Waterproofing Under Green (Garden) Roofs – Part 2 of 2

May 15, 2005

This is part two of a two-part
series on waterproofing membranes
under green (garden)
roofs. The first part, published
last month, traced the history
of waterproofing membranes
under plazas and earth-covered, belowgrade
spaces, and discussed the various
types of waterproofing systems currently
marketed and their advantages and disadvantages
for use under green roof systems.
This part covers the attributes of candidate
membranes and offers a list of minimum
physical properties proposed to satisfy the
specific needs for those membranes
exposed to continuous moist environments,
aggressive chemicals, root invasion, and
abusive maintenance. Additionally, it discusses
failures and offers case studies to
illustrate them.
Unless otherwise noted, all photographs
and illustrations are taken from The Manual
of Below-Grade Waterproofing Systems by
Justin Henshell, published by John Wiley &
Sons, Inc. in 2000, or were taken by the
As noted in the first section of this article,
waterproofing membranes differ from
roofing in that they must perform in a continuously
moist environment. Moreover,
very few, if any, waterproofing membranes
are UV resistant.
Waterproofing membranes under GRSs
differ from those used for waterproofing
hardscaped plazas in that they must resist
root intrusion, fertilizers, fungus, and bac-
Figure 1: Exposing membrane for repair.
F E B R U A RY 2005 I N T E R FA C E • 2 7
teria in soils and abuse from landscapers.
Performance in low temperatures is rarely a
In selecting a waterproofing membrane
under a GRS, the designer
should exercise the same prudence
as he or she would use
a membrane
under a
wearing course that is
to be installed in a
mortar setting
bed – the maintenance
and repair are
difficult and costly (Figure 1).
Primary desirable attributes
for waterproofing for use
under green vegetative
• Satisfactorily
per form
under moist conditions and alternate
wetting and drying.
• Resist acids, alkalis, and other
chemicals, including those commonly
contained in fertilizers.
• Resist fungus and bacteria in soils.
• Possess low water absorption.
• Have low permeance
to water vapor.
• Resist root intrusion
(Figure 2).
• Resist puncture
(critical during
• Act as self-flashing
or use a flashing
system that minimizes
• Utilize flashing that
is capable of resisting
UV degradation.
• Be easily repaired.
Secondary attributes:
• Possess a moderate
degree of elongation
and elasticity (e.g.,
crack bridging ability
is important, but
lead sheets with soldered seams
have all the primary attributes for a
good GRS, but have very little elasticity).
• Fire resistance. Of minor importance
since membranes are protected
by the overburden.
• Low temperature flexibility. After
installation over occupied spaces,
the membrane will not be subjected
to significant low temperatures.
Notably omitted from these
attributes is cost. The difficulty
and commensurate costs of
removing the overburden
to investigate
and repair
w a t e r –
problems are
always high
enough to offset
the difference
between a better
performing but more costly membrane
and one that is marginal but
less expensive. Track records count more
than cost.
Table 1 contains suggested minimum
physical properties for waterproofing membranes
in green vegetative roofs. In addition
to possessing the usual properties for plaza
(hardscape) waterproofing, consideration is
given to the membrane’s exposure to fertilizers,
root growth, and other hostile elements
in the roof environment. Some of
these properties were obtained from membrane
manufacturers’ published literature
and some from ASTM D-6630, Low Slope
Insulated Roof Membrane Assembly Performance.
Carl Cash cautions against putting
undue emphasis on physical properties
“because they don’t predict good future performance.”
However, he also points out that
conversely, poor test performance will usually
predict poor membrane performance.
In addition to meeting the requirements
in Table 1, low slope membranes should
pass a flood test per ASTM D-5957. Where
flood testing may be inappropriate because
of concerns that leaks may damage underlying
building components and furnishings,
low or high voltage systems can be used
instead. See Remo Capolino’s article in the
August 2004 Interface.
Although this paper focuses on the
waterproofing membrane for use in a GRS,
it would be remiss not to mention three of
the more important components of the
usual GRS assembly that relate to the
Insulation may be required by local
codes. Simple calculations using steady
state values to achieve the resistance
required by them may be inaccurate
because they do not account for the effects
experienced over a full year. A discussion of
the thermal resistance of a GRS is beyond
the scope of this article.
Interested readers are
referred to “Engineering
Performance of Rooftop
Gardens through Field
Evaluation” by Karen Liu,
PhD, for the National
Research Council of
In addition to its use
for improving the thermal
resistance of a GRS, insulation
is desirable for
retarding premature, false
growth in the northern tier
of states. It can also be
useful as a protection or
early warning to landscapers
who are digging to
replace dead plantings.
