Hail Damage vs Blistering: How to Tell Them Apart on Shingles

By The Roofing Brief Team | Last reviewed: October 2026

Hail damage vs blistering comes down to direction, pattern and corroboration. A hailstone pushes into the shingle from outside, leaving random hits with crushed or embedded granules, a sloped crater and matching dents on gutters and vents. A blister pushes out from inside, leaving clustered, steep-walled pits with flat bottoms and no storm evidence anywhere else on the property. The distinction matters because a homeowners policy may cover hail but generally excludes blistering as deterioration or a defect.

Key numbers at a glance

These are the figures that decide most hail-versus-blister disputes on asphalt shingles: the hail size that starts doing damage, the size of a typical blister, the heat that causes one, and the sample area adjusters count. Each comes from a named engineering, research or code source and can vary with shingle age, product and temperature.

Figure	Value	Source
Field threshold hail size for damage to 3-tab asphalt shingles	About 1.00 in. (25 mm)	Marshall, Herzog, Morrison and Smith, Haag Engineering, “Hail Damage Threshold Sizes for Common Roofing Materials,” AMS conference paper P3.2 (Table 3, after Morrison 1997)
Field threshold hail size for 30-year laminated shingles	About 1.25 in. (32 mm)	Same Haag Engineering paper, Table 3
Typical blister crater size	About 1/4 in. (6 mm) or less	InterNACHI Inspection Narrative Library, “Blisters vs Hail Damage”
Shingle temperature associated with heat blistering	Over 60°C (140°F), usually from poor ventilation	InterNACHI Inspection Narrative Library
Size of one adjuster test square	10 ft x 10 ft = 100 sq ft (one roofing square)	Test-square method from Marshall and Herzog, Haag Engineering (1999); 10 x 10 ft standard as described by BRG in Claims Journal, May 2026
Minimum attic net free vent area	1/150 of the vented space (1/300 with listed exceptions)	2021 International Residential Code, Section R806.2

Hail damage vs blistering: side-by-side comparison

The fastest way to tell hail damage from blistering is to check five things in order: crater shape, granule condition, distribution across slopes, directionality, and collateral evidence on soft metals. A genuine hail hit usually passes all five; a popped blister usually fails at least three. The table below sets out each characteristic with the source behind it.

Characteristic	Hail damage	Blistering	Source
Direction of force	Inward: an impression pushed into the shingle	Outward: a raised bubble that may later pop	Knight’s Solutions adjuster guide (2023); InterNACHI narrative library
Crater profile	Sloped bottom that follows the curve of the ice	Flat bottom with steep, rigid sidewalls, often with mat visible	InterNACHI narrative library
Granules in the mark	Often still present, pressed into the asphalt if shingles were warm; crushed at the impact point	Absent from the crater; surrounding granules intact	InterNACHI narrative library; Knight’s Solutions
Mat condition	Can be fractured (a “bruise”); may feel soft when pressed	Not fractured; no bruising of the asphalt	Marshall et al., Haag Engineering; Knight’s Solutions
Size	Varies with stone size; a single storm leaves a size range tied to collateral dents	Usually about 1/4 in. or less, sometimes 1 mm	InterNACHI; Knight’s Solutions
Distribution	Random spacing, heavier on slopes facing the storm	Clustered or widespread, often by zone (hot areas, one production lot), on slopes regardless of direction	Hogan, IIBEC Interface, Oct. 2006; Knight’s Solutions
Collateral evidence	Dents in gutters, vents, flashing, HVAC fins; spatter marks on oxidized metal	None that is storm-related	Hogan, IIBEC Interface, Oct. 2006
Timing	Tied to a datable storm	Develops with heat exposure, often early in the roof’s life	Knight’s Solutions; InterNACHI
Typical recovery path	Homeowners insurance claim, subject to policy terms	Manufacturer warranty or contractor, subject to exclusions	ISO HO 00 03 policy form; IKO U.S. Limited Warranty (2025)

For the full visual catalog of hail marks on every roof material, see our guide to what hail damage looks like on a roof.

What does a roof blister look like?

An intact blister is a small raised dome in the shingle surface. A popped blister is a small round pit, usually about 1/4 inch or less across, with clean, steep edges, a flat floor, and dark asphalt or glass mat showing at the bottom. The granules around the pit stay in place, which is why a field of popped blisters looks like neat pinholes rather than smeared impact marks.

Lyle Hogan, writing in IIBEC’s Interface journal in October 2006, describes a form called “rash blistering”: very small blisters, seen on occasion with lightweight “utility-grade” shingles and particularly likely with substandard attic ventilation. The tiny blisters can rupture under even very light contact, leaving what looks like small pockmarks. Hogan states that rash blistering is “completely unrelated to unusual weather events” because it can occur with no hail at all.

