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20 D+D JUNE 2014
Maintenance + Renovation
Vegetative Roof Durability: Lessons From SandyTwo New Jersey
case studies suggest that establishedplantings can survive even
hurricane-force winds.
hey’re proven to reduce heat-
ing and cooling loads, along
with stormwater runoff, and
to mitigate urban heat-island
effects. They can enhance a
building’s aesthetics and
even prolong the life of the roof. Yet for
all we know about vegetative roofs
placed over conventional roofing sys-
tems, there’s much we don’t know.
Vegetative roofs, also known as green
roofs, have been popularized in North
America only during the past few
decades. While the base of research into
the performance of North American veg-
etative roofs is growing rapidly, we have little data regarding
their performance during catastrophic
weather events. The roofing industry is left to ask, “What
happens to vegetative roofs in high-wind con-
ditions?” And “How do I know what is engineered is going to
work? … Is this system over-engineered?”
To answer these questions completely will take years of research
and testing. In the meantime, an-
ecdotal evidence can provide some lessons about the durability
of vegetative roofing in high-wind
situations.
By Matthew Barmore, Firestone Building Products, and Elaine
Kearney, Columbia Green Technologies
T
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Superstorm Sandy, with its landfall occurring in one of the
fastest-growing U.S. markets for vegetative roofs —
metropolitan
New York City — provides an unprecedented in situ opportunity
to
learn more about vegetative roofs and their behavior during
storms. Specifically, the effects of wind — uplift, scour, shear
and
so on — can be seen clearly through images of vegetative
roofs
taken before, during and after the October 2012 storm.
What we’ve learned is promising. We studied two vegetative
roofs in New Jersey, one being installed at the time the storm
hit
and the other installed just four days before the storm. Their
expe-
riences suggest that green roofs with a variety of
construction
types can emerge from major weather events intact.
Maintenance + Renovation21
Cincinnati-based Green City Resources installed a multilayered
vegetative system at Cincinnati Children’s Hospital Medical Center
in 2013. Art accents the hostas,daffodils, alliums and sedum tiles.
Photo courtesy of Firestone Building Products.
On the roofing membrane, installers place the vegetative roof
assemblies— either modular trays, as shown here, or built-in-place
systems. Traystypically interlock, making them resistant to wind
uplift. Photo courtesy of Firestone Building Products.
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Vegetative Roof Basics Vegetative roofs are engineered systems
designed to support
plant life on top of conventional roofs. They are commonly
divided
into two categories: extensive and intensive.
Extensive vegetative roofs contain 6 inches (15 cm) or less of
grow-
ing media. Common goals of an extensive vegetative roof
include
stormwater management, creation of amenity space, enhanced
aes-
thetics, extended roof-membrane life and LEED certification.
Intensive roofs are 6 inches or greater in depth and can
support
a wider range of plant material, such as large shrubs or
even
trees. Intensive roofs, often called roof gardens, may
incorporate
elements such as lawns, decks, promenades and trellises,
which
exist primarily for people to enjoy.
We can further distinguish between the types of vegetative roof
as-
semblies, generally described as modular/tray or built-in-place
systems.
Modular tray systems are high-density polyethylene (HDPE)
trays, typically 2 by 2 feet by 4 inches (61 by 61 by 10 cm) or
1 by 2
feet by 4 inches (30 by 61 by 10 cm), in which lightweight
engi-
neered growing media and plantings have been placed. Modular
tray systems can be installed with pregrown vegetation or
assembled
with growing media and plants on the roof.
Built-in-place systems (also known as loose-laid or built-up
sys-
tems) consist of a drainage layer, typically made of
polyethylene; a
moisture-retention layer of either inorganic/aggregate media
or
polyethylene; and a filter layer, which is typically a
geotextile. A
root barrier is also sometimes necessary, and it is installed
directly
above the roofing membrane.
Together, these products are typically referred to as the
“hard
goods” portion of the vegetative roof assembly. The
installer
places a lightweight engineered growing media blend of
organic
and inorganic matter over the hard goods at the specified
depth(s).
Plantings are then placed in or on the growing media,
depending
on the type of plants specified (plugs, mats, tiles, cuttings,
etc.).
Regardless of system type, all vegetative roof systems have
at
least the following generic components:
• Vegetation, to stabilize the growing media,
evapotranspire water and prevent wind scour.
• Growing medium, to provide moisture and nu-
trients for plants, as well as retain stormwater.
• Moisture retention, to provide additional
moisture-retention capabilities especially with
thin, extensive soil profiles.
