HAIL DAMAGE TO ROOF SHINGLES
By
Christian McGarry
Hail forms when frozen water drops are lifted in
turbulent wind regimes during thunderstorms. The frozen drops of water
increase in size and eventually fall to earth as hail having been
driven by a combination of gravity and wind forces. Hail stones vary in
size from pea size (1/4 inch diameter, little roof damage), through
marble size (3/4 inch in diameter, threshold damage to roof materials)
to golf ball size (11/2 inch in diameter, typically severe damage to
roofing materials). Hail size distributions tend to be localized with
some roofs damaged, while others are not. Wind direction plays an
important role, as well as roof pitch. A direct impact of hail on a
shingle is more damaging than that of a glancing blow. Figure 1a
illustrates the dynamics of
Figure 1a
hail impact on two different roof pitches. The home on the
left would sustain more roof damage than that on the right, because of
the more direct impact on the left as opposed to the less damaging,
glancing, blow to the right. Wind conditions can distort damage to a
home as illustrated in Figure 1b. The home on the left has more
glancing blows to the roof from the
Figure 1b
wind, while the windward wall comes under attack from
direct hail impact. The right wall is protected and undamaged from the
hail. The home on the right will more likely sustain roof damage on the
windward side because of the more direct impact. Assessing hail
damage is often accomplished by a roof inspection, which usually occurs
several days to several months after the hail event. Determination of
whether hail actually fell at a site can be made through statements and
weather reports. Inspection of thin, aluminum fixtures helps verify
hail impact. Figure 2a shows denting on a roof vent cap indicating that
hail had struck the vent at one time.
Figure 2a
Figure 2b
Figure 2b shows denting on a flue vent indicative of hail
impact at one time. The denting on these two fixtures does not impede
their function.
Figure 2c
Figure 2c shows deformation to condensing unit fins on an air
conditioning system, as a result of a driving hailstorm. This damage
does impair operation of the unit, in that air flow through the coils
is reduced, causing higher refrigerant temperature, lost performance
and lost reliability.
Figure 3 shows shredded and pock-marked paint from a driving hailstorm.
The base wood was undamaged but repainting was required.
Figure 3
Figure 4
Figure 4 shows mild impact marks on a cedar shingle,
consistent with hail impact. These marks will typically dissipate over
time with virtually no effect on shingle life. Figure 5 shows more
severe shredding from hail impact. If the impact marks are causing
severe splits, then shingle life will most likely be adversely
affected.
Figure 5
Figure 6 shows impact damage to siding as a result of wind
driven hail. Some shredding and minor splitting has occurred but the
siding appears structurally sound.
Figure 6
Figure 7
Hail damage to asphalt shingles includes severe granule loss, material
removal at the edges of the shingles and penetration. Figure 7 shows
grit loss from asphalt after pounding from a hailstorm. New asphalt
shingles are more resistant to hail impact than older shingles since
the asphalt becomes more brittle with age. Warmer shingles are more
compliant and more resistant to fracture from hail when compared with
colder shingles. Shingles rely on a structurally sound substrate for
hail damage resistance. Flexible roof sheathing allows the shingles to
flex during impact causing damage, while more rigid roof sheathing
supports the shingle during impact resulting in less shingle damage.
In recent years, roof shingle manufacturers have been listing products
that conform to Underwriters Laboratories test standard UL 2218, which
classifies the resistance of a particular product to hail damage. The
test entails dropping steel balls on samples and observing the damage. A
Class 4 rating is the most resistive to hail damage, while the Class 1
rating is the least resistive. Some insurance companies are reducing
premiums when Class 4 shingles are used. The roofing industry takes
issue with some of the testing methods, indicating that shingle aging
and thermal environments are not taken into account.
Most hail related damage is cosmetic and does not affect the useful
life of roof shingles. In other cases severe wood splitting,
significant granule loss, penetration of the shingle and fracture may
require shingle replacement.
