Holy hail!

Holy
hail!
Editor’s note: Following is the second in a
two-part series addressing hail damage to
roof systems. Part one appeared in the May
issue, page 34, and focused on steep-slope
roof systems.
Following hailstorms, roofing professionals,
insurance professionals and building
owners often automatically assume roof
systems need to be repaired or replaced.
However, though hail can inflict serious
damage on roof systems, this is not always
the case.
To identify hail damage on low-slope
roof systems, you must understand basic
properties of various low-slope roof system
types and be able to recognize the effects
of other natural perils, inherent manufacturing
issues and typical weathering issues.
As I mentioned in part one, a roof system
is not considered functionally damaged
by hail unless the roof’s weather
resistance capabilities are diminished or
the roof’s expected service life is reduced.
And whether a roof system is functionally
damaged by hail depends on the hail size
and hardness and roof system type and
condition.
General rules
There are some general rules to consider
when investigating a hail-damaged lowslope
roof system.
First, the thicker the membrane, the
greater the hail resistance. For bituminous
and thermoplastic roof membranes,
the more weathered the roofing material,
the more brittle the membrane and, therefore,
the less impact resistance it will have.
The stiffer the underlying substrate, the
more hail-resistant the roof membrane.
Gravel surfacing affords substantial hail
protection—more and larger aggregate surfacing
typically means greater resistance.
Fractures caused by hailstone impacts
typically start in membranes’ undersides
where tensile strains are greatest and propagate
toward membranes’ topsides. Base
flashing usually is damaged by hail before
other parts of a low-slope roof system because
of its exposure and typically less
solid underlying support.
When looking for hail damage on lowslope
roof systems, remember:
• Over time, fracture edges become progressively
more rounded and fracture
surfaces more weathered; hail-caused
damage does not disappear.
• The cutting and removing of samples
from a roof membrane permits an inspector
to examine the roof covering’s
underside where impact-caused fractures
initiate. (Such destructive testing
may not always be possible.)
• Roofing samples can be transported
to a laboratory and further examined
under magnification. Bitumen can
be desaturated from roofing and reinforcements
extracted; thermoplastic
membranes can be backlit by high-
38 June 2009 www.professionalroofing.net

Identifying hail damage on roof systems
by Scott J. Morrison, P.E.
intensity light and scrutinized visually;
roofing materials can be X-rayed (in
instances where protective surfacing is
other than granules or gravel ballast)
and fractures detected; and magnetic
resonance imaging can be performed
to determine the position of fractures
within the material’s thickness.
• Impacts can be made against roof
assemblies with simulated hailstones
and compared with areas of interest
on roof systems.
• Infrared thermography and impedance
meters can help identify whether water
has migrated through hail-caused openings
and into roof assemblies.
Damage thresholds
Hail must be a certain size and hardness to
functionally damage low-slope roof systems.
However, the size required varies depending
on low-slope roof system type and
condition.
The figure lists the smallest size thresholds
for common low-slope roof system
types. The thresholds, which are the culmination
of more than 45 years of laboratory
testing with simulated hail and tens
of thousands of field inspections, are provided
given the following set of assumptions:
hard hail, perpendicular impacts,
average support, and roof systems in relatively
good or midlife condition.
Built-up
A built-up roof (BUR) system is composed
of a series of reinforcements bonded together
with a waterproofing material such
as asphalt, coal tar or polymer-modified
bitumen. Hail of sufficient size, speed and
hardness can puncture, tear or bruise a
BUR membrane.
Hail-caused fractures always are visible
on a membrane’s underside (unless obscured
by insulation remnants or adhesives).
Although some fractures may not
be obvious in roof surfaces, they can be
detected by hand and feel like localized
soft spots or bruises on an apple. The softness
results from broken reinforcements in
the membrane.
When a BUR system is smooth-surfaced
(surface flooded with bitumen or coated
with aluminum, polymer or emulsion),
impact marks and resulting punctures and
fractures are detectable when examined
closely and tactilely. If a BUR system is
aggregate-surfaced, you must first sweep (or
blow) the loose surfacing aside. This will
expose any nonweathered, black-colored
bitumen where sizeable hail has dislodged
and ejected the flood coat with surfacing
from the impacted site.
A BUR system’s expected service life
can be shortened when protective surfacing
is dislodged by hail. Examples include
the flood coat bitumen or gravel embedded
in the flood coat broken away or aluminum,
polymer or emulsion coatings
disrupted.
ProfessionalRoofing June 2009 39

