Green Roofs - Living Architecture Monitor - Winter 2019 - 30

RESEARCH

CAN GREEN ROOFS PROTECT
AGAINST HAIL DAMAGE?

BY OLIVER GILLINGS-PECK

Within the last decade the negative impact climate change inflicts
on our infrastructure has started to become more noticeable around
the world. Every year there are multiple destructive weather events
that strike urban and rural dwellings, often resulting in billions of
dollars in damage to public and private property.

I

n an attempt to mitigate these impacts, it would be beneficial to
design infrastructure that is more resistant to extreme weather.
One often-overlooked example of this, due to its widespread
and less severe nature, is the damage caused by hailstorms. In
2017 alone, hailstorms in the US caused $22 billion in damage to
buildings, people, cars and crops as reported by the Insurance Institute for Business and Home Safety. From 2000 to 2013 insurers
paid out almost 9 million claims amounting to $54 billion dollars
in damage. Green roofs may provide a solution to hail damage on
flat roofs.
Every year thousands of hailstorms occur around the world but
only a handful of them produce hailstones that are big enough to
cause damage. Hailstones that are considered to be medium to
large in size are anywhere between one to four inches in diameter
and can travel up to speeds of 72 mph (115 kmph). The formation
and precipitation of larger hailstones is a direct result of a supercell thunderstorm that characteristically produces a strong updraft
within the storm, causing water droplets to rise above the freezing
level and thus turn into hail. The stronger the updraft the more
time a water droplet has to collect water and grow in size, until
it eventually becomes too heavy for the updraft to support and
falls to the ground as a hailstone. Hailstorms are expanding in
reach. Over the past decade, hailstorms are now occurring more
frequently outside of Hail Alley, an area made up of Colorado,
Nebraska, and Wyoming.

LIVING ARCHITECTURE MONITOR / WINTER 2019 / 30

The UL 2218 is the current
standard testing protocol for impact resistance of prepared roof
covering materials. This standardized test involves dropping
a steel ball, with various weights
and from various heights, onto
a given roofing material to
determine which class of impact
resistance it falls into. There are
one to four class designations
with class four being the highest
impact resistance. Roofing companies recommend installing a
roof that has at least a class four
UL2218 rating in order to sufficiently protect against a hailstorm. This means that in order
for green roofs to be proven to
mitigate the damages caused by
hailstorms they need to score a
class four rating. The actual test
for class four rankings involve
a one to two pound steel ball
being dropped from a height of

12-20 feet in the same location
of the material. Unfortunately
the test is designed for traditional sheet or panel rooftops
that have multiple layers and
are on inclines exceeding 25 per
cent, but with that being said it
would be quite easy to replicate
the test on a green roof to determine its impact resistance class
ranking. Students and faculty at
the University of Colorado have
recently completed research on
green roofs and hail damage,
soon to be published.
Although the standardized
test has not been officially
administered to a green roof
there is direct field evidence
that suggests that green roofs
are in fact a better option for
hailstorm protection than
traditional roofs. On May 8th
2017, a supercell formed over
the city of Denver, Colorado
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Green Roofs - Living Architecture Monitor - Winter 2019

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