Practical Considerations For The Design And Installation Of Rooftop Gardens: The Waterproofing Challenge

PRACTICAL CONSIDERATIONS FOR THE DESIGN AND
INSTALLATION OF ROOFTOP GARDENS – THE
WATERPROOFING CHALLENGE
DOUGLAS C. FISHBURN, RRO
FISHBURN BUILDING SCIENCES GROUP, HORNBY, ON
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ABSTRACT
Roof gardens, commonly referred to today as “green roofs,” have been used in Mexico for
centuries. Sod roofs were introduced in Canada by the Vikings and later by the French
colonists in Newfoundland and Nova Scotia. Earth dwellings have been used in the southwest
by Native Americans, and sod roofs on homesteads were used during the settling of
western North America.
Over the last 40 or 50 years, roof landscaping has been used over parking decks and
podiums to improve aesthetics and create market appeal for both commercial and residential
buildings. A green roof does not demand an entirely different roof design approach. The
sound principles involved in the design and construction of a conventional or protected roof
membrane can be modified and/or adapted to green roofs. Green roofs offer many operational,
financial, environmental, and social benefits. These benefits can be short lived if the
waterproofing assembly fails to provide its principal function: a waterproof environment.
The environment to which it is exposed, its design, its method of construction, and the
frequency of maintenance can impact the durability of any waterproofing system. Improper
design, poor construction practices, and lack of proper maintenance have been found to
result in premature roof failure. There is no reason to think that such factors would have a
different impact on green roof applications. In order to mitigate the risk of failure and to
improve longterm
performance, specific considerations must be paid to load requirements,
slope and drainage, thermal performance, design of the details, waterproofing membrane,
testing, and requirements for maintenance.
This paper focuses on some of the factors impacting the design of waterproofing for
green roofs, particularly intensive green roofs, and suggests methods of design and construction
that can help achieve longterm
watertight service.
SPEAKER
DOUGLAS C. FISHBURN, RRO — FISHBURN BUILDING SCIENCES GROUP, HORNBY, ON
Acknowledged as an expert in his field, DOUG FISHBURN has investigated numerous
roofing, waterproofing, and buildingenvelope
failures and has appeared as an expert witness
in many highprofile
litigation cases. Considered a leading authority on greenroof
waterproofing and design, he has authored and presented papers addressing greenroof
design and waterproofing issues for the NRC and RCI at numerous conferences throughout
Canada and the U.S. Fishburn is a member of the Professional Engineers of Ontario (PEO),
National Roofing Contractors Association (NRCA), Toronto Construction Association (TCA),
Construction Specifications Canada (CSC), Ontario Industrial Roofing Contractors,
Concrete Institute of Canada, Ontario Building Envelope Council (OBEC), Canadian
Government Specifications Board, RCI, and Green Roofs for Healthy Cities.
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PRACTICAL CONSIDERATIONS FOR THE DESIGN AND
INSTALLATION OF ROOFTOP GARDENS – THE
WATERPROOFING CHALLENGE
Green roofing can range from a carpet of
flowers to grasslands to woody shrubs.
Green roofing has been refined over the
years and is generally divided into three
types: intensive, semiextensive,
and extensive.
Intensive green roofing is characterized
by its higher weight, which is due to the
depth of growing medium (150 mm/6 in) or
more required to accommodate larger
shrubs and trees. These systems weigh 290
to 967 kg/m2 (50 to 200 lb/ft2).
Semiextensive
green roofing is characterized
by a depth of growing medium of
approximately 150 mm (6 in). The weight of
a semiextensive
system can vary from 169
to 290 kg/m2 (25 to 50 lb/ft2).
Extensive green roofing is characterized
by its lower weight, which is due to
reduced depth of growing medium (150mm
/6 in or less), saturated weights between 72
and 169 kg/m2 (12 to 25 lb/ft2), and the use
of smaller plants.
The following are the major benefits and
disadvantages of green roofing compared to
traditional conventional roofing applications.
Note: Most tangible benefits are projectspecific.
TANGIBLE BENEFITS
• May expedite municipal approvals
• Increases the roof membrane life
expectancy
• Decreases maintenance of the membrane
and membrane flashing
• Reduces cooling costs
• Food production
• Increased market value
INTANGIBLE BENEFITS
• Reduced heatisland
effect
• Reduced water run off
• Aesthetic appeal
• Improved air quality
• Reduced sound transfer
• Qualifies for LEED1 points
DISADVANTAGES
• Higher cost of construction due to
increased load capacities and increased
height of flashing
• Higher cost of construction due to
landscaping and planting requirements
• Higher operation cost due to landscape
maintenance
• Higher cost of roof replacement
• More difficult and costly to find and
repair leaks
• Repairs’ impact on the aesthetics,
since mature trees and shrubs are
typically replaced with immature
ones
In general terms, roofs can be classified
as watershedding,
weatherproof, or waterproof.
Watershedding
roofs use gravity to
keep water out. Weatherproof and waterproof
systems employ waterproof membranes
to provide this function. The prime
difference between weatherproof and waterproof
membranes is that waterproof membranes
must remain watertight when
exposed to hydrostatic pressure. Trade
associations such
as the Canadian
Roofing Association
(CRCA) and
the National Roofing
Contractors Association
(NRCA)
recommend that
only waterproof
membranes be
used in the construction
of green
roofs.
