The Science of Hail: Why a 1-Inch Stone Can Destroy a Commercial Roof

Every year, hail causes billions of dollars in property damage across the United States. Yet walk through most commercial districts after a significant hailstorm and you will find property managers who have no idea their roofs just took a serious hit. There are no broken windows. There is no obvious debris. Business carries on as usual. The damage is there, though, working silently beneath the surface until the first heavy rain turns it into a very expensive problem.

Understanding why hail is so damaging, and why it is so easy to miss, starts with understanding what hail actually is and what happens in the fraction of a second when it strikes your roof.

How Hail Forms

Hail begins as a small water droplet or ice crystal carried upward by the powerful updrafts inside a thunderstorm. Severe thunderstorms, the kind capable of producing damaging hail, can have updrafts exceeding 100 miles per hour. These vertical wind currents suspend droplets high in the atmosphere where temperatures are well below freezing, typically between -10°F and -40°F depending on altitude.

As a droplet circulates through layers of supercooled water and ice crystals, it accumulates layer after layer of ice, growing outward in concentric shells much like the layers of an onion. Each trip through the updraft adds more mass. When the hailstone finally becomes too heavy for the updraft to support, gravity wins, and it falls to earth.

The size of a hailstone at impact depends on two primary factors: the strength of the updraft and the time the stone spends cycling through the storm. A weak thunderstorm might produce pea-sized hail measuring a quarter inch in diameter. A violent supercell thunderstorm, the type common across the Southern Plains and stretching into the Southeast, can produce hailstones the size of baseballs or larger. The record hailstone recovered in the United States measured over eight inches in diameter and weighed nearly two pounds.

The Physics of Impact

When a hailstone strikes a roofing surface, the damage is governed by kinetic energy. The formula is straightforward: kinetic energy equals one-half times mass times velocity squared. The squared relationship with velocity is the critical point. Doubling the speed of a hailstone quadruples its energy at impact. Tripling the speed produces nine times the energy.

Terminal velocity for hail varies with size. A small pea-sized stone might fall at 25 miles per hour. A golf ball-sized stone, roughly 1.75 inches in diameter, can reach 70 miles per hour. Baseball-sized hail can strike at over 100 miles per hour. When you calculate the kinetic energy of a two-ounce hailstone hitting at 70 mph, you are looking at an impact force comparable to a hammer blow delivered by a skilled carpenter. Multiply that by thousands of strikes across your roof surface in the span of a few minutes, and the cumulative effect becomes clear.

What makes hail uniquely damaging compared to other impact threats is the combination of mass, hardness, and irregular surface texture. Unlike a rubber ball that distributes energy across a wider area, a hailstone concentrates its impact through edges and protrusions that cut and fracture roofing materials at the point of contact.

What Hail Does to Different Roofing Systems

The type of damage hail produces depends heavily on the roofing system it strikes.

Single-ply membranes (TPO, PVC, EPDM) are somewhat resilient to small hail due to their flexibility, but they are not immune. Larger stones, particularly those with irregular shapes or jagged edges, can puncture or split the membrane directly. More commonly, hail causes what is known as bruising, a compression of the membrane that is not immediately visible as a puncture but weakens the material at that point. Over subsequent seasons, UV exposure and thermal cycling cause bruised areas to crack and fail.

Modified bitumen and built-up roofing systems are particularly vulnerable to hail impact. The granule surface that provides UV protection can be knocked loose by even moderate hail, exposing the underlying asphalt to direct sun. Once granules are stripped away, UV degradation accelerates dramatically, reducing the expected lifespan of the system by years. The impact craters left behind also collect water, accelerating deterioration at those exact points.

Metal roofing panels dent rather than crack under hail impact. While a dented metal panel does not immediately leak, denting can compromise coating integrity at the point of impact, exposing raw metal to moisture and accelerating corrosion. Severe hail can also damage seams and fastener points where panels connect, creating entry points for water.

Flashings, sealants, and penetrations are among the most vulnerable components on any roof during a hailstorm. The rigid nature of metal flashings means they can crack or dislodge under direct impact. Caulking and sealants around pipes, drains, and curbs can be fractured by hailstone impacts in a way that leaves no visible exterior evidence but opens small gaps that water will eventually find.

Why the Damage Is So Hard to See

This is the frustrating reality of hail damage from a property owner's perspective: most of it is not obvious. You are not going to climb onto your roof after a storm and see gaping holes. What you will see, if you know what to look for, are subtle depressions, scuffed surfaces, fractured granules, and small cracks at seam edges. To the untrained eye, the roof looks fine. To an experienced inspector, those signs represent thousands of dollars in developing damage.

The invisibility problem is compounded by the fact that damage does not always manifest immediately. A hail-impacted membrane might perform reasonably well for six to twelve months before the weakened areas begin to crack, separate, or allow water infiltration. By the time you see a stain on your ceiling, the original hail event may have happened the prior year, making it difficult to connect cause and effect without documentation.

Why Professional Post-Storm Inspection Is Non-Negotiable

After any hailstorm with stones measuring three-quarters of an inch or larger, a professional roof inspection is not optional, it is essential. The documentation from that inspection serves multiple critical purposes.

First, it establishes the condition of your roof immediately after the storm event. This documentation is what your insurance carrier needs to process a claim, and it is what protects you from having your claim denied on the grounds that damage was pre-existing. Without a post-storm inspection report, you have no defensible timeline.

Second, early identification of hail damage allows for targeted repairs while they are still straightforward. A puncture repaired within weeks of a storm is a minor expense. That same puncture, left through a Florida wet season, becomes a moisture intrusion event that may require membrane replacement, insulation removal, and deck repair.

Third, consistent documentation of your roof's condition over time builds a maintenance record that strengthens your position with both your insurance carrier and with warranty providers. Manufacturers and insurers alike respond better to property owners who can demonstrate proactive stewardship of their assets.

What 4 Star General Contracting Does After a Hail Event

Our post-storm inspection process is systematic and thorough. We document hailstone impact patterns, measure affected areas, photograph representative damage across all roof sections, and assess all penetrations, flashings, and edge metal components. Where the damage warrants, we deploy infrared or nuclear moisture scanning to determine whether water has already infiltrated beneath the membrane.

We work directly with insurance adjusters and can provide the technical documentation needed to support your claim. We also carry out repairs under the same quality standards we apply to new construction, using manufacturer-approved materials and methods that preserve your warranty coverage.

If your property was in the path of a storm this season, do not assume your roof came through unscathed. Contact our team for a post-storm evaluation and protect your investment before the next rain event finds the vulnerabilities that the hail left behind.