Where blisters show up relative to ventilation

Blisters tend to concentrate wherever the shingle runs hottest. InterNACHI lists poor or inefficient ventilation as the most common cause, plus areas that receive both direct and reflected sun and shaded areas suddenly exposed to hot afternoon sun. On a real roof, that usually means the upper courses near a ridge with little or no exhaust venting, slopes over attic sections with blocked soffits, and areas under or beside reflective walls or dormers.

That location pattern is the opposite of hail. Hail falls on a storm track, so hits follow the wind direction across all exposed slopes, while heat blisters follow the attic’s hot spots. Hogan also notes blistering can be severe in some areas and moderate in others because of lot-to-lot variation in the shingle product or differences in ventilation effectiveness.

What causes shingles to blister?

Shingles blister when gas or moisture trapped in the asphalt expands under heat and pushes the surface up. Inspection references name heat from poor ventilation as the most common trigger, followed by moisture trapped during storage or installation, volatile compounds in the asphalt, tree resin, and solvent-based adhesives. Hail does not cause blistering.

The InterNACHI Inspection Narrative Library lists these conditions under which asphalt shingles typically blister:

• Poor or inefficient ventilation that lets shingles exceed about 60°C (140°F). InterNACHI marks this as the most common cause.
• Wet shingles stored or installed without protection, which trap moisture.
• Wet roof boards that release moisture suddenly into the shingle.
• Resin dripping from trees, which softens the asphalt.
• Too much solvent-based adhesive when hand-sealing, or incompatible adhesives. InterNACHI notes some caulks contain 20 to 50% xylene and benzene, which can soften asphalt quickly.
• Shaded areas suddenly exposed to hot sun, and areas receiving both direct and reflected heat.

Volatile gases escaping from the asphalt layer are the manufacturing-side explanation. The Knight’s Solutions adjuster guide attributes this to hydrocarbons left in the asphalt after refining, which can blow granules off once the installed shingle reaches a high enough temperature. Whether a given blister traces to the product or to the attic is the question that decides which warranty clause applies.

The 2021 International Residential Code, Section R806.2, sets the minimum net free ventilating area at 1/150 of the vented attic space, or 1/300 when a Class I or II vapor retarder is installed in Climate Zones 6 to 8 or when 40 to 50% of the vent area sits within 3 feet of the ridge with the balance at the eaves. An attic below those minimums gives a manufacturer grounds to blame ventilation. ARMA’s current technical bulletin library (checked October 2026) has no bulletin dedicated to blistering, so manufacturer warranty language and the code are the practical references.

What does hail damage look like on asphalt shingles?

Functional hail damage on asphalt shingles is a fracture, puncture or tear of the shingle mat, or a displacement of granules that exposes the asphalt underneath. Haag Engineering’s research defines damage as a reduction in water-shedding capability or in the expected long-term life of the roofing. A cosmetic mark that does neither is generally not treated as functional damage.

Haag’s ice-ball tests found that 11-year-old organic 3-tab shingles were damaged half the time by 1-inch ice stones, aged glass-fiber 3-tab shingles needed 1.25-inch stones, and aged laminated shingles needed 1.5-inch stones. Field surveys after natural storms put the onset of damage at about 1 inch for 3-tab and 1.25 inches for 30-year laminated shingles. Hogan adds that much of the hail falling in the U.S. measures 3/8 inch or less, which he says should not compromise a functional covering.

Small hail is not irrelevant, though. A 2025 peer-reviewed study by the Insurance Institute for Business and Home Safety (Meisenzahl, Giammanco and Hedayati, Frontiers in Materials) found that repeated high concentrations of sub-severe hail, under 1 inch, can cause granule loss, speed natural aging and reduce a shingle’s resistance to a later large-hail event. That damage is gradual and hard to separate from age, which is exactly where blister-versus-hail disputes get murky. Our pillar guide covers hail damage to roof shingles in full, and the size thresholds by material are in what size hail can damage a roof.

Other conditions mistaken for hail damage

Blistering is the most common hail lookalike, but five others show up in claim disputes: mechanical damage, foot-traffic scuffing, manufacturing variation, age-related granule loss, and nail pops. Each has a tell that hail does not produce, such as straight edges, a fixed location, or a pattern that ignores the storm’s direction.