Lowslope
roofs
are divided into
three types: conventional,
where
the roof membrane
is placed above the
roof insulation;
protected, where
the roof membrane
is placed below the
insulation; or cold
(vented) roofs,
where the insulation
is located under the roof deck.
While conventional roofs can employ
extensive green roof technology, typically
intensive green roofs incorporate protectedroofmembrane
designs.
PROTECTED ROOF MEMBRANE
In a protectedroofmembrane
design,
the roof membrane is placed on the deck or
overlay under the insulation. With this configuration,
regardless of the roof finish (gardens,
pavers, or gravel), the roof membrane
is shielded from temperature extremes of
the environment and protected from roof
traffic following construction.
In a protectedroofmembrane
design,
the membrane serves the functions of
waterproofing, air barrier, and vapor barrier.
An example of a protected membrane
roof is shown in Figure 1.
CONVENTIONAL ROOF MEMBRANE
In a conventional roof, the membrane is
placed above the insulation and provides
Figure 1
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the function of waterproofing. When green
roofs utilize a conventional roofing system,
once protection boards, growing medium,
and plants are in place, they have many of
the features and benefits of protected roof
membrane assemblies. An example of a
conventional roof is shown in Figure 2.
COLD (VENTED) ROOF ASSEMBLY
In cold or vented roofs, the insulation is
located below the roof deck. A cold (vented)
roof assembly is not recommended for a
greenroof
application unless a detailed
buildingscience
and engineering review is
completed.
These systems are typically not
designed with the loadcarrying
capacity to
support green roofs and are subject to creep
deflection that results in ponding water.
The lack of a proper air/vapor barrier
and inadequate ventilation of the roof cavity
leads to a moisture buildup
that can
result in deterioration of the wood framing
and mold growth. Converting cold/vented
roofs to conventional or protected roof
assemblies is recommended
as a way to
mitigate the risk of failure.
An example of a
cold/vented roof assembly
is shown in
Figure 3.
ROOF DECK AND LOAD
REQUIREMENTS
The roof deck must be designed to carry
the anticipated dead and live loads, including
temporary loads imposed by construction
equipment and stockpiling of materials.
A number of roofdeck
types such as concrete,
steel, or wood planks can be utilized
in the construction of both intensive and
extensive green roofing, provided they are
designed to carry the anticipated loads.
Pouredinplace
or precast cellular concrete
decks typically do not have the structural
capacity or robustness to accommodate the
installation of green roofs.
When structural concrete decks are left
Figure 2
Figure 3
Figure 4
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Figure 5 – Courtesy of the Bank of
Canada, Ottawa, Ontario.
exposed and used as staging areas, consideration
should be given to using additives in
the concrete mix (to reduce water absorption)
or the use of epoxycoated
rebar (to
reduce the risk of corrosion of the steel
reinforcement).
It is recommended that the preparation
of a loading plan that shows the spare load
capacity of various areas of the deck be completed
early in the design stage. An example
of a loading plan is shown in Figure 4.
The use of lightweight materials in the
design of green roofing increases the potential
for having green roofs on both new and
existing buildings. Building up the planting
area with polystyrene insulation in lieu of a
full depth of growing medium, using
drainage panels in lieu of a heavy layer of
gravel, and using plant varieties that can
grow in a minimum depth of growing medium
will contribute to reduced weight.
If larger trees are incorporated into the
design, one solution to address dead loads
is to use planter boxes for larger shrubs and
locate them over columns or at the roof
perimeter, as shown in Figures 5 and 6.
Concretetopped
insulation, rubber
walkway pavers, stepping stones, or wood
or plastic walkways in traffic zones are all
designed to reduce the dead loads on the
roof assembly. An example of steppingstones
used to reduce weight is shown in
Figure 7.
If wood walkways or paving stones are
incorporated in the design, they should be
installed to allow easy removal in order to
gain access to the waterproofing system.
The greater weight of green roofs compared
to conventional roofing systems can be a
major limitation from both a cost and a
functional point of view.
While the structural requirements can
be easily accounted for during initial construction,
owners may not be willing to pay
the additional costs to upgrade the structure
to carry the additional load capacity
required for green roofs.
Figure 6 – Courtesy of the Minto Hotel,
Ottawa, Ontario.
While existing protectedmembrane
roofs may be viewed as good candidates for
green roofing, the load capacity needs to be
carefully considered. Many protectedmembrane
roofs that are more than 20 years old
could have the necessary spare load capacity
to install extensive green roofing. This is
due to the fact that these roofs were typically
designed with the insulation being
bonded to the roof membrane. These roofs
were positively ballasted to prevent insulation
flotation.
The weight of the gravel ballast was typically
installed at a minimum of 48.8 kg/m2
for 50 mm of insulation or less. The weight
was increased at a rate of 24.4 kg/m2 for
every 25 mm of additional insulation. Roofs
installed with 100 mm of insulation are typically
ballasted at 107 kg/m2.
