What Hailstone Research Tells Us About Insurance Claims

When your insurance company denies a hail damage claim, they usually point to weather reports. “The National Weather Service reported quarter-sized hail in your area. That's not large enough to damage your roof.” It sounds reasonable. It is not. The actual science of hailstones — how big they really are, what shape they take, and how they behave on impact — tells a very different story.

Between 2012 and 2014, the Insurance Institute for Business & Home Safety (IBHS) ran the most comprehensive hailstone measurement program of its kind, deploying a mobile field team that collected and measured 2,557 individual hailstonesfrom 33 separate thunderstorms across the Great Plains. What they found should change how every insurance adjuster, engineer, and homeowner thinks about hail damage.

Hailstones Are Not Perfect Spheres

When you picture a hailstone, you probably imagine a round ball of ice — like a marble or a golf ball. That is not what falls from the sky. The IBHS research documented that the great majority of measured hailstones were spheroidal (roughly egg-shaped or slightly flattened), with smaller proportions conical(pointed on one end) and fully irregular (jagged, lobed, or asymmetric).

Why does this matter? Because every standard impact testused by roofing manufacturers and insurance engineers uses perfectly round ice spheres. The test standard — UL 2218, also known as FM 4473 — fires machine-made ice balls at roofing materials from a controlled height. These ice balls are smooth, uniform, and dense. Real hailstones are none of those things.

Reported Hail Size Is Not the Whole Story

Within a single hailstorm, the size distribution is wide. The IBHS field data shows stones ranging from sub-pea size to over 7 cm in a single dataset, with the largest stones substantially exceeding the typical or mean size in their respective storms. That means if a weather report describes a storm as producing roughly one-inch hail, individual stones in that same storm can be larger — sometimes much larger.

Think about what this means in practice. When a weather report says “quarter-sized hail” (roughly one inch), that is describing the representativestone size — not the largest ones. The same storm likely produced stones materially larger than that. And a larger hailstone carries dramatically more energy than a smaller one.

So when your insurance company says “the reported hail size in your area was only one inch, which is not large enough to damage your shingles,” the correct response is to ask what the upper tail of the size distribution at your address actually was, not just the reported size.

Real Hail Has Less Mass Than Lab Ice Balls

The researchers weighed every hailstone they collected and compared the results to what a perfect ice sphere of the same diameter would weigh. The finding: natural hailstones consistently have less mass than a perfect ice sphere of the same maximum diameter. This makes sense — real hailstones have air pockets, irregular surfaces, and layered internal structures that reduce their density compared to solid lab ice.

This has a critical implication for impact testing. When a roofing manufacturer tests their shingles against a 1.5-inch ice ball and the shingle passes, they are testing against a projectile that is denser and heavierthan a real 1.5-inch hailstone. That sounds like it would make the test harder to pass — and it does. But here is what matters for your claim: it means the test results do not directly translate to real-world hail performance.

What “Impact Resistance” Ratings Actually Mean

Because solid lab ice balls are denser than natural hailstones of the same diameter, the IBHS field data has been used to develop mass-equivalent calibrations — that is, figuring out what size of natural hailstone has roughly the same kinetic energy as the smaller, denser ice balls used in lab testing. The general direction of this calibration is consistent: a lab ice ball of a given diameter is mass-equivalent to a somewhat larger natural hailstone of the same kinetic energy. This is why a roofing product rated to withstand a 1-inch ice ball in a controlled lab test cannot be assumed to resist a real 1-inch hailstorm. The lab test result and the field exposure are not directly equivalent.

Specific mass-equivalent conversion factors are used by IBHS and ASCE in developing the new asphalt-shingle impact test protocol. The takeaway for policyholders does not depend on the precise conversion: whatever the exact ratios, the comparison favors policyholders on a typical hail claim. A storm reporting “quarter-sized hail” in a weather summary almost certainly included some larger stones, and those stones strike with more energy than the lab projectiles used to rate the roofing product. The larger the hailstone, the more oblong and irregular it becomes, and the greater the difference between lab conditions and reality.

Bigger Hail Is More Oblong

The researchers measured the “shape factor” of each hailstone — the ratio of its shortest dimension to its longest. A perfect sphere has a shape factor of 1.0. They found that as hailstones get larger, they become more oblong. Small hailstones are relatively round. Large hailstones are increasingly flattened or elongated.

This matters because an oblong hailstone concentrates its impact energy differently than a round one. When a flattened stone hits a roof, the impact is not distributed evenly across a circular area — it may strike edge-first or with a smaller contact surface, focusing the force on a narrower point. Lab tests with perfect spheres do not replicate this effect.

Why This Matters for Your Insurance Claim

Insurance companies and their hired engineers use weather data and lab test results to make arguments about whether hail could have damaged your property. These arguments typically go like this:

• “Weather reports show only quarter-sized hail in your area.” A reported size is representative, not maximum. Field data shows that within a single storm the size distribution is wide, with the largest stones substantially exceeding the reported typical size.
• “Your shingles are rated Class 3 impact resistant — they can withstand 1.75-inch hail.”That rating is based on perfectly round, solid ice spheres fired in a lab. Real hailstones of 1.75 inches are lighter, more irregular, and behave differently on impact. The rating does not mean your roof is undamaged — it means the shingles passed a specific lab test that does not perfectly replicate natural hail.
• “Our engineer found no evidence of hail damage consistent with the reported hail sizes.” If the engineer is comparing your roof damage to expected impacts from perfect ice spheres, they are using the wrong baseline. Natural hail impacts look different from lab impacts because the stones are shaped differently.
• “The hail wasn't big enough to cause damage.” Ask them: big enough according to what? The average from a weather report? A lab test with artificial ice balls? The actual peer-reviewed science says both of those metrics understate the real-world hail exposure your roof experienced.

The Broader Point

Hail damage claims are not simple. The interaction between hailstone size, shape, mass, velocity, angle of impact, and roofing material properties creates enormous variability in real-world damage. Insurance companies prefer to reduce this complexity to a single number — “the hail was X inches” — because a single number is easy to use in a denial letter.

The IBHS field research undermines this simplification. Hailstorms produce a wide range of stone sizes, the largest of which can substantially exceed the typical or representative size used in weather reports. Real hailstones behave differently from lab test projectiles. And the farther you get from controlled laboratory conditions, the less predictable the damage becomes.

None of this means every hail claim is valid. But it does mean that a denial based solely on “the reported hail size was too small” is not supported by the science. If your insurer is making that argument, they owe you a better explanation.

About This Research

The findings discussed in this article come from the Insurance Institute for Business & Home Safety (IBHS) Hail Field Research Program, which conducted mobile field measurements of natural hailstones across the Great Plains between 2012 and 2014. The program’s methods and results are documented in IBHS annual field summary reports and in conference papers presented at the American Meteorological Society Conference on Severe Local Storms by Tanya M. Brown-Giammanco, Ian M. Giammanco, and collaborators. Related lab impact-test calibration work has been published by IBHS researchers in ASCE’s Natural Hazards Review.

IBHS is a nonprofit research organization funded by property insurers and reinsurers. This research was funded by the insurance industry itself — which makes its findings particularly difficult for insurers to dismiss when they contradict the simplistic arguments used in claim denials.

This article is for informational purposes only and does not constitute legal advice. Insurance policies and applicable law vary by state and by policy form. Consult with a licensed professional regarding your specific situation.

Written by Leland Coontz III, Licensed Public Adjuster, CA License #2B53445.