Condition	What it looks like	Where it appears	How to rule hail in or out	Source
Blistering (heat or volatiles)	Small round pits, steep walls, flat floor, no crushed granules	Hot zones, clusters, any slope	No collateral dents; pits too uniform and too small for the storm	InterNACHI; Hogan (2006)
Mechanical damage (tools, hammer)	Granules pulverized to powder; marks can be linear or rectangular	Near work areas, ladders, satellite dishes, recent repairs	Hogan: a 10x hand lens shows the powdered granules; hail of hammer-head size does not pulverize them	Hogan, IIBEC Interface (2006)
Foot-traffic scuffing	Scraped, smeared granule loss rather than round pits	Along walking paths, around vents, chimneys and access points	Scuffs track human routes, not the storm direction	Hogan (2006)
Manufacturing variation	Patches of heavier blistering or granule loss	Following bundle or lot boundaries, sometimes one section of a roof	Hogan reports lot-to-lot variation in the same product on the same roof	Hogan (2006)
Age-related granule loss	Thin, even wear; some products lose edge granules first	Across the roof, worst on the sunniest slopes	Hogan calls edge-first loss “a product attribute, completely unrelated to hail activity”; granules in gutters alone are not proof of hail	Hogan (2006)
Nail pops	A raised bump or small hole in the overlying shingle	Directly over fasteners, in rows along nail lines	Aligned spacing at nail lines is a fastener pattern, not a storm pattern; lifting the tab shows the backed-out nail	Standard inspection practice; Knight’s Solutions adjuster guide on nail pops (2024)

How to tell hail damage from blistering on your roof, step by step

Start on the ground and work up, because collateral evidence usually settles the question before anyone touches a shingle. If gutters, vents and HVAC fins show fresh dents, hail is likely and the shingle marks need counting. If nothing metal is marked, the shingle pits deserve a much harder look as blisters or another lookalike.

1. Confirm a storm happened. Note the date and check the NOAA Storm Events Database or local storm reports for hail near your address. Our 2026 hail and storm loss database compiles NOAA Storm Prediction Center hail events alongside IBHS, NICB and state insurance department data.
2. Check soft metals at ground level. Look at aluminum gutters, downspouts, HVAC condenser fins, mailbox and light fixtures. Hogan calls deformed HVAC cooling fins “the sure signature of hail exposure.”
3. Look for spatter marks. Hail knocks oxidation off aged metal and paint, leaving shiny marks against duller surfaces. Their size gives a rough maximum stone size, and their location gives the storm’s direction.
4. Compare slopes. Hail damage should be heavier on slopes facing the storm. Blisters show no such directional bias.
5. Examine the marks with raking light. Shine a flashlight across the surface at a low angle. A depression with a sloped floor and crushed granules points to hail. A pit with vertical walls and a flat floor points to a blister.
6. Press gently on a suspected hit. A fractured mat may feel soft or give slightly compared with the shingle around it. A blister crater does not.
7. Check the attic. Look for blocked soffit vents, missing ridge exhaust, bathroom fans vented into the attic, or heat and moisture staining. Poor ventilation supports a blister diagnosis.
8. Get a credentialed inspection before filing if unsure. Walking a roof can add scuffs and damage. If you are not trained or the pitch is steep, hire an inspector or roofer to do the on-roof steps.

Which is it? A decision table

Use this table to map what you see on the roof and around the property to the most likely cause and the right next step, whether that is an insurance claim, a warranty claim or a ventilation fix. It is a screening tool, not a coverage decision; the insurer or manufacturer makes that call under the policy or warranty terms.

What you see	Most likely cause	Next step
Random hits, sloped craters, fresh dents on gutters and vents, heavier on storm-facing slopes	Hail	Photograph, mitigate leaks, notify your insurer
Small uniform pits with flat floors, clustered near the ridge or hot zones, no collateral dents	Heat blistering	Inspect attic ventilation; contact the shingle manufacturer and installer
Blisters on a newer roof across many slopes, ventilation meets code	Possible product issue	Open a manufacturer warranty claim within the notice window; keep samples if requested
Fresh collateral dents plus older uniform pits on the same slopes	Both hail and pre-existing blistering	Ask the inspector to mark and count only hits that match the storm’s size and direction
Powdered granules or straight-edged marks near a recent repair	Mechanical damage	Raise with the contractor who did the work
Bumps or holes in rows along nail lines	Nail pops	Re-fasten and seal; generally a maintenance or workmanship item
Even, overall granule thinning, worst on sunny slopes, on an older roof	Age and weathering	Plan replacement; not usually an insurance item

Does insurance cover roof blistering?

Generally, no. Many U.S. homeowners policies follow the ISO HO 00 03 form, which excludes loss caused by “wear and tear, marring, deterioration” and by “mechanical breakdown, latent defect, inherent vice.” Blistering from heat, moisture or a product flaw usually falls under those exclusions, while hail is a sudden, accidental loss that the same form may cover. Your own policy wording controls.