On low-slope green
roofs, insulation should be
installed over the membrane to avoid problems
with condensation from occupied
spaces below. However, when roofs are
sloped over 3:12, resistance to slippage
becomes a real concern. Insulation cannot
be loose-laid or slippage will occur.
Mechanically-attached insulation should be
Figure 2: Root intrusion.
28 • I N T E R FA C E F E B R U A RY 2005
* In “Testing for Fungal Growth in Building Products,” ASTM International Standardization News, July 2004, Pamela Hargrove discusses two standards, ASTM G-21,
Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi; and ASTM D-3273, Resistance to Growth of Mold on the Surface of Interior Coatings. She
notes the concern that the G-21 method may give false negative results because the direct inoculation severely wets the sample, whereas the test method in D-3273 represents
a more realistic exposure environment.
** FLL Guidelines for the Planning and Upkeep of Green-Roof Sites, Procedure for Investigating Resistance to Root Penetrations at Green-Roof Sites, 1995
Property Standard Criteria
Static Indentation Resistance D-5602 Section 11 watertight at 250N (56 lbs) over concrete @ -18˚C (0˚F)
Dynamic Indentation Resistance D-5635 Section 12 specimen to be watertight at 10 Kg (22 lbm)
over concrete
Vapor Permeance E-96 <5.7ng/s•m2 pA (<0.1 perms)
Water Absorption in Plastics D-570 <3% by weight
Water Absorption in D-95 <0.1% by weight
Bituminous-based Materials
Resistance to Hydrostatic Pressure D-5385 no leaks at 103 kPa (15 psi) (34.65′ head)
Linear Dimension Change D-1204 <2%
Low-Temperature Flexibility and Crack C-836 Pass
Bridging for Liquid-Applied Membranes
Low-Temperature Flexibility and Crack D-5849 Test Condition Pass
Bridging for Modified Bituminous Membranes for 500 cycles
Resistance of Plastics to Fungi G-21 or D-3273 (Tests currently under review for applicability)*
Resistance to Deterioration from Organisms E-154, Section 13 <10% increase in water vapor permeability
and Substances in Contacting Soil
Resistance of Plastics to Bacteria G-22 No effect
Resistance to chemicals contained ASTM D-896 (undiluted No delamination, blistering emulsification, or
15 N/5P/5Potash) deterioration of adhered membranes
Resistance to root penetration FLL Guidelines ** No penetrations
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F E B R U A RY 2005 I N T E R FA C E • 2 9
Table 1: Suggested minimum physical properties for waterproofing membranes in green vegetative roofs.
located under the membrane to avoid puncturing
it. Fully-adhered insulation may be
placed under the membrane or over it.
However, the latter is not feasible in a looselaid,
single-ply waterproofing system.
On steep roofs, the drainage medium
and root barrier must also be restrained
from slipping. Mechanically anchoring or
adhering them to insulation or the waterproofing
membrane is impractical. A possible
alternative is attaching or adhering
them to sleepers that have been installed
over the membrane and are individually
waterproofed or installed on butyl pads.
Protection Layers
Protection boards and sheets compatible
with the membrane are a prerequisite to
ensuring the watertight performance
of the membrane.
When vegetation dies, it
must be replaced, often by
gardeners wielding squareedged
spades that can cause
immeasurable damage to the
membrane and its components.
It may be tempting to
use the insulation in lieu of
sheet or board protection,
but when consideration is
given to sequencing and the
potential for damage during
the interval between the
membrane completion and
the insulation installation,
prudence would dictate that
a separate protection course
be provided.
Root Barriers
Root barriers are a critical part of the
GRS assembly to prevent intrusion that can
lead to leaking. There are two basic types of
root barriers used in green roof assemblies:
physical and chemical.
• Physical root barriers can vary from
a slab of lightweight concrete to
sheets of metal foil or plastic. Sheets
require fused or taped seams; otherwise,
they may be subject to root
intrusion, although some GRS marketers
claim that laps of 1.5m (5
feet) or greater are sufficient.
• Chemical root barriers are usually
non-woven polypropylene geotextiles
coated or embedded with the herbicide
trifluralin. Two manufacturers
are Tex-R Root Barrier, manufactured
by Texel; and Biobarrier, manufactured
by Reemay Inc.
• One built-up membrane manufacturer
claims to incorporate a
root barrier in its assembly
and several incorporate a
root barrier blended in with
the protection sheet. As a
matter of interest, Dick
Fricklas points out that
coal tar pitch is a natural
enemy of root growth, mold,
and mildew, as demonstrated
by its use in mothballs and
below-grade pipe wraps.
