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Hailstones impacting a roof result in primarily two types of damage – aesthetic
and functional. Aesthetic damage, which affects only the appearance of the
roof, is by far the most common type of damage from hailstorms. For asphalt
shingles, this is usually in the form of negligible loss of granules, which
will have minimal impact on the life of the shingle. For other roof coverings,
the aesthetic damage may include discolorations and/or dimples. While neither
of these scenarios is very attractive to building owners, the result is minimal
to no impact on the life expectancy of the roof covering.
Functional damage affects the expected performance characteristics of the roof.
This type of damage is defined as the loss of water-shedding ability or a
reduction in the expected service life of the roof. Functional damage levels
will vary from roof to roof depending on the type and age of the roof covering
material.
Identifying Hail Damaged Roofs
The majority of roof coverings for residential roofs are asphalt shingles, wood
shingles and shakes, and roof tiles. In fact, asphalt shingles are used on more
than 80% of residential roofs. The following sections examine hail damage to
various types of roof coverings that has been documented from field
observations.
Asphalt Shingles
Although standard asphalt shingles generally perform well under a variety of
weather conditions, hailstones impacting an asphalt shingle roof covering
influence the expected performance of this roof covering more than any other
roof covering. Damage to asphalt shingles from hailstones is typically one of
two modes – rupturing of the reinforcing mat and loss of granules that exposes
the underlying bitumen. Rupturing of the reinforcing mat represents a potential
loss in the shingle’s water-shedding ability in that a ply of roof covering is
removed by the rupture. The loss in water-shedding ability increases the
potential for water to reach the roof fasteners, causing corrosion, or the
butted joints in the sheathing, permitting water to enter the interior of the
building. The loss of granules represents the potential for a reduction in the
expected service life of the shingle.
Visible identification of rupture of the mat includes bruising or puncturing. A
bruise on a glass fiber composition shingle from hail impact is shown in Figure
8. More substantial damage of asphalt shingles where significant loss of
granules has occurred, exposing much of the underlying bitumen, is shown in
Figure 9.
Figure 8 Burise from Hail Impact
(Courtesy Haag Engineering)
Figure 9 Significant Loss of Granules
(Courtesy William Mason, Second Opinion Home Inspection, ASHI)
Field observations and some laboratory experiments tend to indicate that for
bruising or puncturing of asphalt shingles to occur, hailstones must be 1 inch
in diameter or larger. However, for older shingles that show some deterioration
or weathering, or those that are not effectively supported (flexible deck),
hailstone sizes as small as ¾ inch in diameter have been documented to cause
damage. These are general ranges, and as discussed previously, there are
factors other than hail size that also affect the level of damage to the roof
covering.
Wood Shingles and Shakes
Wood shingles and shakes are damaged by hailstones typically by split or
puncture in the wood resulting from the impact of the hailstone. Figure 10
shows a split in a cedar shake from an impact from a hailstone. The impact mark
is readily identifiable by the circle inscribed around the mark.
Figure 10 Hail-caused split in a cedar shake
(Courtesy Haag Engineering)
A wood roof that has been impacted by hailstones is typically
easily recognized. As the hailstones impact the surface, impact marks, where
gray-colored oxidation and organic surface growths have been removed, are
recognizable new features in the roof’s appearance. However, impacts to wood
roofs that do not split or puncture the wood are not considered to cause
damage. Studies have shown that wood shingles and shakes that are impacted by a
hailstone and do not split immediately upon impact are not prone to future
splitting.
Tests conducted on wood roofs generally demonstrate that hailstones
1 ¼ inches in diameter and larger will split wood shingles that are not in good
condition. Hailstones 1 ½ inches in diameter and larger are required to split
wood shakes in good condition. [Haag Engineering]. However, older shakes and
shingles, or ones that have been severely affected by weathering, are more
vulnerable to damage from hailstones that are smaller.
Hail Damage Assessment
When analyzing a roof for hail damage, it is important to
differentiate between damage caused by hail and damage caused by natural
weathering. Additionally, manufacturing defects, damage during construction,
and/or damage occurring during transportation can often look much like hail
damage. A detailed description distinguishing between hail-induced and natural
weathering damage is outlined in the paper titled Protocol for Assessment of
Hail-Damaged Roofing, by Haag Engineering.
A roof should be inspected in a manner that permits an intimate and
careful examination of the roof system. It will also be beneficial to diagram
the roof and damaged areas, in addition to taking photographs and written notes
concerning the observations.
To quantify the extent of hail damage to a roof it has been
suggested [Haag Engineering] that a test area on each slope of the roof be
used. Test areas should measure approximately 100 ft2 (equivalent to one
roofing square). The roof covering should be examined thoroughly within the
test area. The amount of damage in a test area will typically suffice for
determining the extent of the hail damage and may also be used to determine
whether repairing or replacing the roof covering is the more economical choice.
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