Controversies can arise with regard to how asphalt
shingles are damaged by hail, and what hail damage
actually looks like. More specifically, questions have
been raised as to whether granules removed from
asphalt shingles during a hailstorm will reduce the
expected life or water shedding ability of the roof
shingles. In this paper, the authors will review the
definition of hail damage to asphalt shingles and
explain the characteristics of such damage. We also
will present the results of our ten-year study on
granule loss to asphalt shingles as well as review the
methodology to assess hail damage to an asphalt
The authors have inspected thousands of asphalt
shingle roofs and found that damage inspectors
frequently mistake various shingle anomalies such as
foot scuffs, adhesive spots, etc., as hail damage. We
have also inspected numerous roofs where people
have tried to simulate hail damage by using a variety
of tools or other objects in order to defraud an
insurance carrier. Therefore, the last part of this
paper will focus on various shingle anomalies that are
frequently misidentified as hail damage and explain
how to differentiate between intentional and
unintentional roof damage.
2. ASPHALT ROOF SHINGLE COMPONENTS
Asphalt roof shingles are one of the most common
and affordable roof coverings on the market today.
Base mat materials are either paper (organic) or
glass-fiber (inorganic). The mats are coated with an
asphaltic mixture composed of asphalt, limestone
powders, and other mineral stabilizers (fillers).
Granules are applied to the shingle surfaces to give
them color, add weight, and to block the underlying
asphalt from deleterious effects of the sun. Most
granules are crushed stone coated with a ceramic
material. The ceramic gives color to the granules.
Generally, one third of the shingle weight is granules,
one-third asphalt, and one-third filler. The mat is a
small fraction of the total weight.
Asphalt roof shingles come in various sizes,
shapes, and thicknesses. Generally, the thicker or
heavier the asphalt shingle, the more it costs. The
most common asphalt shingles are three-tab and
Three tab shingles contain slots or joints that give
the appearance of a common brick pattern when
installed on the roof. Laminated type shingles are
comprised of one full shingle and half a shingle
bonded together with asphalt to give them thicker
look, similar to that of wood shingles or slate.
Corresponding author address: Timothy P. Marshall,
2455 McIver Ln., Carrollton, TX 75006. Email:
2. HAIL DAMAGE DEFINITION
Morrison (1999) defined damage to roofing as a
diminution of water-shedding capability or a reduction
in the expected long-term life of the roofing material.
Marshall and Herzog (1999) more specifically defined
functional hail-caused damage to asphalt shingles as
punctures, tears, or fractures (bruises) in the shingle
mats (Figure 1).
Figure 1: Hail damage to asphalt shingles: a) broken edges,
b) bruise, c) puncture,
d) torn edge.
Shingle bruises are an indentation
with fracture in the mat that feels soft like that of an
apple bruise. The bruise is usually obvious as
granules are also dislodged from the impact area
exposing the asphaltic mat.
Marshall et al. (2002) presented their ice impact
test results that employed a mechanical launching
device. Ice stones were launched at standard
velocities against roofing products that included
various 11-year-old, naturally aged, asphalt shingles.
Impacts were oriented perpendicular to the shingles.
The study concluded that aged organic mat-based
asphalt shingles were damaged half of the time by
one-inch diameter ice stones, whereas it took 1.25 in.
(3.1 cm) diameter ice stones to damage the aged
glass-fiber mat asphalt shingles. Thicker, aged
laminated type shingles were damaged by 1.5 in. (3.8
cm) ice stones. Greenfeld (1969) and Koontz (1991)
had presented similar results in conducting ice ball
impact tests on asphalt shinglesHowever, there remains a controversy whether
granules removed by hail, without visible asphalt
exposure, constitutes hail damage. Many asphalt
shingle manufacturers have issued "technical
bulletins" about hail and granule loss, stating that if
granules are lost from the shingle due to hail, the
shingle has lost life. However, there are no published
scientific studies to validate this statement.
3. GRANULE LOSS STUDY
In order to determine how many granules, if any,
must be removed in order to affect the service life or
water shedding ability of the shingle,
Figure 3: Same view as Figure 2 only at the conclusion of the ten-year study
Figure 2: Test panel at the beginning of the study showing percentage of granules removed. "C" is the control.
the authors' firm
conducted a granule loss study on asphalt shingles.
