT" marked above that spot was being used to indicate granule loss due to foot traffic.
Details about chalk codes used to mark on roofs during inspections and to indicate observations are in the article above at Convention for roof damage chalk marking codes.
Question: Expanding the definition of hail damage to include loss of roofing material - not just "dents"
One thing to look at with regards to shingle scouring is if there are any chunks of roofing material (that broke away from the area due to the hail hitting it) left on the surface of the roof after the hail storm. I do not believe that hail damage is only defined as if there was an indentation in the shingle due to the hail. The impact of the hail, in combination with a rapid temperature drop in the shingle due to the icing effect of the hail can cause the shingle to delaminte and loose parts of the shingle when impacted by a hail stone. - Martin Rizzo 6/21/11
Thanks for the helpful comments about hail damage Martin. What you say makes a lot of sense. We agree.
Question: resolution of hail damage case?
Is it known whether this man and his situation with blisters vs hail damage mentioned above ever won his case with his insurance co? I too have the same problem. - Tom McLaughlin 8/22/11
No Tom, we have not had more feedback, though the case described above was unusually well documented.
Question: Phoenix hail storm - inspector claims damage due to weather and acid from bird droppings
There was a hail storm in Phoenix, AZ, in October 2010. I was out of the country at the time. I have returned in April 2012. I am in the process of determining if there were hail damage on my roof currently. The insurance inspector claim there was no hail damage on the roof except on the AC condenser and some of the window screen. The inspector claimed the damages seen on the roof were mainly due to weather and acid from the bird dropping. Is is possible for me to send some photos to you to obtain your opinion? Thank you in advance for your time and help in this matter. - PSF Chan 5/24/12
xI'd like to see some sharp photos both of the whole roof, all slopes, and of the slope where the damage seems apparent to you, and sharp closeups of the shingle damage area details. Use the CONTACT US link found on any website page.
Question: What about other roof products that are more resistant to hail damage
Why are there no concrete roof tile products mentioned under hail resistant roofing? Are they not more durable than asphalt shingles? - Robert Joseph 7/10/12
Robert, we agree that concrete roof tiles are more hail resistant than thinner softer products such as asphalt shingles. This article is about recognizing hail damage to asphalt shingles and distinguishing hail damage from other types of asphalt roof wear such as granule loss, blistering, foot traffic, cracking, &c. The issue of distinguishing hail from other wear on concrete roof tiles has not arisen, though I speculate that in extreme cases such as baseball sized hail that occurs in some locales, all roof materials, including concrete, tile, slate and stone can suffer damage as well. See Hail Damage Resistant Roofing Products.
Question: when is entire roof replacement vs just individual shingle repacement justified ? How many hail stone hits per square justify a new roof.
What warrants replacement of entire roof versus repairing only damage shingles from hailstones? How many hail stone hits per square warrant entire roof replacement? Thanks - Anon
When testing for hail stone hits, how do you conclude which area to test? From an adjuster's view, do you choose the best square or most damaged square? - Anon
If the roof is worn out and leaking it needs replacement;
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