It will come as no surprise to building
pathologists who investigate waterproofing
that, despite claims to the contrary, plazas
(both hardscaped and landscaped) have a
long history of leaking. A majority of the
leaking appears to be caused by failed
expansion joints and bitumen-clogged
weepholes in drains. Some leaks are due to
root intrusion of trees and large shrubs into
membrane seams. Others are caused by
improper flashing of sprinkler and conduit
penetrations. Still others are caused by
water entry above cap flashings where they
are saturated by sprinklers.
Case Histories
Designer error is one of the chief
sources of waterproofing failure. Specification
of an inappropriate system accounted
for the failure of a membrane in a police
administration building in Florida. Pedestrian
and planted plazas were constructed
on several levels over occupied spaces. The
waterproofing system under the planted
areas consisted of sheets of
polyethylene covered with
gravel and sod. The plastic
sheets were minimally
secured to the flanges of
area drains. When the
sheet was exposed, it
appeared to have virtually
deteriorated to the extent
that it was no longer a
viable, watertight membrane.
The author also investigated
a failure attributed to
inadequate detailing at expansion joints
and penetrations. This occurred in the
waterproofing under an intensively planted
garden atop a plaza above the street spanning
between an office tower and the World
Trade Center plaza. Originally, the entire
plaza was waterproofed with a hot-applied
rubberized asphalt membrane that leaked.
It was replaced with a polyester reinforced
membrane that also leaked. Investigation
indicated that the leaks were caused by the
failure of the flashing at the expansion
joints at each end of the bridge, at the glass
railings on the sides of the bridge (Figure 3),
at the drains, and at conduits feeding lighting
fixtures in the planted area. At all of
these locations, the detailing was either
inadequate or absent.
The author investigated another failure
in the waterproofing at a landscaped plaza
above a garage in Florida due to an ill-
30 • I N T E R FA C E F E B R U A RY 2005
Figure 3: Flashing failure at glass railing.
conceived drainage system. The planted
areas were drained by perforated pipes
extending from leaders through the slab.
There were no conventional drains. Because
the hubs on the pipe fittings and the radius
of the elbow connecting the lateral pipes to
the leaders raised the pipes above the slab,
there was a reservoir of standing water
more than an inch deep (Figure 4). Sheet
rubber expansion joint covers had been
installed flush with the waterproofed slab.
The rubber was bonded to the liquidapplied
membrane, creating a plane of
weakness that was exploited by the hydrostatic
head. The inadequate drainage and
failure to elevate the expansion joint cover
caused the joint to leak.
A poor choice of materials caused leaking
in the garage below planters in a courtyard
in a hotel in Michigan. The planters
were separated from the walks with low
concrete masonry walls which also divided
the deeper planters from shallower ones.
The courtyard was waterproofed with a
single-ply butyl sheet membrane that was
carried up and over the lower two courses of
the concrete masonry units. The masonry
walls were reinforced with vertical rebars.
The butyl sheet membrane was flashed to
the rebars, but the deformations in the bars
prevented the sheet flashing from tightly
sealing around them. Virtually every bar
Raymond Wetherholt, RRC, CPWC, PE,
investigated a leaking planter lined with a
self-adhering rubberized asphalt waterproofing
membrane. He determined that the
leaking was caused by root intrusion into
the seams. There was no protection board
or root barrier in the assembly. He also
observed the same condition on another
project where the waterproofing membrane
consisted of two plies of APP modified bitumen
applied in cold adhesive with heat
welded joints. Water had wicked into the
reinforcing and disbonded the seams.
Phil Haisley wrote of investigating a
badly leaking 40-year-old terrace garden
and recreation plaza in Hawaii. The plaza
was originally waterproofed with a tar modified
coating that was interrupted by planter
walls and curbs, resulting in many exposed
edges. Leaking was pervasive. Hollow core
concrete planks were found to be filled with
water, with many concealed paths for moisture
He corrected the leaking condition by
drilling the planks to allow drainage, stripping
walls, curbs, and planters down to the
structure, and applying a new liquidapplied
polyurethane coating over the entire
deck. Only this time it was made
continuous beneath all walls
and curbs, with positive seals
around all penetrations and
rebars. He reports that the repair system
has performed well for more than 12
years, except at one spot where aggressive
shrub roots penetrated a lap joint in the
coating application. This spot was excavated
and patched, adding Biobarrier for
root protection.
