Re-evaluating Sub-Severe Hail Effects on Asphalt Shingles: A Technical Response to Forensic Interpretations of Damage by Raymond Gessner, P.E.
1. Purpose and Scope
This paper evaluates the interpretation of sub-severe hail effects on asphalt shingle roofing
systems in light of recent experimental research and subsequent forensic position statements.
Specifically, it examines whether restrictive definitions of “damage,” as presented in certain
forensic engineering interpretations, accurately reflect the current body of experimental evidence
regarding cumulative material degradation.
This paper argues that restricting damage to conditions involving immediate functional failure is
inconsistent with established engineering principles and does not reflect experimentally observed
material behavior under cumulative hail exposure.
Recent peer-reviewed research demonstrates that repeated exposure to sub-severe hail,
particularly when combined with natural weathering, can result in measurable physical changes
to asphalt shingles, including granule loss and reduced resistance to subsequent impacts. In
contrast, certain forensic interpretations maintain that such changes do not constitute damage
unless they produce immediate functional impairment, such as rupture or water intrusion.
The purpose of this paper is not to dispute the existence of established forensic evaluation
criteria, but to assess whether those criteria fully capture the physical and mechanical behavior of
asphalt shingle systems under cumulative environmental loading. This discussion is limited to
engineering principles, material behavior, and interpretation of experimental data, and does not
address insurance policy language or claims practices.
2. Background and Existing Research
Asphalt composition shingles are the most widely used residential roofing material in North
America. Their performance is influenced by environmental exposure, including ultraviolet
radiation, thermal cycling, moisture, and mechanical impacts such as hail. Historically, hail
damage has been associated with impacts from larger hailstones capable of producing immediate
functional impairment. Smaller hailstones have generally been considered unlikely to produce
such effects in isolation.
However, asphalt shingles are not static materials. Over time, natural weathering leads to
oxidation, embrittlement, and progressive loss of protective granule surfacing. These changes
alter the mechanical response of shingles and influence their resistance to impact loading.
Recent experimental research conducted by the Insurance Institute for Business and Home Safety
investigated the cumulative effects of sub-severe hail. The study incorporated repeated impacts
from smaller hailstones, natural weathering between impact sequences, and subsequent exposure
to larger hail impacts. The results demonstrated measurable increases in granule loss,
amplification of degradation due to weathering, and reduced resistance to later impacts.
These findings indicate that sub-severe hail contributes to progressive material degradation
rather than immediate failure.
In contrast, forensic engineering interpretations, including those presented by Haag Engineering,
emphasize that damage should be defined by demonstrable impairment of water-shedding
capability or serviceability. These interpretations acknowledge the physical mechanisms
observed in experimental research but conclude that such changes do not constitute damage
unless they result in immediate functional consequences.
The distinction between these positions is not based on disagreement about observed physical
phenomena, but on how those phenomena are defined and interpreted within an engineering
framework.
3. Engineering Definition of Damage and Application to Asphalt Shingles
In engineering and materials science, damage is understood as a physical alteration that degrades
performance, durability, or expected service behavior. Failure represents one endpoint of damage
progression, but it is not the only meaningful manifestation. Materials commonly undergo
measurable deterioration long before complete loss of function occurs.
Examples across engineering disciplines illustrate this principle. Fatigue cracking in metals,
microcracking in concrete, corrosion in steel, and oxidation in polymers are all recognized as
forms of damage even when systems continue to function. The continued ability of a system to
perform its intended function at a given moment does not negate the presence of underlying
degradation.
Asphalt shingles should be evaluated within this same framework. A shingle may remain
watertight while having undergone physical changes that reduce impact resistance, accelerate
aging, or shorten service life. Limiting damage to conditions involving immediate leakage or
visible rupture excludes significant stages of material deterioration.
A critical distinction must be made between functional failure and degradation. Roofing systems
are intended to provide durable performance over time, not merely to remain watertight at a
single point of inspection. A condition that measurably reduces durability or performance
capacity represents a meaningful physical change, regardless of whether terminal failure has
occurred.
Granule surfacing plays a central role in this evaluation. Granules are not merely aesthetic; they
serve as a protective layer that shields the asphalt coating from ultraviolet radiation, reduces
thermal degradation, and contributes to overall durability. When granules are displaced, the
underlying asphalt becomes more susceptible to accelerated aging and embrittlement. Granule
loss therefore represents a change in a protective system, not a purely cosmetic condition.
It is acknowledged that some level of granule loss occurs during manufacture, installation, and
natural weathering. However, the relevant engineering question is not whether granule loss
exists, but whether a specific event produces measurable displacement beyond expected baseline-conditions
in a way that alters performance. Experimental findings demonstrate that repeated sub-severe
hail impacts can produce such increases, particularly when combined with environmental aging.