Within the last 15 years, many protected
roofs were designed as lightweight systems.
With protected lightweight systems,
the insulation was looselaid
with a waterpermeable
fabric installed over the insulation.
While greater ballast weights were
required at the roof perimeters and corners
to offset wind loads, the ballast in the field
of the roof was typically installed at a weight
of 48.8 kg/m2. The insulation was expected
to float under ponding water conditions and
the waterpermeable
fabric was expected to
keep the insulation boards in alignment like
a raft floating on water if the roof periodically
ponded water.
Designers and contractors must proceed
with caution when substituting the
gravel ballast on lightweight protectedroof
membrane assemblies and installing an extensive
greenroof
cover, since the weight of
the growing medium and plants may be insufficient
to prevent flotation, and, as a result,
the insulation may become dislodged.
National and regional building codes are
not static and change periodically to reflect
increases or decreases in live loads imposed
by rain or snow. A reduction may, in some
cases, allow additional load capacity for the
installation of green roofs. The replacement
of a builtup
gravel roof with a lighter modified
or singleply
membrane will also provide
additional spare capacity.
The weight of a builtup
roof membrane
can be reduced to approximately that of a
modifiedroof
membrane by substituting
the bitumen and gravel surfacing with a ply
of modifiedmembrane
cap sheet. Depending
on the design of the system, this reduction
in weight is approximately 25 kg/m2.
These systems are generally referred to as
hybrid roof membranes and are reviewed
elsewhere in this paper.
FIRE RESISTANCE
Building codes require that roofs meet
UL or ULC (in Canada) requirements for
external fire resistance. The risk of external
fire propagation will increase with roof
slope. Due to insufficient test data, it is recommended
that local fire marshals and
insurance underwriters approve designs
early in the design stage. Tall grasses and
Figure 7 – Courtesy of the Minto
Hotel, Ottawa, Ontario.
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Figure 8
woody shrubs and trees pose an elevated
fire risk in comparison to lowgrowing
plant
material such as sedums.
Where there is an elevated risk of fire,
consider providing firebreaks or firewalls
within the greenroof
system. Increasing the
height of firewalls above that required by
regulatory requirements will provide additional
protection. The location and width of
the fire breaks will depend upon the fire risk
imposed by plant material.
On large continuous roof areas, firebreaks
of 1200 mm (4 ft) at approximately
every 30 m (100 ft) may be used as a rule of
thumb.
Providing and increasing the width of
vegetationfree
zones (0.9 m or 3 ft wide)
adjacent to walls and openings in the roof
(such as drains, skylights, rooftop equipment,
etc.), providing sprinkler irrigation,
and deadheading
vegetation will assist in
minimizing the risk of fire. Hot exhaust
from production equipment, kitchen hood
exhausts, and equipment that expel material
onto the roof (such as lint from dryer
vents) also pose higher fire risk.
Increasing the size of the vegetationfree
zones, together with the use of fireresistant
materials such as concrete curbs vs. wood
curbs, would increase the margin of safety.
It is important to review increased risk
potential with owners and endusers
and
establish maintenance procedures to ensure
leaves and other debris are cleaned
from vegetationfree
zones around equipment
on a regular basis. The maintenance
plan should include regular inspection and
maintenance of grease traps and cleaning of
the interior of ducts that can carry firehazardous
materials onto the roof.
When the roof is required to accommodate
a concentration of mechanical equipment
that would require extensive coverage
of vegetationfree
zones to provide adequate
fire protection, provide a roof divider and
use a standard (nonvegetative)
conventional
or protected membrane roof in these
areas. Roofs covered with vegetation can be
tested for interior fire exposure according to
current prescribed test procedures.
SLOPE AND DRAINAGE
Green roofs have demonstrated the ability
to control stormwater runoff through
absorption, the slow release of water into
the storm drainage system, or evaporation.
In colder regions, a buildup of snow on the
roof will retard surface drainage, and
increased water runoff may occur when the
soil becomes frozen and snow cover is minimal.
Waterretention
drainage panels and
waterretention
mats installed under the
growing medium will further reduce water
runoff. Waterretention
drainage panels not
only promote drainage, but also allow water
vapor to migrate out of the system after
water has receded. The use of controlled
flow drains and ponding of water under
drainage panels should be avoided, due to
the risk imposed by increased loads, potential
washout
of growing medium and
plants, and floating insulation on protectedmembrane
roofs.
A minimum of one drain plus one overflow
drain or scupper is recommended for
each roof area. There is a higher risk of roof
collapse, should blockage of the drainage
system occur. The prudent design decision
may be to install drains at closer intervals
rather than providing a few large ones.
The installation of green roofs on buildings
that do not have adequate slopetodrain
could result in ponding water, particularly
during severe rain events. Regular
inspection and maintenance must be provided
to keep drainage paths and drains in
good operating condition.
Due to reduced water flow at scuppers,
scupper outlets should be designed larger
than internal drains by a factor of three.
Larger screens designed to maximize water
flow are recommended for both scuppers
and internal drains. An example of a larger
Figure 9
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Figure 10
screen for a scupper is shown in Figure 8.