The recovery path for blistering is the shingle manufacturer’s limited warranty, but read the exclusions before filing. IKO’s U.S. Limited Warranty for shingles installed on or after February 1, 2025, covers only manufacturing defects that significantly affect the shingles’ water-shedding performance. It excludes damage caused by hail, damage from inadequate ventilation (including blocked or nonoperative vents), and the effects of resins or drippings from trees. It also asks owners to contact IKO Warranty Services within 30 days of becoming aware of the concern.

That creates a gap worth knowing about. If blistering traces to an under-ventilated attic, the insurer may call it deterioration and the manufacturer may call it a ventilation problem. In that case, the contractor who installed the roof and venting, and any workmanship warranty they gave, may be the remaining route. Check your own manufacturer’s warranty, since terms differ by brand and by installation date.

For how hail claims move from first notice to payment, see our guide to the hail damage roof insurance claim process.

How adjusters test a roof for hail

Adjusters and engineers typically mark a 10-by-10-foot test square on each directional slope, circle every qualifying hit with chalk, and count hits per square. They then check soft metals to estimate stone size and direction and confirm the hits match the storm. The method traces to Haag Engineering’s procedures, published formally by Marshall and Herzog in 1999.

Chalk circles are where blistering gets screened out. A trained inspector should circle only marks that fit the storm: hits with crushed granules or mat fracture, sized consistently with the collateral dents. Popped blisters, scuffs and nail pops should be left uncircled. What to expect during that visit, and how to prepare, is covered in our guide to the insurance adjuster roof inspection.

Why one test square can mislead: the arithmetic

A test square is a sample, and hail hits land randomly. Giancarlo Grant of BRG made this point in Claims Journal in May 2026, modeling hits with a Poisson distribution. Our own arithmetic, using that same standard model, shows how much a single square can swing. Carriers set their own count guidelines; the 8-hit figure below is a hypothetical example, not a published standard.

True average hits per 100 sq ft on the slope	Chance one square shows 0 hits	Chance one square shows fewer than 8
1	37%	Over 99%
3	5%	99%
5	0.7%	87%
8	Under 0.1%	45%
10	Under 0.01%	22%

Roofing Brief estimate: Poisson probabilities, P(0) = e^−λ, computed for each average (λ). Assumes hits land independently and every hit is correctly identified.

Two practical takeaways follow. A square with zero hits still leaves a 5% chance that the slope averages 3 hits per square, which is why a single empty square does not prove “no hail.” And on a slope that truly averages 10 hits, one square reads below 8 about 22% of the time, but the total of four squares falls below 32 only about 9% of the time. Asking for more than one square per slope makes the count more reliable, whichever way it goes.

Is it worth claiming hail damage?

It can be, when the damage is functional and the repair cost clearly exceeds your deductible. Hail is generally a covered peril, but many policies in hail-prone states carry a separate wind and hail deductible set as a percentage of dwelling coverage, and some add cosmetic-damage exclusions or depreciation schedules for roofs. Read your declarations page before you file.

A worked example shows the math. A 2,000-square-foot (20-square) architectural shingle roof at $5 to $8 per square foot installed, the range in our 2026 roofing cost report, costs about $10,000 to $16,000 to replace. With a hypothetical 1% wind and hail deductible on $300,000 of dwelling coverage, the deductible is $3,000; at 2% it is $6,000. An actual-cash-value roof policy would also subtract depreciation. If the hits are few and the roof is old, the net payout may be small. Our guide on how much hail damage it takes to replace a roof covers repair-versus-replace thresholds.

Should shingle blisters be popped?

No. Popping a blister does not repair anything; it strips the granule cap and exposes asphalt to sunlight. IBHS notes that once granules are displaced, the asphalt is exposed to UV radiation and becomes more brittle, speeding natural aging. Popping blisters also destroys the evidence a manufacturer may want to examine during a warranty claim.

If blisters are spreading, the useful steps are to fix the ventilation, photograph the condition, and contact the manufacturer and installer. Isolated popped blisters that do not expose the mat are often monitored rather than repaired, but a roofer should judge whether any shingle needs replacing.

What photos to take

Good photos separate hail from blistering for an adjuster or a warranty reviewer. Take them before any repair, include something for scale, and capture the context around each mark, not just the close-up. A U.S. quarter is 0.955 inch across, per the U.S. Mint, which makes it a handy scale reference next to a hit or a dent.