• Drainage, to remove excess water from the veg-
etative roof system and direct it to the roof drains.
• Roofing/waterproofing, to provide a water-
tight barrier between the interior and exterior
of the building.
The projects profiled in this study are located
in Jersey City and Woodbine, N.J. They used dif-
ferent types of vegetative roofs: namely trays
and built-in-place assemblies. However, these
systems followed the same basic installation ap-
proach, and they are representative of the typi-
cal types of vegetative roofing on the market.
Following the completed installation of the
roofing system, installers set in place on the
roofing membrane either individual layers
(water retention, filter and drainage layers, in
the case of the Jersey City project) or trays (in
the case of the Woodbine project). Next, they
distributed lightweight growing media and installed plants.
Fi-
nally, they placed edge metal around the perimeter of the
com-
pleted vegetative roof system.
Plantings were installed on one of the vegetative roofs, using
pre-
grown sedum mats. The other roof was being installed when
the
storm occurred.
Performance Standards and Wind UpliftWind performance guidelines
and testing for vegetative roofing are
still in their infancy. The American Society for Testing and
Materials
22 D+D JUNE 2014
Superstorm Sandy provided an unprecedented in situ opportunity
to learn more about vegetative roofsand their behavior during
storms. Photo courtesy of Firestone Building Products.
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Maintenance + Renovation23
(ASTM) has been working to propose an industry-standard wind
uplift test method, but until such a standard is released, there
is no
widely accepted test method for green roofing in the United
States.
In the absence of a standard testing method, industry groups
have
created voluntary guidelines. Factory Mutual Insurance Co. has
a
January 2007 Property Loss Prevention Data Sheet addressing
green
roof systems. The Single Ply Roofing Industry stepped forward
in
June 2010 with a voluntary design guideline addressing
vegetative
roofing and wind uplift. ANSI/SPRI-RP-14 Wind Design Standard
for
Vegetative Roofing Systems is modeled on ANSI/SPRI RP-4 Wind
De-
sign Standard for Ballasted Single-ply Roofing Systems. It is
intended
to provide a minimum design and installation reference for
those
who design, specify and install vegetative roofing systems.
Users of
RP-14 can select a category of vegetated roofing to satisfy a
maxi-
mum allowable wind speed based on a building’s height,
roof-edge
(parapet) height and wind exposure category.
It is important to note that RP-14 treats vegetative roofs as
ballast;
it applies principles gleaned from experience in testing
ballasted roof
assemblies to these systems. Therefore, vegetative roof systems
are
categorized as No. 4 ballast or No. 2 ballast, as determined by
their
minimum dry weight and construction methodology. The ballast
def-
initions include a provision for the additional wind uplift
resistance
provided by interlocking modular trays. Once a designer has
deter-
mined whether she should use a system 1, 2 or 3 green roof
design
to meet their maximum allowable wind speed, RP-14 directs her
to
use minimum perimeter and corner setback allowances in
combina-
tion with green roof ballast category No. 2 or No. 4.
The modular tray system profiled in the case studies to
follow
count as No. 2 ballast (the maximum wind uplift ballast
protection)
under RP-14, because they are “Interlocking contoured fit or
strapped
together trays containing growth media spread at minimum dry
weight of 13 psf (64 kg/m2) of inorganic material plus organic
mate-
rial.” Furthermore, the trays are attached to each other using
poly-
ethylene pins, creating a monolithic assembly that displays
considerable resistance to wind uplift even when empty.
Manufactur-
ers use various interlocking tray designs to mitigate wind
uplift.
As RP-14 suggests, vegetative roofs act as ballast over the
installed
roofing system. Since vegetative roofs vary widely in design
(depth of
media, types of plantings, additional securement, etc.), it is
impossible
to state an average weight for vegetative roofs. However, most
vege-
tative roof systems weigh more than 20 pounds per square foot
when
fully saturated, and many weigh 25 to 35 pounds per square foot.
Be-
cause fully saturated, healthy vegetative roof systems usually
are sub-
stantially heavier than traditionally installed ballast
(typically at least
10 pounds per square foot), we can expect them to remain in
place
more effectively than ballast. Additionally, when a vegetative
roof’s
rooting is established, it provides additional resistance to
wind scour.
Two Roofs, One SuperstormHurricane Sandy became the
second-costliest hurricane in U.S.
history when it struck the Atlantic Coast in October 2012.