Roof decks should be designed to shed
water effectively. A slope of 2 percent or a
quarterinch
fall per linear foot should be
considered the minimum requirement. Stabilization
measurements are required on
roof slopes exceeding 16 percent to keep the
roofing system and landscaping in place in
order to prevent shear failure. Do not rely
on friction or adhesive alone. A restraint
system must transfer the gravity load to the
structure. Restraint systems can be installed
at the eaves, at the ridges, or within
the field of the roof.
If insufficient slope is provided, longterm
creep deflection of the structure or
oversights in construction can collect silt
that washes out of the
growing medium and collects
at low points, thereby
blocking drainage
paths.
To aid in obtaining
positive roof drainage in
addition to positive slope,
roof drains should be installed
in a sump that will
allow them to be set below
roof level. Drainage sumps
on concrete decks should
be a minimum of 1200
mmx 1200mmand should
slope gradually from general
roof level to a minimum
of 19 mm at the roof
drain. An example of a
sump found in a concrete
deck on a protectedmembrane
roof is shown in Figure 9. An example
of a roof drain for a conventional roof is
shown in Figure 10.
Excessive slope at the drain sump can
cause wrinkling of the roof membrane and
break adhesive bonds between membrane
layers. The sump should be designed to
accommodate variations in construction
and ensure clamping rings do not restrict
water flow. Clamping rings with drainage
slots on the underside of the clamping ring
are preferred, as shown in Figure 11.
Most landscape architects are well
aware of the requirements for irrigation and
drainage for plant survival; however, often
little consideration is paid to the impact of
water on the performance and durability of
the waterproofing system.
In some cases, landscape architects
may attempt to minimize the impact of
ponding water on plants by installing a
thicker (50 mm to 100 mm) drainage layer.
The impact of ponding water as it relates to
live loading and the impact on the performance
of the membrane cannot be ignored.
Typically, an increase in the moisture
content of the membrane will erode its performance
characteristics and shorten its life
expectancy. Increased rainfall, low temperatures,
and slow drying conditions characterize
late fall days in most of the northern
states and Canada. Wet soil will become
frozen during long periods of freezing temperatures,
a development that will prevent
topside drying.
Good slope and drainage will minimize
the impact of moisture on the surface of the
roof membrane. The installation of waterretention
drainage board immediately above
the membrane on a conventional roof will
increase drainage and promote drying at
the membrane level, thereby improving the
system’s response to moisture control. This
is achieved by providing continuous
drainage and venting paths at flashings and
roof drains.
The drainage paths can be vented to the
roof surface at vegetationfree
zones, thereby
promoting drying. Providing insulation
with drainage grooves has the same effect
and allows drainage and venting at the
membrane level on protected membrane
roof assemblies. Additional information on
the use of drainage grooves is provided in
the section addressing insulation. At the
bottom of slopes and when relatively thin
drainage panels are
installed, a drainage pipe
is required to collect and
move water to the drain.
Three types of drainage
pipes are available:
round, square, and triangular.
The latter two are
preferred, due to their
ability to evacuate water
and increase watercarrying
capacity at a lower
drainage plane.
Due to their exposure
to moisture and to the
corrosive nature of some
fertilizers, the use of
drains that are made of
corrosiveresistant
material,
such as copper or
Figure 11 stainless steel, should be
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considered. If corrosiveresistant
drains are
not available, preference should be given to
fitting drains with stainlesssteel
bolts,
clamping rings, and strainers. Roof drains
must be accessible for regular inspection
and maintenance. Roof drains in intensive
landscaping may be located a meter or more
below the surface, and access wells need to
be provided, as shown in Figure 9.
Access wells also serve as relief for ventilation.
Roof drain strainers should be
designed to facilitate inspection and cleaning.
If traditional castiron
drains are used,
they should be painted or coated to improve
their durability. The coating must be compatible
with the roof membrane.
Bolts used to secure strainers should be
stainless steel wiped with Teflon Dope to
minimize rusting and to make future
removal easier. Attention to these details
will increase strainers’ durability and minimize
the need to replace roof drains at considerable
risk and cost when the waterproofing
system is eventually replaced.
Heat loss and the collection of silt due to
soil washout
can result in retained moisture
around drains, providing an ideal environment
for vegetation growth, as seen in
Figure 12.
The use of zinc strips
at drain screens will
retard moss and other
vegetation growth and
assist in keeping drainage
paths open. Underscoring
the insulation to
provide routes for drainage
and reducing the insulation
thickness at the
drains will also improve drainage and drying
of the subsurface components. This
subject is reviewed in more detail under the
heading of “Roof Insulation.”
ROOF INSULATION
Dry growing medium has a higher Rvalue
than growing medium saturated with
water or frozen material. Saturated
growing medium provides a greater
heat sink than dry, which may reduce
cooling loads. The geographic location
of the building, type and depth of
growing mediums, and moisture content
of the growing medium are factors
that will impact heating and cooling
loads.
In addition, the type of vegetation
will impact heat gain or loss. It has
been stated that the evaporation from
one gallon of water equals 8,000
BTU.2 The effectiveness of plants to provide
cooling should not be underestimated.
