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What the Hail?

May 15, 2008

Hailstones usually consist of
water ice measuring
between 1/4 and 2 inches
in diameter (the size of a
small pea to the size of a
golfball), with larger stones
coming from severe thunderstorms. Hail
forms on dust, insects, or ice crystals when
supercooled water freezes on contact. Once
a hailstone becomes too heavy to be supported
by a storm’s updraft, it falls out of
the cloud. When a hailstone is cut in half, a
series of concentric rings reveals the number
of times the hailstone had traveled to
the top of the storm before falling.
Hail can sometimes grow to 6 inches
and weigh more than half a pound. It can do
serious damage to cars, skylights, and
crops. Massive hailstones have caused concussions
and fatal head trauma. The last
known hail fatality in the United States was
Juan Oseguera, a 19-year-old who died
after being hit in the head by a softballsized
hailstone in Lake Worth, Texas, on
March 29, 1990. Hail-producing clouds
may be green in color.
Hail damage to a slate roof is easily
identifiable. Although slate tends to be
durable and resistant to hail, there are
three primary conditions that will maximize
damage to such a roof: 1) very large hail; 2)
thin, soft, or deteriorated slates that are relatively
fragile (such as older Pennsylvania
soft black slates); and 3) slates installed in
a side-lap style where much of the roof has
only a single layer of slate. The combination
of unusually large hail and unusually fragile
slates, especially on a side-lap slate roof,
creates the highest expectation of slate roof
Hail is frozen water propelled
through the air by the force of wind
and gravity, and it can damage a slate
roof upon impact. The effect of that
impact is to either break or puncture
the slates and/or dent metal flashings.
In general, slate can be perforated by
objects from either inside or outside a
roof. Two common examples of impact
damage from inside a roof include bullets
shot through a roof from the interior
of the building, often seen on barn
roofs where farmers shoot at pigeons in
the rafters; and nail heads in the roof
deck working loose and slowly pushing
through the slate above. External
impacts include hail, rocks, golf balls,
bullets shot from outside, etc. Thought
different, both types of perforations are
easily identifiable. When slate is perforated,
the impact leaves a clean hole on
the impact side and a cratered hole on
the opposite side of the impact (Figure
1). Therefore, by looking at the hole,
one can easily determine whether the
impact originated from outside or
Another way to assess the amount
of hail impact on a roof is to examine
the metal components for indentations.
For example, Figure 2 illustrates hail
impact indentations on low-slope copper
roofs after severe hail events. These
roofs are in New Orleans and Chicago.
Figure 3 illustrates hail damage to a
graduated slate roof in Indianapolis.
Although the slates were 1/2 in to 3/4
in thick Vermont slates only 75 years
old, some were perforated by huge hailstones.
Figure 1 – When slate is perforated, the impact
leaves a clean hole on the impact side and a
cratered hole on the opposite side of the impact.
(All photos by Joseph Jenkins.)
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Although hail can damage a
slate roof, in most cases the roof
can be repaired. This author
examined at a 100-year-old
Vermont sea-green side-lap barn
roof after a severe hailstorm and,
looking up from inside, it resembled
a planetarium – points of light
were everywhere. But a day’s work
for a couple of experienced slaters
replaced all of the perforated
The larger problem with hail
events is that roofing contractors
with little experience working on
slate roofs and who see money to
be made on an insurance claim
will condemn a roof that has been
hail damaged rather than propose
to repair it. Many good slate roofs
have been lost
because of this.
Part of the fault
lies with insurance
who are quick to
fork out money
to have a slate
roof replaced before
getting an
expert opinion on
the roof’s actual
condition. It’s
possible that soft
slate on an old
roof that has
been impacted
by large hail
could be fatally
damaged, but in
most cases, hail
damage on a
slate roof should
simply be repaired.
Figure 2 – Hail impact indentations on metal roofing
are not typically a cause of leaks and can often
simply be ignored.
Figure 3 – Hail-perforated slates should simply be removed and
replaced. Hail damage rarely justifies replacement of an entire slate
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Such repair involves the replacement of
damaged slates, which can be either perforated
or broken. In some cases, damaged
flashings may also have to be replaced.
When replacing damaged slates, the
replacement slates should be of the same
size (length, width, and thickness), type
(i.e., origin, such as Vermont sea-green),
shape (e.g., square-cornered, beveled, scalloped),
and approximate age, since new
slate (with few exceptions) rarely blends in
well on an old roof.
This article was originally published by
Joseph Jenkins, Inc. in the 2007 issue of
Traditional Roofing.
Joseph Jenkins, president of Joseph Jenkins, Inc., near
Grove City, PA, has worked in the roofing trades since 1968.
He provides slate roof consulting services nationwide; slate
and tile roof contracting services in northwestern
Pennsylvania; and slate roofing publications, tools, and supplies
distributed internationally. He has personally worked on
over a thousand slate roofs, both installing and restoring
them. Jenkins authored and self-published the award-winning
Slate Roof Bible in 1997, now in its second edition and
available on RCI’s publications list (, and publishes Traditional
Roofing Magazine. He founded the Slate Roofing Contractors Association of North
America in 2005 and maintains informative and interactive Web sites at,,, and
Joseph Jenkins
Beginning in 2008, the Tile Roofing Institute (TRI) will expand its
certification efforts by offering two separate one-day programs designed
to meet the specific educational and professional development needs of
its audience.
TRI will now offer a one-day certification course designed to
provide a broader, more detailed knowledge of tile, including
tile markets, design features and benefits, materials
specifications, code regulations, estimating, ordering,
and improved management of roof tile jobs.The TRI
Certified Specialist designation will target designers,
specifiers, consultants, and any building professionals
interested in learning more about the rapidly
expanding tile roofing market.This course can also be
used as a refresher for inspectors who want to stay up
to date on codes and proper procedures.
This will be offered in addition to TRI’s Installer
Certification Program, a one-day course designed to provide training
on how to install tile roofs in compliance with current code requirements,
based on the ICC-ES approved Installation Manual for Moderate
Climate Regions.
The TRI certification programs address broad-based tile roofing
practices and techniques that impact estimating, planning, and execution
of commercial and residential projects. The courses also address
regional practices in each market where the training is being conducted.
Since launching its Installer Certification Program in April 2006,
TRI has helped more than 500 contractors, consultants, manufacturers,
and owners to build and expand their businesses
with proper tile roof installation knowledge.
Anyone interested in learning more about the TRI
Specialist or Installer Certification program should
visit the Tile Roofing Institute Web site at, call (312) 670-4177, or email for the latest in registration
The Tile Roofing Institute (TRI) is a nonprofit trade
and membership association open to those involved in the production,
distribution, and installation of clay and concrete tile roof systems.
TRI has provided technical expertise to roofing professionals and other
building industry constituents since 1971. TRI manufacturer members
produce 95 percent of the tile roofing installed in North America.
The world’s largest moose (known as an elk in Sweden) will soon straddle
the border of two northern counties in that country. The 148-ft-tall structure
atop the Vithatten Mountain will hold a restaurant and a concert hall, with
spectacular views from the moose’s antlers, reached by elevator in an adjacent
tree-shaped tower accessed through the moose’s mouth.
Founder Thorbjörn Holmlund, owner of the Svansele wilderness center, has
organized safaris for visiting tourists for many years. The oversized moose has
been nicknamed Stoorn, or “The Big One.” To be constructed from local wood
and steel tubing, Stoorn is expected to be completed by the summer of 2009.
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