Remediating leaks in GRS is similar to
remediating leaks in hardscaped plazas,
except that locating the leak source and
repairing it is complicated by the fact that
there are more layers in the assembly to
remove and planting must be stockpiled
and protected. Since most leaks occur at
penetrations and terminations, including
drains and expansion joints, locating leak
sources can be a fairly routine exercise to
the seasoned building pathologist. Since
flood tests are ineffective on a membrane
covered with overburden, the investigator
must rely on his experience with correlating
leak sites with probable leak sources that
are visible GRS components or those that
can be inferred from drawings. Once a suspected
leak source is identified and excavation
has begun, the investigator must be
prepared to encounter ponding and arrange
to have the exposed membrane drained,
dammed, and dried in order to view and
probe the suspect leak source.
When leaks are caused by root intrusion
into the membrane, the investigation can be
a major undertaking. Where intensive
planting includes trees and large shrubs
that must be removed, wholesale excavation
becomes extremely difficult and costly.
Stockpiling plants and soil assumes a major
logistical challenge. Sometimes, in order to
avoid overloading the structure, the entire
overburden—soil, shrubs, trees, and all—
must be removed and lowered to grade.
Patching becomes further complicated by
the need to prevent water from flowing into
the area to be patched, keep wind-blown
soil off adhesive applications, and seepage
from surrounding soil away from patches.
To overcome this problem, Haisley has
attempted, with some success, to stop leaking
in an intensively planted plaza by injecting
the slab from below. Injection might
mend holes in an existing membrane if it is
Figure 4: Elevated drainage pipes.
32 • I N T E R FA C E F E B R U A RY 2005
in reasonable condition. This probably
works better with adhered membranes than
with those that are loose-laid and is of limited
use where leaks are due to root intrusion.
In these areas, Haisley has recommended
excavation and removal of larger
trees and plants with aggressive root systems
before patching the membrane from
above. Unless root intrusion is permanently
corrected, this form of remediation simply
defers patching or replacing the membrane.
Ed Snodgrass points out the danger
that inappropriate plants pose to most
membranes. Bamboo, Johnson grass, and
similar plants with rhizome roots, which
have arrow-shaped points can easily penetrate
the most root-resistant membrane and
root protection layers. Ed recalls a failed
GRS where the contractor had substituted
local plants for the specified sedum.
Unfortunately, some of the plants were
Johnson grass and the roots invaded the
• Specify membranes manufactured
for waterproofing, not those adapted
from roofing membranes.
• Specify membranes with proven
track records that have at least ten
years of successful use in belowgrade
waterproofing applications.
• Prefer membranes that are seamless
or those whose seams are fusion
• Select membranes that have proven
resistance to burial in soil and exposure
to fertilizers.
• Specify systems that incorporate
root barriers with root-resistant
• Specify that liquid-applied system
be at least 3mm-thick (120 mils) dry
• Specify that PVC membranes be at
least 2mm (80 mils) thick.
• Design the membrane to be continuous
under all elements and components
of the GRS above it.
• Extend flashings and terminations
not less than 8″ above the top of the
soil, not the membrane or the wearing
Baskaran, Bas, “Not all Green Roofs are
Garden Roofs,” Roofing/Siding/Insulation,
January 2004.
Breckenridge, Mary Beth, “Gardening
on the Rooftop,” Akron Beacon
Journal, Akron, OH, 2004.
D’Antonio, Peter, “Thermoplastic Waterproofing
Membranes in Green Roof
System Construction,” Interface,
February 2004.
FLL, The Landscaping and Landscape
Development Research Society,
Guidelines for the Planning, Execution
and Upkeep of Green-Roof
Sites, 1995.
Franz, Janie, “Root Barriers Prevent
Costly Damage,” Soil Erosion &
Hydroseeding, June 2001.
Fricklas, Richard L., “Harvest the Vegetated
Roof,” Roofing/Siding/Insulation,
July 2004.
Friedberg, M. Paul, “Roofscapes,”
Architectural & Engineering News,
September 1969 (A seminal article
that discusses all aspects of green
Garden Roof Assembly, American
Hydrotech, Inc., 2000.
Green Roof Roofscapes, Barrett Company,
Green Roof Systems, Sarnafil, Inc., circa
Green Rooftops, Metropolitan Urban
Roof Consultants Institute
1500 Sunday Drive, Suite 204, Raleigh, NC 27607-5151
Phone: 800-828-1902 • Fax: 919-859-1328 • www.rci-online.org
A monthly* publication
from the industry’s commercial
roofing experts, featuring the
latest technical information on
today’s roofing issues:
• avoiding trapped moisture
• energy efficient roof designs
• sustainable building products
• roof warranties
• protected membrane systems
• steep roofs vs. low-slope roofs
• bituminous roof systems
• cold-applied roof systems
• insulating cellular concrete
• wind resistance
• and more…
the technical journal of the
Roof Consultants Institute
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MAIL OR FAX TO: *RCI publishes 11 issues of Interface a year
F E B R U A RY 2005 I N T E R FA C E • 3 3
Small Sites, BMP Manual, Metropolitan
Council/Barr Engineering
Haisley, Phil, Arch. D, AIA, Private correspondence.