Given the number of variables, it is recommended
that the calculation of heating
and cooling loads be based primarily on the
insulation component of the roof assembly,
a factor that can be easily calculated. A
design professional should determine the
requirements for cooling and heating. The
thermal resistance of common construction
materials should be calculated based on
information provided by ASHRAE.
Green roofs designed with a protected
membrane place great demands on the
insulation component of the system. The
type of insulation used must have good
physical and moistureresistant
properties.
Extruded polystyrene with a minimum density
of 40 PSI is recommended. Use of a
highdensity,
60or
100PSI
material
should be considered when the roof is to be
subjected to increased loads such as roof
planters, waterfalls, or heavy traffic during
or following construction.
It is recommended that the insulation
be looselaid. Looselaid
insulation will
speed construction and allow for salvage
and reuse when repair or replacement is
required, thereby reducing
cost and lessening
the impact on the environment.
While the drainage
from protected membrane
roofs covered with
vegetation has not been
extensively analyzed, on
protected membrane
roofs covered with gravel
ballast, it is believed that approximately 80
percent of water drains above the insulation
and 20 percent at the membrane level.
Drainage under the insulation is a slower
process and is retarded by the offset of
insulation boards one to another, water tension,
and irregularities in the membrane
(including side laps in membrane running
Figure 13
across the slope) causing water to dam.
Where possible, side laps of singleply
membranes should be laid with the slope in
order to improve drainage. The use of insulation
with drainage grooves will improve
drainage at the membrane level.
Drainage grooves should be installed
around the perimeter and in the field of
each insulation board. The drainage
grooves can range from 13 mm to 19 mm
wide and deep and can be installed by the
insulation manufacturer or contractor.
Polystyrene insulation manufacturers
do not recommend the use of a drainage
mat or protection board under the insulation,
since convective currents could reduce
the effectiveness of the insulation and
change the location of the dew point.
Depending upon the construction schedule
and anticipated loads from construction
traffic, the underscoring of insulation may
eliminate the requirements for a drainage
mat and protection board on some systems.
An example of drainage grooves is shown in
Figure 13.
In order to promote drainage, reducing
the thickness of the insulation at roof
drains is recommended, since more water is
shed from the surface of the roof insulation
than below it. Reducing the insulation
thickness and increasing the thickness of
gravel ballast at the drains will tend to offset
flotation forces at the low points of the
roof.
This will also increase the heat loss
adjacent to the roof drains, an increase that
will assist in keeping drainage paths open
in the winter months. An example of reducing
the insulation at roof drains for landscape
roofing is shown in Figure 14.
When the roof membrane is installed
above the insulation, such as in a conventional
roof assembly, the insulation should
be installed with a highdensity
cover board
in order to improve the roofmembrane
resistance to damage from construction
traffic.
Figure 12
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Figure 14
WATERPROOF MEMBRANE
The incorporation of a root barrier was
not generally included in most early landscape
roofing designs. The need for protection
of the waterproofing membrane from
root penetration is now receiving general
acceptance.
Some membranes, such as polyvinyl
chloride (PVC) and thermoplastic polyolefin
(TPO), provide a natural root barrier. Waterproof
membranes incorporating organic
material such as asphaltbased
products
are susceptible to microorganic
activity
and root penetration.
Roots can infiltrate small deficiencies in
the membrane and lap joints, resulting in a
breach of the waterproof membrane. Protection
from root penetration can be provided
with sheet root barriers. Other membranes,
such as modified membranes, can
be manufactured with copper films or be
chemically altered to deter root penetration.
To prevent roots from plugging drains
and drainage pathways on protectedmembrane
roofs, a root barrier should be used
above the insulation, as shown in Figure 15.
The root barrier must be of a type to
allow the passage of water vapor. The installation
of a polyethylene sheet to provide this
function is not recommended, since it will
prevent topside venting of the insulation
and may cause the polystyrene insulation
to absorb water. Waterretention
mats
installed directly in contact with the topside
of the insulation are not recommended for
the same reason.
The level of protection against root penetration
must be assessed with each project,
since some plant varieties have more
aggressive and deeper root systems than
others. Planting shrubs and trees that have
aggressive root systems in concrete planters
is one approach to root containment.
To be effective, a sheet root barrier must
be sealed at overlap and around penetrations
(such as vent pipes) and carried up
flashings at parapets, walls, and curbs.
During the design stage, chemically altered
membranes or root barriers must be verified
to be compatible with other components,
such as metal flashings built into the
membrane layer. They must also address
environmental concerns. Figures 15 and 16
provide examples of where to terminate the
root barrier at a parapet wall and vent pipe.
Countries such as Germany have adopted
standardized membrane testing for root
penetrations. For example, root penetration
is tested under the German FLL greenroof
guidelines over a threeto
fiveyear
period.
Figure 16
Figure 15
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While testing for root membrane penetrations
is now under review by ASTM, to date
there is no Canadian test standard.
MEMBRANES
While most singleply
membranes, such
as looselaid
thermoplastic or elastomeric
sheets, have been used for green roofs,
great care must be employed in their design
and application. These systems do not have
the same redundancy offered by multiply
systems, and they have a tendency to be
more easily damaged by construction traffic.
