How Big Does Hail Have to Be to Damage a Commercial Roof?

In short: There is no magic hail size that guarantees damage. The National Weather Service calls hail "severe" at 1 inch, and that's a sensible trigger to inspect — but membrane type, roof age, wind speed, and impact angle matter just as much as size. New research shows even repeated sub-1-inch hail can measurably weaken a roof. When in doubt after a hailstorm, document and inspect.

Why is there no single "damage size" for hail?

It's tempting to look for a cutoff — if the hail was bigger than X, call an inspector; if smaller, don't worry. Unfortunately, hail damage doesn't work that way. A 2-inch stone can harmlessly bounce off a thick, flexible membrane, while a storm of smaller stones driven by 60-mph wind can pepper an aging roof into an insurance claim.

Think of it in terms of energy and vulnerability. The stone's size sets the energy of each impact, but the roof's condition decides what that energy does. A new, flexible TPO membrane on solid insulation absorbs impacts very differently than a 20-year-old, sun-brittled modified bitumen roof with ponding water. The stone is only half the story — the roof is the other half.

That said, some sizes are useful mental benchmarks. Let's walk through them.

What does 1-inch hail do to a commercial roof?

One inch (25.4 mm) is the National Weather Service's threshold for classifying hail as "severe," and it's a practical line for property managers: hail this size gets noted in storm reports, which is exactly the kind of third-party documentation that supports an insurance claim. If a storm report shows 1-inch hail at your building's address, that record helps establish a storm date for your claim.

As for physical effects, 1-inch hail can:

• Dent soft metals — exhaust vents, HVAC housings, flashing, and gutters. These dents are important twice over: they're damage in themselves, and they're the corroborating evidence adjusters trust most when evaluating the membrane.
• Bruise aging membranes — older or brittle roofing surfaces can show impact bruises at this size, where the surface layer is compressed or fractured even if it doesn't puncture.
• Strip surface granules — on granule-surfaced systems, a concentrated 1-inch hail event can knock loose protective granules, exposing more of the surface to UV and weathering.

On a healthy, mid-life membrane, a single 1-inch hailstorm may leave little more than cosmetic marks. But if the same roof has taken several of these storms over the years, the cumulative effect adds up — which brings us to what recent research uncovered.

Can small hail really damage a roof, or is that just a sales pitch?

This is the honest question every property manager should ask when a contractor points at pea-sized hail damage. For years, the industry consensus was that small hail was mostly cosmetic: engineers generally agreed that most shingle-damage events came from stones 1.5 inches or larger, and insurers treated sub-severe hail (under 1 inch) as a minor concern.

That consensus was challenged in September 2025, when IBHS — the Insurance Institute for Business & Home Safety, the property insurance industry's own research center — published a peer-reviewed study in Frontiers in Materials (authors Brenna Meisenzahl, Ian Giammanco, and Faraz Hedayati). The researchers fired 0.7-inch and 1-inch ice spheres at roof samples with a hail cannon, let them weather naturally for a year, then hit them with larger 2-inch hail. Their findings:

• Repeated sub-severe hail impacts "significantly reduce the roof cover's resistance to future large hail events" and accelerate natural aging.
• The cumulative granule loss from repeated sub-severe impacts exceeded the granule loss from a single 2-inch hailstone impact on the same products.
• Sub-severe-exposed shingles that were weathered and then hit with 2-inch hail suffered dramatically worse damage than new shingles exposed to the same 2-inch hail.

To be precise about what the study does and doesn't say: it did not claim 0.7-inch hail routinely punctures or tears roofing. The measured problem was cumulative — granule loss, degraded performance, accelerated aging, and greater vulnerability to the next storm. Small hail wears the roof down; the next big hail finishes the job.

For commercial property owners, the takeaway is practical: a roof that's lived through several years of small-hail events may be far more vulnerable than its age alone suggests. And if your hail damage inspection shows heavy granule loss or surface erosion after seasons of small hail, that's real degradation worth documenting — not a sales pitch.