Hendricks, Nico A., “Designing Green
Roof Systems: A Growing Interest,”
Professional Roofing, adapted from a
presentation given at the NRCA
107th Annual Convention & Exhibit,
San Francisco, CA.
Henshell, Justin, The Manual of Below-
Grade Waterproofing Systems, John
Wiley & Sons, Inc., 2000.
Labs, Kenneth, “Technics: Roofs for
Use,” Progressive Architecture, July
Liu, Karen, “Engineering Performance of
Rooftop Gardens Through Field
Evaluations,” Interface, February
Peck, Steven W., “The Greening of North
America,” Professional Roofing,
March 2004.
Roofmeadow Systems, Roofscapes, Inc.,
October 12, 2000.
Tremco Green Roof System, Tremco
Sealant/Weatherproofing Division,
January 26, 2004.
Wark, Christopher and Wendy, “Green
Roof Specifications and Standards,”
Construction Specifier, August 2003.
Wetherholt, Raymond, private correspondence,
The author is indebted to associates and
colleagues who reviewed this paper and
provided input and constructive criticism;
however, the expressed views are the
author’s responsibility. Reviewers include:
• Timothy M. Barrett, Barrett Co.
• Matthew F. Carr, American Hydrotech,
• Carl G. Cash, PE, Simpson Gumpertz
& Heger
• James E. Corcoran, Sarnafil, Inc.
• John M. Easter, AIA, William
McDonough & Partners
• Richard Fricklas, consultant
• Phi Haisley, Arch. D., AIA, Architectural
Diagnostics, Ltd.
• Kevin J. Kehoe, Tech-Rep
• Ralph M. Paroli, National Research
Council, Canada
• Ed Snodgrass, Green Roof Plants
34 • I N T E R FA C E F E B R U A RY 2005
Two standards addressing high-wind test methods and uplift
resistance have been included in the 2004 International Building
Code (IBC) Supplement. After 14 years of research and analysis
measuring the performance of asphalt shingles in high-wind situations,
these standards form the foundation for a simple classification
method for matching asphalt shingles to wind-speed zones.
Class D shingles are suitable for use in 90 mph wind zones; Class G
shingles for 120 mph; and Class H for 150 mph.
Shingle manufacturers should begin testing and labeling shingles
through approved laboratories or testing facilities for the new classifications
in the coming months.
“The asphalt roofing industry has worked long and hard developing
the testing and engineering analysis that led to these groundbreaking
high-wind standards, says Russell Snyder, executive vice
president of ARMA, the Asphalt Roofing Manufacturers Association.
The key to determining the high-wind classification of an asphalt
shingle is based on its measured resistance to the uplift force of
wind at differing speeds. The standards introduced recently were
ANSI/UYL 2390, “Test Method for the Wind Resistance of Asphalt
Shingles with Sealed Tabs,” and ASTM D-6381, “Measurement of
Asphalt Shingle Mechanical Uplift Resistance.” They take into
account such variables as wind speeds, building height, building
exposure, sealant uplift resistance, and the specific fastening recommendations
of the shingle manufacturers.
A study by the Foundation of the
American Subcontractors Association
(FASA) concludes that the practice of
retainage, widespread in the construction
industry, “reduces competition and
increases the cost of a project,” inducing
general contractors to increase contract
prices by 2.2% and subcontractors to
increase contract prices by 3.6%.
The practice of withholding approximately
10% of a fee to make sure a project
is finished and all punchlist items
resolved is longstanding – dating to the
1840s in England. But the recent study by
Dennis Bausman, Ph.D., assistant professor
in the construction Science & Management
Department at Clemson University,
claims that retainage negatively impacts
project relationships. Responses from
more than 1,000 owners, architects, construction
managers, general contractors,
and subcontractors found wide disagreement
about retainage among the different
construction team members.
The study is available online at
Justin Henshell, FAIA, CSI, FASTM, is a registered architect
and partner in Henshell & Buccellato, Consulting Architects,
specializing in water-related issues in the building envelope.
He is the author of numerous technical articles on roofing
and waterproofing, an ASTM standard on waterproofing
details, and The Manual of Below-Grade Waterproofing
Justin Henshell, FAIA, CSI, FASTM
Shingle Classes Now
Match Wind Zones