Because singleply
membranes characteristically
do not have
the mass of multiply
membranes, defects in
the deck, such as trowel
ridges in concrete decks
or small stones tracked
onto the working surface
can puncture the membrane
from below. If a
looselaid,
singleply
system
is used on a concrete
surface such as in a protectedroof
assembly, it
should be installed over a
moistureresistant
underlay
such as polyester
felt to protect the membrane.
Thicker singleply
membranes have improved
physical characFigure
17
teristics and may carry
longer manufacturers’
warranties. Singleply
membrane should be
a minimum of 60 mils thick; 80mil
thermoplastic
and 90mil
elastomeric sheets are
also available. In short: the thicker, the better.
While changes in technology have
improved the performance of field seams in
elastomeric membranes, the use of a cover
strip over the seam is recommended for
green roofs.
Singleply
membranes can be solidly
bonded to the substrate or installed with
water cutoff
to limit the spread of water
under the membrane, should a leak occur;
however, field experience has shown that
singleply
membranes have only limited
success unless increased care is provided.
Not all solidly bonded membranes have the
same ability to restrict the flow of water
under the membrane, should a leak occur.
Multiplelayer
systems that are solidly
bonded to the deck, such as a hotrubber,
builtup
membrane using kettlemodified
SEBS mopping asphalts, or prefabricated
modified asphalt membranes, offer good
undermembrane
resistance to water flow.
In multiplelayer
systems, additional
layers of membrane can be added if needed
to build up low points to eliminate or reduce
water ponding on the membrane surface.
Should a leak occur, the disruption and
cost to remove the landscaping in order to
gain access to the roof membrane could be
substantial. To avoid this, it is prudent to
increase the number of plies of membrane
beyond that normally recommended for
conventional roofing use. Increasing the
number of plies will have a minor impact on
cost but can have a
major impact on longterm
performance and
waterproofing service life.
When constructing a
builtup
membrane, the
use of fiberglass or polyester
felts is recommended.
In addition, the installation
of a cap sheet,
such as 250 gm/m2 or
350 gm/m2 over a bituminous
builtup
or hotrubber
membrane, will
improve the membrane
crackbridging
ability,
tensile strength, and
puncture resistance.
The use of a granulesurfaced
cap sheet as
compared to a smoothsurfaced
sheet also provides
a slipresistant
work surface, prevents the insulation from
becoming adhered to the membrane, and
aids in drainage by reducing water film tension.
Due to the ability to spot physical
damage, should it occur, lightcolored
cap
sheets are recommended.
Because asphaltbased
products such
as membranes constructed with asphalt or
hot rubber are subject to root penetration,
the use of a modified cap sheet that has
been chemically formulated to deter root
penetrations as the top layer will substantially
improve the durability of the system.
Figure 17 shows a modified cap sheet being
installed over a builtup
membrane.
In order to restrict drainage, should a
leak occur, large areas of the roof and highrisk
areas such as water features should be
separated from one another with the use of
area roof dividers.
The separation allows for precise moisture
control, according to the requirements
of any given section, and enables a wider
variety of plants to be successfully established,
which can add to overall aesthetics.
On conventional roofs, dividing large roof
areas into smaller sections can reduce the
total thickness of insulation required to
achieve the required slope within each section,
thereby reducing the cost of insulation
and the need to raise the height of parapet
walls. In addition, this approach will reduce
the cost of repairs, should the need arise,
since the leaks would be contained within
smaller areas.
When incorporating ponds and waterfalls
into landscape roofing designs, an
additional, independent waterproofing system
should be installed. The use of a protected
membrane roof is not recommended
when constructing water features.
While drainage mats and root barriers
are important elements in a landscape roofing
design, they can also contribute to trapping
moisture in the roof assembly.
Trapped moisture within protectedroof
assemblies due to restricted topside venting
can increase the moisture content of the
insulation, even if extruded polystyrene
insulation is used. Good drainage and topside
venting are prerequisites if longterm
performance is to be achieved.
More study on the negative impact of
root barriers and drainage mats on topside
venting is required.
FLASHINGAND
VEGETATIONFREE
ZONE
Flashings typically represent 70 percent
of all waterproofing problems. The detailing
of flashings on roofs or podiums often poses
increased challenges. In addition to landscaping,
flashings often incorporate rooftop
mechanical equipment and may also incorporate
conduits for electrical and mechanical
services, lightning protection, railings,
fall arrest systems, and davit arms for windowwashing
equipment.
Because rooftop equipment penetrates
the moisture and thermal plane, flashings
must be designed not only to be watertight,
but also to prevent condensation and air
leakage and to be insulated to provide thermal
continuity.
Flashings at roof access points are of
particular concern. Either flashing heights
must be raised to accommodate the depth
of growing medium, or curbs must be provided
to separate the flashings from plantings
or patio areas.
The use of curbs can allow deeper
depths of growing medium without sub1
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stantially increasing flashings heights,
examples of which are shown in Figures 18
and 19.