Varying quantities of granules were removed with a
wire brush from new, three-tab, glass-fiber mat
shingles. The shingles then were exposed naturally
to the weather in Dallas, Texas for a period of ten
years. The quantities of granules removed were
none (control), and approximately 6, 15, 45, and 70
percent of the total granules on the shingles. Another
shingle was installed upside down such that the
asphaltic mat was exposed to the weather. The
shingles were installed conventionally over a plywood
deck on a 4:12 pitch that faced south. The shingles
were examined at intervals throughout the ten year
period as well as at the conclusion of the study
(Figures 2 and 3).
In one year, the exposed asphalt had oxidized
grey but there was no visible evidence of surface
cracks or erosion. After five years, areas of exposed
Figure 4. Close up views of new shingle and tenyear weathering with both 70 percent of the granules removed and no granules, respectively.
asphalt had oxidized but this did not affect the
function of the shingles to shed water. Surface
erosion was visible on the shingle without granules,
and some of the glass fibers had become exposed.
After ten years, no significant change was noted in
the shingles except for the shingle without granules.
More glass fibers were exposed on this shingle due to
erosion; but, the shingle continued to shed water
The quantity of granules lost from the roof shingles
during a hailstorm is a relatively small amount.
Generally, about one-third the weight of an asphalt
shingle is granules such that a 25 square roof
covered with three-tab shingles would have about
one ton of granules. Granule loss is expected from
the moment shingles are manufactured, shipped,
installed, and during the weathering process.
Granules are part of the wearing surface on the
shingle and exposure to hail is part of the wearing
process that is actually built into the design. Thus,
more granules are initially placed on the shingles
than needed to cover the mat.
In our study, we found between 12 to 15 percent
of the surface granules had to be removed from new
shingles before the asphaltic mat was exposed. The
amount of "excess" granules on new shingles varied
by plus or minus ten percent. We would expect that
the quantity of granules lost during a hailstorm
generally would fall within the normal variation of
granules placed on a shingle.
Therefore, it seems logical to conclude that the
small quantities of granules removed from shingles
during a hailstorm does not shorten the life of the roof
or adversely affect its water shedding ability as long
as the impacted areas are not bruised or punctured,
and remain covered with granules. This conclusion
agrees with the work done by Morrison (1999).
4. ASPHALT SHINGLE ANOMALIES
There are usually a number of anomalies on an
asphalt shingle roof not related to hailstone impact.
Some of these anomalies may take rounded forms
that can be mistaken as hail damage. Understanding
how shingles are manufactured, installed, and
weather is important when properly differentiating
between non-hail conditions and hail damage.
Figure 5. Various shingle manufacturing defects:
lack of granule adhesion on a three-tab,
c) asphalt exposed in lower laminate
d) lines of missing asphalt and granules.
Asphalt shingles are manufactured in a high
speed, fully automated process. Occasionally,
certain defects involve insufficient granule or asphalt
coverage, or the use of poor quality asphalt. Shingle
manufacturers should "cull" or remove such defects
before the shingles are shipped. However, the level
of quality control of shingle products varies. Thus, it
is not unusual to find shingle defects on a roof that
involves bands or spots of missing asphalt and/or
granules (Figure 5).
As asphalt shingles age, their components break
down. The extent of aging depends upon many
factors including the quality of the asphalt, shingle
color, roof pitch, slope direction, and attic ventilation.
Common deficiencies inherent with aged asphalt
shingles are blistering, splitting, cupping, clawing,
crazing, and flaking. In many instances, these
anomalies are not discovered until after a hailstorm;
however, this does not mean they were created or
aggravated by the storm
(Figures 6 and 7).
Shingle blisters occur from a combination of poor
quality asphalt combined with heat. These blisters appear as
small bubbles in the shingle surfaces where a portion
of the granule surface is raised. Eventually, the
shingle bubbles rupture exposing steep-sided voids in
the shingle surfaces that frequently extends down to
the shingle mat. Shingle blisters are usually 1/4 in.
(.6 cm) in diameter or less and are not caused by
Figure 8. Shingle installation deficiencies: a)
b) edge scuffing, c) elevated fasteners, d) adhesive spots.
Figure 7. Various shingle anomalies not caused by
hail: a) cupping, b) clawing, c) crazing, d) flaking.
