Hail Damage Thresholds: What Size Hail Actually Damages Your Roof

When an insurance company sends an engineer to inspect your roof after a hailstorm, there is a good chance that engineer works for Haag Engineering. Haag is the most widely used forensic engineering firm in the property insurance industry. Their inspectors examine thousands of roofs every year, and their reports are routinely used to justify claim decisions — including denials.

What most homeowners do not know is that Haag's own researchers published a study establishing the minimum hail sizes needed to cause functional damage to common roofing and siding materials. These thresholds are based on controlled testing and field observation. They are the same benchmarks Haag trains its own field inspectors on. And they often contradict the conclusions in the very reports Haag produces for insurance companies.

The Damage Thresholds

In 2002, Haag Engineering researchers presented their findings at the 21st American Meteorological Society Conference on Severe Local Storms. They tested and documented the minimum hail sizes required to cause functional damage— meaning damage that compromises the material's ability to do its job — for the most common roofing and siding materials in North America.

Look at the first row. Standard 3-tab asphalt shingles — the most common roofing material in America — can sustain functional damage from hail as small as one inch. That is quarter-sized hail. When your insurer tells you quarter-sized hail cannot damage your roof, Haag's own published research says otherwise.

“Functional Damage” vs. “Cosmetic Damage”

This distinction is at the heart of many hail claim disputes. Insurance companies increasingly argue that hail damage is “cosmetic only” — meaning it affects the appearance of the roof but does not compromise its performance. Some policies now include cosmetic damage exclusions that allow the insurer to deny claims for dents, bruises, and granule loss that do not immediately cause leaks.

The Haag study defines functional damageas damage that compromises the material's ability to shed water and protect the structure — even if the damage does not look dramatic to an untrained eye. A shingle with a fractured mat beneath the surface may still appear intact from a distance. But the fracture allows water infiltration, and the shingle will fail prematurely.

This is important because many hail damage denials are based on a visual inspection from the ground or a brief walk on the roof. Functional damage often requires close inspection, including pressing on the shingle to feel for mat fractures beneath the granule surface. An inspector who does not perform this test may miss the damage entirely.

Impact Angle Changes Everything

The Haag researchers noted that the angle at which hail strikes the roof significantly affects whether damage occurs. In a real thunderstorm, hail rarely falls straight down. Wind drives the stones at an angle — sometimes a steep angle. This is called “windblown hail,” and it is the norm, not the exception.

When hail hits at an angle, the force is concentrated differently than a vertical drop. Depending on the angle and the roof slope, an angled impact can be more damaging than a direct vertical strike because the hailstone may slide and gouge the surface rather than bouncing cleanly. This means that the threshold sizes in the table above are based on controlled vertical drops— in a real storm with wind, damage can occur from hailstones even smaller than the listed thresholds.

Using the Carrier's Own Expert Against Them

Here is where this research becomes a powerful tool for homeowners. When an insurance company hires Haag Engineering to inspect your roof, and the Haag engineer reports that the hail was “not large enough to cause damage,” you can ask a pointed question: does that finding align with Haag's own published damage thresholds?

If the weather data shows quarter-sized hail (one inch) and your roof has 3-tab shingles, Haag's own research says damage is possible. If the engineer's report says no damage occurred, the engineer needs to explain the discrepancy — not just say “I didn't see any.”

The same logic applies to aged shingles. Older shingles have less granule adhesion, more brittle mats, and reduced flexibility. The thresholds published by Haag apply to shingles in reasonable condition. A twenty-year-old shingle may sustain functional damage from hail even smaller than the published minimums.

Why Insurers Rarely Mention This Research

Haag Engineering is the insurance industry's go-to firm for forensic roof inspections. Their reports carry significant weight in claim decisions, appraisals, and litigation. But when a Haag report favors the insurer, the insurer rarely mentions that Haag's own published research may support the homeowner's position.

This is not a conspiracy. It is simply how adversarial claim handling works. The insurer presents the evidence that supports their position. It is your job — or your public adjuster's job, or your attorney's job — to present the evidence that supports yours. And Haag's own threshold research is some of the strongest evidence available, precisely because it comes from the firm the insurer chose to hire.

About This Research

The findings discussed in this article are based on the paper: “Hail Damage Threshold Sizes for Common Roofing and Siding Materials” by Timothy P. Marshall, Richard F. Herzog, Scott J. Morrison, and Steven R. Smith (Haag Engineering Co., Dallas, Texas). The paper was presented at the 21st Conference on Severe Local Storms, hosted by the American Meteorological Society, in 2002.

Haag Engineering has been providing forensic engineering services to the property insurance industry since 1924. Their damage threshold research remains one of the most widely referenced studies in hail damage assessment and is used by engineers, adjusters, and attorneys on both sides of claim disputes.

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.