While
the media focused on Sandy’s catastrophic flooding,
sustained
winds during the storm reached 69 mph (60 knots) in some
loca-
tions, and wind gusts peaked at nearly 90 mph (78 knots).
Fully saturated, healthy vegetative roof systems usually are
much heavier thantraditionally installed ballast. A vegetative
roof’s established rooting providesadditional wind resistance.
Photo courtesy of Firestone Building Products.
The potential benefits of vegetative roofs include stormwater
retention, prolonged roofing material life, energy conservation and
enhanced aesthetics.This system is installed at York Place
Apartments, Edina, Minn. Photo courtesy of Firestone Building
Products.
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24 D+D JUNE 2014
Jersey City and Woodbine, N.J., where our study’s two cases
are located, are 121 miles and 35 miles (195 km and 56 km)
re-
spectively from the epicenter of Superstorm Sandy’s
landfall.
The Beacon Apartments are located in Jersey City. The Art
Deco-
style building was erected in the early 1930s. It is 1.7 miles
inland
from Upper New York Bay, directly northwest of Ellis Island
and
Liberty Island. The vegetative roof is located above the eighth
floor.
The roofing system at the Beacon consists of a
styrene-butadi-
ene-styrene modified bitumen roofing membrane and base ply
adhesively attached to a glass mat gypsum board and polyiso-
cyanurate insulation. The roof height is 65 feet (20 m), and
the
parapet around the rooftop ranges from 24 to 48 inches (0.6
to
1.2 m) above the deck.
The vegetative roof was being installed using a
built-in-place
system. The hard goods and approximately 4 inches (10 cm) of
growing media were in place; however, installers had not yet
placed plant material at the time of the storm. This built-up,
ex-
tensive vegetative roof assembly is typical of those found
throughout the United States and Europe. It consists of
three
primary layers beneath the growing media and plants:
Drainage layer, 0.375 inches (0.95 cm) of extruded polyester
woven into an entangled cuspate geometric patterned matrix
with heat-welded junctions.
Filter layer, 2.0 ounces per square yard of nonwoven
polypropylene attached to the drainage layer.
Water-retention layer, a half-inch of high-loft, nonwoven
geot-
extile consisting of thermal bonded polyester fibers treated
with
insoluble polymer resins to form an evenly distributed,
three-di-
mensional blanket matrix intended for water retention,
drainage
and anchorage points for promoting solid root structures.
The growing media for the project was transported to the
site
using standard 2-cubic-yard-capacity totes, then hand-broad-
Case No. 1: Beacon Apartments
casted with wheelbarrows and graded to approximately 4
inches
deep in those areas completed prior to the storm event
(primarily
the west side of roof).
Sandy made landfall near Atlantic City, N.J., roughly 96
miles
(154 km) south-southwest of the project site. At 9 p.m. on Oct.
29,
Sandy was 15 miles (24 km) northwest of Atlantic City.
We examined wind speed data from Robbins Reef, N.J., and
Bergen Point, N.Y., the two National Oceanic and Atmospheric
Ad-
ministration (NOAA) sites nearest Jersey City. Wind
measurement
instruments at Robbins Reef are 49.8 feet (15.2 m) above sea
level.
Bergen Point instrumentation is located on a relatively
protected
inlet and is 29.8 feet (9.1 m) above site elevation.
Wind data acquired roughly 4.5 miles (7.2 km) from the
project
site at NOAA’s Robbins Reef buoy during the most extreme
por-
tions of Sandy showed sustained winds of more than 46 mph
(40
knots), with sustained wind gusts of more than 69 mph and
peak
wind gusts near 90 mph.
Wind data acquired roughly 7.1 miles (11.4 km) from the
project
site at NOAA’s Bergen Point buoy during the most extreme
por-
tions of Sandy showed sustained winds of more than 34 mph
(30
knots), with sustained wind gusts of more than 46 mph and
peak
wind gusts nearing 58 mph (50 knots).
As mentioned, the Beacon roof was in various stages of
comple-
tion at the time of the storm. The webcam images above
illustrate
the effects of the storm.
In the lower-left section, we observe roll-up of system
compo-
nents where installation of the edge metal system was
unfinished,
as well as displacement of loose laid insulation. Note the edges
of
the rest of the vegetative roof areas, where at least some
edge
metal was already in place.
In the upper-left section, we see displacement of loose laid
insu-
lation and windblown debris.
In the upper-right section, windblown debris is present, but
we
see little impact to the vegetative roof system.
In the lower-right section, we see no significant impact.