Figure 6. Various shingle anomalies not caused by
a) closed blisters,
b) open blisters including
close-up view in the inset photograph,
d) horizontal splitting.
Diagonal and horizontal splitting of asphalt
shingles involves a combination of asphalt shrinkage,
deck movement, and low tensile strength in the mat.
Ribble et al. (1993) further explain such problems
with asphalt shingles.
Cupping and clawing results from asphalt
shrinkage on the top and bottom surfaces of the
shingles, respectively. The corners and edges of the
shingles are prone to curling or cupping as the mat
shrinks. Crazing of the shingle surfaces also results
from asphalt shrinkage. Eventually, chunks of
granules flake away from the mat leaving the asphaltcoated
mat exposed to the weather.
Additional shingle anomalies can be created
during installation. The most common shingle
installation deficiencies are marring, edge scuffing,
elevated staples, and adhesive spots (Figure 8).
Shingle marring occurs when people walk across
the roof on a day when the shingles are hot, soft, and
pliable. The asphalt in the shingle surface softens to
the point where it is pushed aside along with the
granules and typically forms a ridge on the outside
edge of the mark. Persons walking on the roof can
also remove granules along the bottom edges of the
Elevated fasteners can occur during the
installation of the shingles and can protrude through
or buckle the overlying shingles. The fasteners are
either not driven flush to the shingle or are driven into
joints between the roof decking. Elevated fasteners
are not caused by hail striking the roof.
Adhesive can drip off the shingles onto the other
shingles leaving a round, dark spot that can be
mistaken by some as hail damage. If the adhesive
from one shingle contacts another shingle and bonds
to it, a portion of the shingle surface can be removed
when the shingles are separated leaving a rounded
area of missing granules that can also be mistaken
by some as hail damage.
5. ASSESSING / INSPECTING FOR HAIL DAMAGE
Accurate assessment of hail damage is a step by
step process that involves an examination / Inspection
of the roof
shingles as well as other objects on and around the
An example of 9-year-old hail damage to a
glass-fiber mat asphalt Roofing shingle.
Marshall and Herzog (1999) presented a
methodology on how to quantify hail damage to a roof
through the use of test squares. The number of hail
damaged shingles are counted in each test square on
each directional roof slope and that number
determines whether the roof slope is repaired or
replaced through the use of the DURA formula.
Shingles are particularly susceptible to hail
damage if they have little or no underlying support,
especially along ridges, rakes, eaves, and valleys.
Shingle edges also are vulnerable to being chipped
or broken. Therefore, the entire roof must be
Recently exposed asphalt appears black
or unweathered,whereas asphalt exposed for several
months oxidizes forming a surface film that is a grey
This color difference is one way to tell new hail
damage from old hail damage.
6. CAUSING INTENTIONAL HAIL DAMAGE
Figure 11. Attempts to simulate hail damage with a ball peen hammer.
Figure 10. Intentional roof damage examples:
a) line of impact marks,
b) circular arrangement of impact marks
c) impact marks perpendicular to each affected slope with no marks on the ridge.
On occasion, some people have utilized various
tools or other objects in an attempt to simulate hail
damage on a roof. Popular items have included: 1)
ball peen hammers, 2) claw hammers, 3) coins, and
4) screwdrivers. The authors have recognized a
number of factors that distinguish intentional roof hail damage
from real hail damage. For example, intentional roof hail damage
is not randomly distributed on the roof but usually
occurs in groups or lines concentrated in upper
portions of the roof, away from roof edges.
Impact angles of the tool or object are nearly perpendicular
to the affected roof slope, therefore indicating multiple
impact directions (Figure 10). In contrast, hail would
leave a random distribution of damage on the roof.
The windward slope typically sustains the most
concentrated and direct hail impacts whereas the
leeward slopes have fewer, glancing hail impacts.
Intentional damage is concentrated frequently in
the interior or center portions of the shingles, away
from shingle edges, as it is human nature to hit the
center of an object. Such centered impacts usually
are found on each affected slope, regardless of slope
direction (Figure 11).
Close-up views of mechanically caused impacts to shingles using:
a) a ball-peen hammer,
b) claw hammer,
d) quarter coin.
