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 ever conducted. Researchers collected and measured more than 2,500 individual hailstones from 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 found that 84% of hailstones are spheroidal (roughly egg-shaped or slightly flattened), 10% are conical (pointed on one end like a spinning top), and 6% are completely 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.

The Maximum Is Typically Twice the Mean

Here is the finding that should keep every insurance adjuster honest: within a single hailstorm, the maximum hailstone size is typically about twice the mean (average) size. That means if a storm produces an average hailstone diameter of one inch, individual stones in that same storm likely reached two inches.

Think about what this means in practice. When a weather report says “quarter-sized hail” (roughly one inch), that is describing the typicalstone size — not the largest ones. The same storm almost certainly produced stones closer to two inches. And a two-inch hailstone carries dramatically more energy than a one-inch stone.

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: the average was one inch. Some of the stones that hit your roof were likely twice that 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

The IBHS researchers calculated a “natural hail equivalent” for each standard test ball size. In other words, they figured out what size of real hailstone has the same mass as the perfect ice balls used in lab testing. Here is what they found:

Read the table carefully. A roofing product rated to withstand a 1-inch ice ball in the lab would be expected to resist natural hailstones up to about 1.18 inches — not much more than a quarter. But because the maximum hail size in a storm is typically twice the mean, a storm reporting “quarter-sized hail” likely produced individual stones well beyond what that rating covers.

And at the 2-inch test level, the gap gets even wider: a 2-inch lab ice ball has the same mass as a natural hailstone of approximately 2.65 inches. 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.” But the maximum stones in that storm were likely twice the reported average size. Quarter-sized average means some stones were likely half-dollar to golf ball sized.
• “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 research proves that this simplification is scientifically indefensible. Hailstorms produce a wide range of stone sizes. The largest stones are far bigger than the average. 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 are based on the peer-reviewed paper: “Observations of Hailstone Sizes and Shapes from the IBHS Hail Measurement Program: 2012–2014” by Ian M. Giammanco and Tanya M. Brown (Insurance Institute for Business & Home Safety), Matthew R. Kumjian (The Pennsylvania State University), and Andrew J. Heymsfield (National Center for Atmospheric Research). The study was published in the Journal of Atmospheric and Oceanic Technology (American Meteorological Society).

IBHS is a nonprofit research organization funded by property insurers and reinsurers. Notably, 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.