A hail-caused bruise in a built-up roof membrane with an aluminum coating
Gravel is not driven into the membrane but ejected from the impact
location.
Polymer-modified bitumen
Polymer-modified bitumen membranes are
composed of fiberglass and/or polyester
reinforcement, and their top surfaces are
protected by surfacing such as factoryembedded
granules, factory-bonded foils
or field-applied coatings.
Hailstones of sufficient size and hardness
can puncture, tear or bruise a polymermodified
bitumen membrane where they
strike and reduce the membrane’s weatherproofing
capability. A roof system’s expected
service life can be reduced when hail disrupts
the membrane’s protective surfacing
and exposes its coating bitumen.
The best way to examine a polymermodified
bitumen membrane is to feel
areas with sizeable impact marks by pushing
against the membrane to discern localized
soft spots, which indicate ruptures in
the reinforcement. Fractures in polymermodified
bitumen membranes’ top surfaces
can be curvilinear or a series of fractures
in a concentric pattern. Fractures in
concentric patterns in membrane surfaces
typically are accompanied by fractures in
star-shaped patterns (multiple fractures
radiating from common impact points)
in membranes’ undersides.
Areas without hail-caused punctures, tears
or bruises where granules have been dislodged
must be probed further to determine
whether the exposed polymer-modified
bitumen has additional underlying granules.
In many instances, though some
granules will have been dislodged and
dark-colored bitumen exposed at the impact
site, you will discover additional
granules just below the exposed bitumen’s
surface. These granules protect the polymermodified
bitumen and reinforcement below;
therefore, there is no loss of service life and
no functional damage.
Single-ply membranes
To learn more about hail, log on to www.professionalroofing.net.
Common thermoplastic membranes such
as PVC and TPO typically incorporate
reinforcement in woven forms known as
scrims. The most prevalent thermoset roof
membrane is EPDM, which is produced
as reinforced (with polyester scrims) and
nonreinforced membranes. Hail damage
to thermoplastic membranes includes
fractures or tears in the material body
or reinforcement.
Roof areas most sensitive to damage
typically are at underlying stress plates in
lap seams of mechanically attached systems
or those used for fastening underlying
insulation boards.
Hailstone strikes against membranes at
stress plate edges are known as “anvil strikes”
and result in crescent-shaped fractures 1 ⁄16 of
an inch to 1 inch long. Fractures in thermoplastic
membranes (away from stress plates)
over insulation or deck materials can be
curvilinear, star-shaped or a series of fractures
in a concentric pattern.
Larger fractures in thermoplastic membranes
are plainly visible. Smaller fractures
require closer inspection and hand manipulation
to identify. Pushing on a membrane
will identify dents in underlying insulation
for further scrutiny. Tugging at the membrane
at areas in question will open miniscule
fractures.
When looking for hail-caused damage
in thermoplastic roof membranes, keep in
mind:
• Fractures in membranes generated by
hailstone impacts are immediate and detectable
by a knowledgeable inspector.
• Hail-caused fractures are visible immediately
in all membranes’ undersides.
• Fractures caused by hail are visible
immediately when backlit by highintensity
light.
• Good lighting is important for identifying
smaller fractures in thermoplastic
membranes.
• Water, snow and ice on a membrane
may obscure fractures completely.
• Fractures in some thermoplastics,
especially PVC, may not be readily
40 June 2009 www.professionalroofing.net

Photos courtesy of Haag Engineering Co., Irving, Texas
A star-shaped fracture in a brittle nonreinforced PVC membrane (with
shrinkage)
Fractures in coating and spray polyurethane foam crushed by a hailstone
visible in membranes’ top surfaces
until the membrane has “relaxed.”
• Fractures in brittle nonreinforced PVC
membranes are star-shaped and tend
to propagate when the membrane is
taut.
• EPDM is an elastic material and does
not accumulate stress or fatigue when
struck by multiple hailstones.
• Generally, thermoplastic roof membranes
are more sensitive to impactcaused
damage in colder temperatures.
• Insulation below membranes can be
dented or fractured, which may constitute
functional damage.
SPF
Spray polyurethane foam (SPF) roof systems
are composed of multiple lifts of
polyurethane foam finished with an
elastomeric coating. Depending on hail’s
attributes, resulting damage can range from
fractures in the elastomeric coating to punctures
in coating accompanied by crushing
or fracturing of the polyurethane foam.
Closely inspect SPF roof systems for
hail-caused damage, and feel any areas of
concern by hand. Recommendations for
repairs are available from the Spray Polyurethane
Foam Alliance based on the extent
and frequency of the damage.
Metal
Inspectors will encounter many types of
metal roofing materials. Metal can weather
naturally or be protected by coatings. Panels
are manufactured in a variety of widths
and thicknesses with various types of seam
configurations and applied with exposed
or concealed fasteners.
Hail thresholds for low-slope roof coverings
Roof type
Threshold (inches)
Built-up roofing—smooth 1 3 ⁄4 to 2
Built-up roofing—aggregate-surfaced 2 1 ⁄2
Polymer-modified bitumen membranes 1 1 ⁄2 to 2
Thermoplastic single-ply membranes 1 to 2
EPDM 2
EPDM—ballasted 2 1 ⁄2
Spray polyurethane foam 3
⁄4
Steel panels 2 1 ⁄2
The smallest threshold sizes of hail that can damage various low-slope roof coverings
When investigating hail damage, closely
examine metal roofing panels. These can
be damaged when hailstone parameters are
sufficient to rupture the metal, disengage
lap elements or disrupt protective coatings.
Ruptures in panels and disengaged lap elements
are obvious.
Micrographs have verified that thinning
of metal—even within deep dents—is insignificant.
Analysis of hail-caused dents
with a scanning electron microscope and
analysis of products accumulated within
dents using dispersive X-ray spectroscopy
have demonstrated that factory-applied
coatings—galvanized, Galvalume® and
fluoropolymers—are not damaged by
hailstone impacts even when the metal
is dented. However, field-applied paints
may be vulnerable to hailstone impacts.
Careful conclusions
Hail damage is fairly frequent and can
wreak havoc on roof systems. But in the
event of a hailstorm, try not to make any
hasty conclusions about the need for repairs
or replacement. Take time to carefully
identify hail damage and whether it
is considered functional damage before
deciding whether to perform localized
repairs of damaged roofing materials or
replace the entire roof system.
Scott J. Morrison, P.E., is principal engineer
for Haag Engineering Co., Irving, Texas.
ProfessionalRoofing June 2009 41