The impacts tend to be singular occurring once per shingle. In contrast, hail does not
prefer the centers of the shingles nor strike shingles
once consistently. In our inspections of suspicious roof damage, we utilize a series of
magnification rings to closely photograph the impact marks. Typically, shingles
struck by metal objects will have broken or shattered the ceramic coating on the
granules. This will leave a "powder" residue containing shattered ceramic
material within the impact mark. Any side-to-side or "rounding out" motions will
tend to leave swirl marks within the powder residue and/or leave smudges in the
exposed asphalt surface (Figure 12). When claw hammers are utilized, the metal
peen frequently does not strike the roof slopes exactly perpendicular but tends
to tilt forward slightly leaving a characteristic curving fracture in the
shingles that opens towards the direction of impact. The concave fracture in the
shingle resembles a "frowny" face when looking upslope. Granules closest to the
concave side of the fracture are frequently compressed uniformly into the
shingle mat. Sometimes coins are utilized to leave small divots in the shingle
surface. Quarter coins have small ridges around their perimeters that can leave
a series of ridges in the asphalt under magnification.
7. DOCUMENTING INTENTIONAL DAMAGE
The authors have developed a methodology to
better document intentional damage to a roof. The procedure involves examining objects around and on
the house similar to the hail damage inspection
protocol as explained by Marshall and Herzog (1999).
Hail-caused "spatter" marks are usually found on
faded metal surfaces such as air conditioners,
electrical junction boxes, and metal window frames.
Hail-caused "scuff" marks are recorded on wooden
fences and dents occur in aluminum fins on air
conditioners. Such items provide good estimates of
hail size and direction of hailfall.The same can be
said with the examination of metal items on the roof.
Thus, items on and around the house can provide
evidence of hail size and direction that should match
the size and direction of any alleged hail damage to
the roof shingles.
Roof plan diagram showing the distribution of mechanically-caused impact marks in an alleged hail damage claim.
After a general examination of the building surroundings is performed, a roof
plan diagram is drawn and shingle marks are plotted. Any pattern or grouping of
shingle marks quickly becomes apparent in the diagram (Figure 13). The diagram
will indicate those roof slopes or ridges that are notably without shingle marks
as well as any grouping of marks. Usually, large areas of the affected roof
slopes are without shingle marks as are other, perhaps smaller, roof slopes that
face the same direction. Shingle marks closest to the roof edge are measured.
In this paper, we have explored certain issues with
regard to hail damage on asphalt roof shingles. The
results of our ten-year granule loss study were
presented where it was found that there was no loss
of life or reduction of water shedding ability even with
70% of the granules removed from the glass-fiber
mat shingles. The shingle with 100% of the granules
removed did exhibit more erosion than the other
shingles. Therefore, asphalt roof shingles that lose
some granules during a hailstorm are not considered
damaged as long as the shingles remain covered
with granules. Functional damage to asphalt roof
shingles includes punctures, tears, or fractures
(bruises) in the shingle mats.
We also have shown there are a number of
anomalies on asphalt shingles that occur during
manufacturing, installation, and weathering. Some of
these anomalies take on rounded forms that can be
mistaken by some as hail damage. We also have
discussed how to recognize intentional damage to
asphalt roof shingles where someone attempts to
simulate hail-caused damage. A methodology was
presented to better document intentional damage to a
The authors would like to thank our reviewers:
Greenfeld, S.H., 1969: Hail resistance of roofing
products,Building Science Series #23,National
Bureau of Standards, 9 pp.
Koontz, J.D., 1991: The effects of hail on residential
roofing products, Proc. of the Third International
Symposium on Roofing Technology, NRCA/NIST,
Marshall, T. P., and R. F. Herzog, 1999: Protocol for
Assessment of Hail-Damaged Roofing, Proc. of the
North American Conf. on Roofing Technology,
Toronto, Canada, p. 40-46.
Marshall, T.P, R.F. Herzog, S.J. Morrison, and S.R.
Smith, 2002. Preprints, 21st Conf. on Severe Local
Storms, San Antonio, TX, Amer. Met. Soc., 95-98.
Morrison, S.J., 1999: Long-Term Effects of Hail on
Asphalt Composition Shingles Proc. of the North
American Conf. on Roofing Technology, Toronto,
Ribble, R., D. Summers, R. Olson, and J. Goodman:
From generation to generation: issues and problems
facing the steep-slope roofing industry. Proc. of the
10th Conf. on Roofing Technology, 1-5.