The Forschungsgesellschaft Landschaftsentwicklung
Landschafts bau ev (F.L.L.)
guidelines as used in Germany typically
provide a horizontal (vegetationfree)
zone
between the curbs and flashings. This separation
allows for phased construction and
prevents conflict among the trades during
initial construction. The separation can be
designed to allow for drainage, provide a fire
barrier, and allow for foot traffic to gain
access to the flashings and plant areas. The
width of the gravel bed can be tailored to
each project, but it is typically 500 mm.
When a parapet wall is provided at
perimeters, the height of the parapet and
the type of material used in the vegetationfree
zone must be sufficient to prevent the
roof system and roof gardens from being
dislodged by the wind. Wind problems have
not typically occurred when roof gardens
have been constructed at or near grade
level; however, when roof gardens are
placed on taller structures, wind is more of
a concern. Building code and Factory
Mutual requirements (if applicable) must be
considered early in the design stage. A
windprotection
mat may be used to keep
the growing medium in place in highwind
zones until the plants are established.
F.L.L. recommendations in regard to
providing vegetationfree
zones adjacent to
flashings have merit. However, typically,
parking decks in North America are
designed with the landscaping carried up to
the flashings, with good success.
Given specific requirements of the
design, the width of the vegetationfree
zone
could be reduced
partially
at interior
or
high para
pet walls if a
vertical and
hor i zontal
drainage plane is provided adjacent to all
flashings to encourage drainage away from
these critical points. An example of a wall
detail with a vertical drainage plane as an
alternative to a vegetationfree
zone is shown
in Figure 20.
The drainage plane
at flashings can also be
used to vent moisture
out of the system and
minimize the impact of
the local environment
on the membrane and
flashings. The drainage
plane can be provided
by grooved insulation,
protection drainage
board, or stone.
It is also recommended
that root barriers,
drainage mats, and
insulation a minimum
1,200 mm from the
roof perimeter be
installed in the direction
of the parapet.
This will facilitate ease
of finish and access,
should a leak occur. An
example is shown in
Figure 21.
Where possible, membrane flashings
should be carried over and turned down the
outside face of the building. A minimum
flashing height of 200 mm (8 in) above the
Figure
18
Figure 19 – Courtesy of the Minto Hotel, Ottawa, Ontario.
Figure 20
S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 F I S H B U R N • 1 0 9
Figure 21
mance, helps to
eliminate condensation
traps, and
reduces the need
and frequency of
maintenance. The
benefits of insulating
flashings have
been published in a
previous paper by
this author, titled
“Improving the Performance
of the
Protected Membrane
Roofing Systems.”
3
In order to meet
minimal height
standards, membrane
flashing can
be carried up the
walls and hidden
finished surface is recommended. The top of
all flashings should be sloped to drain to the
building interior. Depending on the type of
membrane flashing, between 4 and 8 percent
slope is recommended.
In order to improve longterm
performance,
covering the vertical portions of the
roof flashing with insulation is recommended.
This approach will not only reduce the
impact of roof traffic and the external environment
on the performance of roof flashings;
it improves the overall thermal perforbehind
siding or
pavers, as shown in Figure 22.
While tradeoffs are common in design
and construction, watertightness
should
not be sacrificed for aesthetics. Due to their
resistance to corrosion, copper or stainless
steel materials are recommended to flash
roof penetrations such as soil pipes or
exhaust stacks that are built into the waterproofing
membrane.
Depending upon their location, highgrade,
prefinished
metal, copper, or stainless
sheet counterflashings are also recommended
for the same reasons. Lightgauge
aluminum flashings are not recommended,
due to poor performance when exposed to
some fertilizers, as well as to their high
thermal coefficient of expansion.
While this article primarily reviews the
application of landscape roofing on protectedmembrane
roofs, the application of
extensive green roofs is also finding acceptance
on conventional roofs.
When a landscape roof is installed on a
conventional roof assembly, many of the
benefits normally associated with protectedroofing
systems (e.g., shielding the membrane
from environmental extremes) can
also apply to the conventional roof. There
are, however, exceptions. Flashings on conventional
roofing have typically been the
“weak link.” Many of the problems associated
with flashings can be mitigated on conventional
roofs by insulating them similarly
to protected membrane roofs, as reviewed
earlier in this paper and covered in a previously
published paper, titled “Protected Membrane
Flashings Designed to Work.”4 A comparison
of a typical conventional roof design
at a parapet wall is shown in Figure 23.
While some membrane manufacturers
may not be in agreement, an alternate
approach as suggested in this paper is
shown in Figure 24.
TESTING
Leaks in green roofs can be costly to
investigate and repair. Most membrane
manufacturer warranties include clauses
that state the cost of removal and replacement
of the overburden in order to gain
access to their membrane is not covered by
the warranty.
Figure 23 – From a report on
environmental benefits and cost of
landscape roofing technology for the
Figure 22 city of Toronto, Ontario.
1 1 0 • F I S H B U R N S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7
Some warranties also give the manufacturer
the right to claim against third parties
to recover the cost to investigate and
remove the overburden, should it be proven
that the leak was not the result of defects in
the membrane. This could apply to leaks
that result from damage caused by others
or to leaks in walls or windows that eventually
find their way to the building interior
and are incorrectly assumed to be breaches
in the waterproofing membrane.