This distinction clarifies the difference between abstract susceptibility and physical degradation.
A system may be more vulnerable due to age or exposure, but once measurable physical
alteration has occurred, the condition is no longer theoretical risk. It is a degraded material state
attributable to specific loading conditions.
Accordingly, damage in asphalt shingle systems should be defined as a hail-caused physical
alteration that measurably impairs performance, reduces durability, shortens expected service
life, or increases vulnerability to subsequent loading beyond ordinary aging alone. This
definition includes both immediate failure and progressive deterioration where measurable
evidence of material change exists.
4. Evaluation of the Haag Critique of IBHS Methodology
The methodological critiques raised regarding experimental research on sub-severe hail effects
focus primarily on variable isolation, measurement techniques, and interpretation of results.
These critiques identify certain limitations but do not invalidate the observed relationships
between cumulative exposure and material degradation.
The experimental design incorporated both environmental weathering and repeated impact
loading. While these variables were not fully isolated in a factorial sense, the combined approach
reflects actual service conditions in which roofing systems experience both aging and repeated
impacts over time. The absence of complete isolation does not negate the observed trends linking
cumulative exposure to increased degradation.
Granule loss was used as a primary metric for evaluating physical change. Although no
definitive threshold linking granule loss to immediate failure was established, the study provides
measurable and repeatable data demonstrating increased loss under cumulative impact
conditions. The absence of a precise failure boundary does not invalidate the relevance of the
measurement itself, as many forms of material degradation are quantified without a single
deterministic threshold.
The use of subsequent large-hail impacts serves as a performance validation step rather than a
standalone damage mechanism. The key finding is that prior exposure alters the response of
shingles to later loading, indicating that earlier cumulative effects influence overall performance.
Measurement techniques involving image processing introduce potential variability, but the
methodology was applied consistently across all specimens and acknowledged as a limitation.
No contradictory data has been presented demonstrating that the results are materially inaccurate.
Differences in hail representation between studies reflect variations in modeling approach rather
than direct contradiction. The incorporation of repeated impacts and environmental exposure
represents an expansion of traditional testing methods toward more realistic service conditions.
Overall, the experimental findings demonstrate consistent relationships between cumulative
exposure and material degradation. The critiques raised identify areas for further refinement but
do not undermine the validity of the core observations.
5. Implications for Engineering Evaluation
The interpretation of sub-severe hail effects has direct implications for how asphalt shingle
systems are evaluated following hail events. A framework limited to immediate functional
failure does not fully capture the progressive nature of material behavior under repeated
environmental loading.
Engineering evaluation should distinguish between pre-existing condition and additional
physical change resulting from a specific event. While aging processes produce baseline
deterioration, event-driven impacts may accelerate that process in measurable ways. Where
physical changes can be attributed to a hail event and are shown to influence performance or
durability, they represent meaningful alterations to the system.
Performance-based considerations provide a more comprehensive understanding of roof
condition. These include reduced resistance to future impacts, accelerated aging mechanisms,
and changes in material properties such as brittleness and energy absorption capacity. Such
changes may not be immediately visible as functional failure but are indicative of altered
material behavior.
A consistent evaluation framework should recognize both immediate failure and progressive
degradation, distinguish between normal aging and event-driven deterioration, and incorporate
both visual and performance-based indicators. Restricting damage recognition to visible rupture
or leakage introduces a discontinuity between observed material behavior and evaluation criteria.
6. Conclusion
Experimental research demonstrates that repeated sub-severe hail exposure, particularly when
combined with natural weathering, can produce measurable physical changes in asphalt shingles,
including increased granule loss, accelerated degradation, and reduced resistance to subsequent
impacts. These findings establish that sub-severe hail contributes to progressive material
deterioration.
Forensic interpretations that limit damage to immediate functional impairment do not refute
these observations but instead apply a narrower definitional framework. The resulting
disagreement is therefore not about whether degradation occurs, but whether it is recognized as
damage.
From an engineering standpoint, measurable, performance-affecting material change constitutes
damage regardless of whether failure has yet occurred. A framework that includes both
immediate failure and progressive degradation provides a more complete and technically
consistent understanding of asphalt shingle behaviour under cumulative environmental loading.
Accordingly, evaluation of hail effects should consider not only visible failure but also
measurable physical alteration, deviation from expected aging, and impact on performance and
service life. Recognizing these factors allows for a more accurate assessment of material
condition and aligns damage evaluation with established engineering principles.
Schedule a Professional Evaluation
If your property has experienced a hailstorm, a comprehensive engineering assessment can determine whether your roofing system has undergone measurable degradation that may not yet be visible.
A qualified structural inspection professional can evaluate the roof framing, load capacity, and overall stability of the structure. Call A Step in Time Structural Engineers at (605)-467-7328 to schedule a forensic structural inspection of your
