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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
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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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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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