Which matters more — hail size, or the type of roof?

Size gets the attention, but roof type often decides the outcome:

• Built-up roofs (BUR) with gravel surfacing handle impacts well — the gravel absorbs and deflects energy. Smooth-surfaced BUR is more vulnerable to penetration.
• TPO and PVC single-ply membranes are generally impact-resistant when new and flexible, but they become more brittle with age and UV exposure. A bruise on a brittle membrane can become a fracture point.
• EPDM tends to rebound from impacts better than many systems, though seams and aged sections are weak points.
• Modified bitumen with granule surfacing can lose protective granules to repeated impacts, exposing the bitumen to UV degradation — a slow, compounding failure.
• Metal roofs rarely fail functionally from hail — they dent. The question becomes cosmetic vs. functional damage, which is exactly where insurance disputes live.

Age may matter most of all. An oxidized, brittle membrane at year 18 behaves nothing like the same material at year 5. If you're responsible for a roof approaching the end of its service life, your inspection threshold should be lower — smaller hail can start failures on a roof that's already degraded. That's also why regular inspection and maintenance matters: a documented maintenance history proves the roof was in sound condition before the storm, which makes it much harder for an adjuster to dismiss everything as wear and tear.

How do wind and impact angle change the equation?

Two storms can drop the same size hail and do very different damage. The variables:

• Wind speed. A 1-inch stone falling straight down carries far less energy than the same stone driven sideways by 60-mph wind. Wind-driven hail also strikes at an angle, which can damage vertical surfaces — parapet caps, wall panels, rooftop equipment — that vertical-falling hail never touches.
• Stone density and hardness. Hailstones aren't uniform. Some are hard and clear; others are slushy. Harder, denser stones transfer more energy on impact.
• Storm duration. Ten minutes of 1-inch hail and an hour of it are different events. Longer storms mean more impacts per square foot, and cumulative impact is exactly what the IBHS research flagged as the under-appreciated threat.

This is why "how big was the hail" is the wrong first question. The right questions are: what size, for how long, with what wind, on what roof, in what condition.

What should you do after any hailstorm?

Forget the size debate and follow a simple rule: if you know hail fell on your building, inspect. Here's a practical sequence:

1. Record the storm details now. Date, time, approximate hail size, wind. Save any weather alerts or news reports. Check whether the National Weather Service logged severe (1-inch-plus) hail at your location — that report is powerful supporting evidence.
2. Do a ground-level walk. Look for dented gutters, damaged signage, cracked skylights visible from below, and damage to parked vehicles. Collateral damage to soft metals is both evidence of hail intensity and an indicator of what the roof took.
3. Photograph everything, dated. Wide shots for context, close-ups for detail. Include the soft-metal dents — adjusters weight this evidence heavily. For a fuller process, see our storm damage documentation checklist and what happens during an inspection.
4. Get a professional inspection. A qualified inspector distinguishes hail bruises from wear, counts impacts, checks seams and flashings, and produces a report that speaks the adjuster's language. The fresher the marks, the clearer the distinction.
5. Don't delay filing. Most commercial policies give you roughly a year from the storm date, and some less. Fresh evidence is always more convincing than weathered evidence.

One caution: don't repair before documenting. Emergency tarping to stop active leaks is expected and appropriate — permanent repairs before the adjuster documents the roof can erase the evidence your claim depends on.

The bottom line for property managers

Stop looking for a minimum hail size and start thinking in terms of risk factors. After any hailstorm: document the event, inspect the roof and the soft metals, and keep the photos and storm reports on file. A single 1-inch hail event on a healthy membrane may mean nothing — but repeated small-hail seasons on an aging roof can quietly push it toward failure, and the research now backs that up.

If you're managing commercial property and a hailstorm has passed through your area, request an inspection before the next storm tests whatever this one weakened. For ongoing protection, learn about our storm response services for businesses.