1. Each elevation and slope, wide. One photo per slope from the ground or a ladder shows distribution and direction.
2. Collateral damage with scale. Gutters, downspouts, vents, HVAC fins and flashing, each with a coin or tape measure.
3. Spatter marks. Shiny spots on oxidized metal, with a compass direction noted.
4. Close-ups at a low angle. Use raking light so the photo shows whether a mark is a depression or a pit with vertical walls.
5. Straight-down close-ups with scale. One per type of mark, so size and granule condition are visible.
6. Test squares, if an inspector marks them. A wide shot of the chalked square and a close-up of several circled hits.
7. The attic. Soffit vents from inside, ridge or roof vents, insulation baffles, and any bathroom or kitchen fan ducts.
8. Paperwork. The shingle wrapper or invoice showing product and installation date, plus any pre-storm photos with dates.

Metal roofs, standing seam and metal accessories follow different rules for cosmetic dents versus functional damage. See our guide to metal roof hail damage before treating dents there as proof of shingle damage.

Frequently asked questions

How can you tell the difference between hail damage and blisters?

Hail pushes in and blisters push out. A hail hit has a sloped crater, crushed or embedded granules, possible mat fracture, random spacing heavier on storm-facing slopes, and matching dents on gutters and vents. A popped blister is a small pit, usually 1/4 inch or less, with steep walls, a flat floor, no crushed granules, clustering in hot zones, and no storm evidence on surrounding metal.

Does insurance cover roof blistering?

Generally, no. Many homeowners policies follow the ISO HO 00 03 form, which excludes wear and tear, deterioration, latent defects and inherent vice, and blistering usually fits those exclusions. The usual route is the shingle manufacturer’s limited warranty, though warranties such as IKO’s 2025 U.S. warranty exclude damage from inadequate ventilation. Your specific policy and warranty wording decide the outcome.

Is it worth claiming hail damage?

It can be, when hail caused functional damage and the repair cost clearly exceeds your deductible. Many policies in hail-prone states use a percentage wind and hail deductible, so a 1% deductible on $300,000 of dwelling coverage is $3,000. Compare that, plus any depreciation, with the repair estimate. A professional inspection before filing helps confirm the damage is hail and not blistering.

Should shingle blisters be popped?

No. Popping a blister removes the granule cap and exposes asphalt to UV, which IBHS research links to brittleness and faster aging. It also destroys evidence a manufacturer may want to inspect for a warranty claim. The better steps are to photograph the blisters, check attic ventilation, and contact the manufacturer and the installing contractor.

What causes a shingle to blister?

Heat expanding gas or moisture trapped in the asphalt. InterNACHI names poor ventilation, which lets shingles exceed about 140°F, as the most common cause. Other causes include shingles or decking that were wet at installation, volatile compounds in the asphalt, tree resin, excess or incompatible solvent-based adhesive, and areas that get both direct and reflected sun.

Can a roof have both blistering and hail damage?

Yes. A roof that blistered early in its life can later take real hail hits, and the two can sit side by side on one slope. Inspectors separate them by counting only marks that match the storm’s size and direction and that have crushed granules or mat fracture. Dated pre-storm photos of the roof make that separation far easier to support.

Sources

• Marshall, T.P., Herzog, R.F., Morrison, S.J., and Smith, S.R., Haag Engineering Co., “Hail Damage Threshold Sizes for Common Roofing Materials,” American Meteorological Society conference paper P3.2 (ams.confex.com/ams/pdfpapers/45858.pdf).
• Hogan, L., “A Proposal for Uniformly Assessing Hail Damage to Roofs,” Interface (IIBEC, formerly RCI), October 2006.
• Meisenzahl, B., Giammanco, I., and Hedayati, F., “Sub-severe hail: the missing piece in assessing asphalt shingle risk in North America,” Frontiers in Materials 12:1603074, Insurance Institute for Business and Home Safety, September 2025.
• InterNACHI Inspection Narrative Library, “Blisters vs Hail Damage” (hinarratives.com).
• Grant, G. (BRG), “Hail to High Variance: Rethinking Test Squares and Roof Damage Assessment,” Claims Journal, May 6, 2026.
• IKO Industries, U.S. Limited Warranty for asphalt shingles installed on or after February 1, 2025 (iko.com/na).
• International Code Council, 2021 International Residential Code, Section R806.2, Minimum vent area.
• Insurance Services Office, Homeowners 3 Special Form HO 00 03, Section I Perils Insured Against.
• Knight’s Solutions, “Shingle Blistering vs. Hail Damage: A Guide for Insurance Adjusters,” September 2023.
• Asphalt Roofing Manufacturers Association, Technical Bulletins library (asphaltroofing.org), reviewed October 2026.
• U.S. Mint, quarter dollar coin specifications.