The vegetative roof at the Beacon Apartments in Jersey City,
N.J., was incomplete at the time of the storm. As webcam images
shot during the storm show,Sandy’s effects ranged from some roll-up
of system components and displaced loose-laid insulation in the
lower-left section, to no significant effects in thelower-right
section. Photo courtesy of the Beacon Apartments.
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Circle No. 22 on Reader Service Card
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26 D+D JUNE 2014
Our second case is an 1,860-square-foot (173-square-meter)
vegeta-
tive roof located on the Sam Azeez Museum of Woodbine Heritage
in
Woodbine. Woodbine is located in far southern New Jersey, 9.8
miles
(15.8 km) inland, and about halfway between Cape May and
Ocean
City. The one-story building with a roof height of approximately
20
feet (6 m) is in a suburban setting, with a roof inaccessible to
the
general public. The parapet ranges from 0 to 18 inches (46 cm)
high.
The vegetative roof was built using an interlocking and
overlap-
ping modular tray system. This modular solution consists of
plastic
trays 2 foot by 2 foot by 4-5/8 inches (61 by 61 by 12 cm),
designed
for use on rooftops.
The vegetative roof consists of a planted-in-place tray
system
over a fully adhered EPDM roofing system. Installers set
empty
trays in place and interlocked them according to the
manufac-
turer’s instructions. Each tray was overfilled with +/- 5.33
inches
(13.5 cm) of growing media. After fully wetting the growing
media,
installers simply laid sedum mats on the growing media. They
edged the vegetated portion of the roof in aluminum and
ballasted
the remainder of the roof with ASTM No. 4 crushed rock. The
team
completed the installation just four days before Hurricane
Sandy.
The site experienced elevated winds during Sandy. The
nearest
data points, taken from the Cape May weather station,
suggest
the area experienced sustained winds of 60 mph (52 knots)
and
gusts of 75 mph (65 knots). Instrumentation at Cape May is 40
feet
(12 m) above site elevation, and is attached to the edge of a
pier
extended 30 feet (9 m) into the harbor.
Representatives of the green roof manufacturer examined the site
on
Nov. 8, approximately one week after the storm had passed and
before
any remedial work. The vegetative roof remained completely
intact.
Observers found no evidence of movement or peel-back of the
vege-
tated sedum mats. While the plants appear slightly
weather-beaten,
Case No. 2, Sam Azeez Museum
they are in good health with no sign of dieback. The underlying
tray
system remained in its original position with little to no
shifting or mis-
alignments observed. The team that visited the site to inspect
for dam-
ages surmised that Sandy’s significant amounts of rainfall
had
saturated the sedum mat and growing media such that it was
very
heavy and therefore had little susceptibility to the high
winds.
Anecdotal FindingsAlthough the installation was completed only
four days before the
storm began, the fully installed vegetative roof system in
Wood-
bine performed better than the partially installed system in
Jersey
City. That was to be expected, as the Jersey City roof was
mostly
unattached, and ballast was achieved only from the
lightweight
engineered growing media that had been installed.
What was perhaps unexpected was that the growing media,
while displaced in proportionally large areas of the Jersey
City
roof, remained intact in others. The Woodbine vegetative
roof
showed no visible signs of stress, scour or displacement
following
the storm. That the installed plants were pregrown,
established
sedum tiles likely contributed to their ability to withstand the
ele-
vated winds without damage.
This article is based on a paper presented by the authors at the
29th
International RCI Convention & Trade Show:
rci-online.org.
About the AuthorsMatthew Barmore has served as a technical
coordinator, national account sales execu-
tive, manager of roofing solutions, manager
of estimating services and, most recently,
as product manager for Firestone Building
Products’ green roof, daylighting, solar and
other green building envelope products. He
holds a B.S. from Indiana State University
and a master’s from Bethel University. Pre-
viously, Barmore was an officer in the U.S. Air Force.
As a designer and project
manager, Elaine Kearney
has been engaged in all
aspects of product devel-
opment and implementa-
tion. She has worked on
award-winning projects
featuring green roofs and
living walls. Kearney
holds a B.A. in economics from Trinity University in Texas and
a
master’s in landscape architecture from Harvard’s Graduate
School
of Design. A registered landscape architect, she is a member of
the
American Society of Landscape Architects. D+D
The green roof on the Sam AzeezMuseum of Woodbine Heritage,
inWoodbine, N.J., shows no visiblescour or displaced materials
inthese post-storm photos. Photoscourtesy of Columbia
GreenTechnologies.
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