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

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.

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.

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.

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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