Sub-Severe Hail: Why Small Hailstones Cause Big Problems

Your insurance company denied your roof claim because the hail was “too small.” Quarter-sized. Maybe smaller. They told you hailstones under one inch cannot damage asphalt shingles. That sounds like settled science. It is not. A 2025 peer-reviewed study by researchers at the Insurance Institute for Business & Home Safety (IBHS) — the insurance industry's own research organization — proves that small hail does something far worse than causing visible damage on the first hit. It makes your roof dramatically more vulnerable to the next storm.

What Is “Sub-Severe” Hail?

The National Weather Service classifies hail as “severe” when it reaches one inch (quarter-sized) or larger. Anything smaller is considered “sub-severe.” The study focused on hailstones in the 0.7 to 1 inch range (about 1.8 to 2.5 cm) — roughly nickel-sized to quarter-sized. This is the most common hail size in North America. Most thunderstorms that produce hail produce hail in this range.

Insurance companies have long treated sub-severe hail as harmless. If the hail report shows stones under one inch, the claim gets denied. The adjuster writes “hail size insufficient to cause damage” and moves on. The IBHS researchers wanted to know whether that assumption holds up under testing. It does not.

Ten Times More Vulnerable

The researchers subjected asphalt shingles to repeated impacts from sub-severe hailstones — the kind your insurer calls harmless. After exposure to these small impacts, the shingles were then tested against subsequent hail. The result: shingles previously hit by sub-severe hail became approximately ten times more susceptible to damage from future hail impacts.

Read that again. Not ten percent more vulnerable. Not twice as vulnerable. Ten times. A shingle that could withstand a given hailstone on a fresh roof failed catastrophically against the same hailstone after being exposed to repeated small-hail impacts.

This is cumulative damage. Each small impact weakens the shingle's protective granule layer. Individual hits may not leave visible marks. But the granule bond degrades with each strike, and eventually the shingle reaches a tipping point where even moderate hail causes failure.

Cumulative Small Hits Outperform a Single Large Hit

Here is the finding that should end the “too small to cause damage” argument forever: the researchers found that cumulative granule loss from repeated sub-severe hail impacts exceeded the damage caused by a single two-inch hailstone. In other words, getting hit by many small stones did more total damage than getting hit once by a large one.

Think about what this means for a real roof. Your area gets hit by a thunderstorm in 2022 that drops nickel-sized hail. The adjuster comes out, sees no visible damage, and closes the claim. Two years later, another storm rolls through with slightly larger hail. This time the roof fails — shingles crack, granules wash off in sheets, leaks develop. Your insurer says the 2024 storm caused the damage. The 2025 IBHS research says otherwise: the 2022 storm weakened the roof, and the 2024 storm finished the job.

Why This Matters for Your Claim

Insurance companies deny hail claims based on size thresholds every day. The standard argument goes: “The hail in your area was measured at three-quarters of an inch. This is below the threshold for damage to asphalt shingles. Claim denied.” This research dismantles that logic in two ways:

• First, sub-severe hail is not harmless. It causes progressive, cumulative weakening of the shingle surface that may not be visible to the naked eye but is measurable and real.
• Second, any roof that has been exposed to prior hailstorms — even storms with “too small” hail — is significantly more vulnerable than a new roof. An adjuster who evaluates your roof as if it were brand new is ignoring documented science.

If your roof has been through multiple hailstorms over the years, even small ones, the cumulative effect may be the primary cause of your current damage. Your insurer cannot simply point to the size of the most recent hail and declare it insufficient.

The Insurance Industry's Own Research

This is not research from a plaintiff's expert or a roofing contractor with something to sell. IBHS is funded by property insurers and reinsurers. Its mission is to reduce losses from natural hazards. When IBHS publishes a finding that undercuts the industry's most common hail denial, that finding carries enormous weight. Insurers cannot credibly dismiss research produced by the organization they fund to study exactly this question.

The Bigger Picture

Most of North America's hail falls in the sub-severe range. The vast majority of hailstorms produce stones smaller than one inch. If this research is correct — and peer review says it is — then millions of roofs across the country are carrying invisible cumulative damage from years of “harmless” hail. Those roofs are time bombs, waiting for one more storm to push them past the breaking point.

The insurance industry has spent decades telling homeowners that small hail does not matter. Their own researchers just proved otherwise.

About This Research

The findings discussed in this article are based on the peer-reviewed paper: “Sub-severe hail: the missing piece in assessing asphalt shingle risk in North America” by Michael Meisenzahl, Ian M. Giammanco, and Saman Hedayati (Insurance Institute for Business & Home Safety). The study was published in Frontiers in Materials in 2025.

IBHS is a nonprofit research organization funded by property insurers and reinsurers. The fact that this research was produced by the insurance industry's own institute makes its conclusions particularly significant for policyholders whose claims have been denied based on hail size alone.

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.