While the use of modular systems may
make the green roof more accessible and
reduces the time and cost of investigating
and repairing leaks, testing of the membrane
system to ensure it is defectand
leakfree
should be incorporated into all
landscape roofing construction prior to
installation of the overburden. Compared
on a persquaremeter
basis, repairs due to
leaks after landscaping and planting is
completed can be four to ten times greater
than the cost of repair at time of initial construction.
In order to minimize inservice
problems, some form of testing is recommended.
Although visual inspection during construction
provides useful information, testing
can also include water testing, infrared,
nuclear, capacitance, electronic fieldvector
mapping, moisture sensors, and air pressure.
These test methods are often used in
concert with one another.
This paper does not review the features
and benefits of the particular test methods,
but it is intended to highlight some of the
systems available.
When flood/water testing is employed,
the test is completed on the exposed waterproofing
membrane by dividing the roof into
zones, capping the roof drains temporarily,
and flooding the roof surface with water to
a depth of approximately 100 mm.
The water is left over a 24or
48hour
period. The watertightness
of the membrane
is determined by a visual inspection
of the building interior. In the case of new
construction, the test should be conducted
prior to the completion of interior finishes to
reduce the possibility of damage, should a
leak occur (both the NRCA and CRCA do not
endorse flood testing). A water spray test of
flashings at walls, windows, and roof openings
to ensure that seals are intact has
merit and can often pinpoint latent construction
defects that are incorrectly
assumed to be roof leaks.
The use of electronic field vector mapping
to test the continuity of the membrane
is relatively new and has proven to be beneficial
in detecting leaks in the waterproof
membrane. The grid wire, if required with
this system, can be left in place to allow for
future inservice
monitoring. Electronic
fieldvector
mapping cannot be used on all
systems. Consult the manufacturer for recommendations.
During construction, wireless
electronic moisture sensors with an
alarm and a telephone interface can also be
installed in a grid pattern under the roof
membrane to monitor performance.
MAINTENANCE
While both quality control and quality
assurance measures implemented during
construction will have a positive impact on
the performance of the roof, when the
architects and contractors have left the site,
the roof becomes the owner’s responsibility
Regardless of the type of system (intensive
or extensive) or membrane installed, all
green roofs require periodic inspection and
maintenance. This section addresses the
maintenance of the waterproofing system,
but not the plant material. Experience has
shown that it is less expensive to provide
periodic maintenance in order to maximize
the system’s service life than it is to have it
fail from neglect.
While there is, in this author’s experience,
usually great care taken in the design
and installation of a green roof, this same
care is not often afforded to roof modifications
(such as the installation of new roof
openings) after the architect, engineers, and
contractors have left the site. To aid in the
maintenance and modification of the roof,
information on the roof construction,
together with the recommended maintenance
procedure, need to be provided to the
owner or building operator at the end of the
project. On larger projects, these records
can be incorporated into the buildingcommissioning
process.
A maintenance inspection is recommended
during both the spring and the late fall, as
well as prior to the lapse of the contract or
manufacturers’ warranties. Most roofing
trade organizations, such as the CRCA and
NRCA, provide information on the frequency
and type of maintenance required for roofs in
general.
The maintenance for green roofs
(excluding plants and growing medium)
should include:
1. An inspection and cleaning of roof
drains, scuppers, and accessible
drainage paths
2. Investigation of areas that appear to
pond water
3. Inspection and removal of all debris,
including dead plant material and
spills of contaminants that can
increase the risk of fire or block
drainage paths
4. Identification and inspection of all
modifications made to the roof since
last inspected, to ensure that the
work has been completed to good
trade practices and does not negatively
impact the performance of the
roof
S Y M P O S I U M O N B U I L D I N G E N V E L O P E T E C H N O L O G Y • NO V E M B E R 2 0 0 7 F I S H B U R N • 1 1 1
Figure 24
5. Inspection of all drains, drain
screens, flashings built into membranes,
and counterflashings for
signs of movement or corrosion;
replacement of any dislodged or
damaged material
6. Inspection and replacement of any
areas or materials that have become
dislodged due to floating, wind, or
system failure
7. Inspection and reseal of all broken
or deteriorated caulking joints or
seals
8. Inspection and repair of leaks when
reported; failure to act in a timely
manner will escalate cost
Additional maintenance information is
available and may form part of most extendedwarranty
agreements available from
membrane manufacturers.
CONCLUSION
Green roofs can provide aesthetic
appeal and improved moisture, thermal,
and sound control. However, the benefits of
green roofing will not be achieved if the
waterproofing system leaks prematurely,
requiring its removal and replacement.
Depending upon its design and accessibility,
the cost of replacement can be four to
10 times the cost of original construction.
While other types of roof assemblies can
be used, roofs of protectedmembrane
design provide the best chance of success,
due to the fact that the insulation covers
and protects the roof membrane.
The roof deck must be designed to carry
the anticipated structural loads and must
be sloped to achieve positive roof drainage.
A minimum slope of 2 percent is recommended.
Steeper slopes need to be given
careful design consideration to avoid slippage
or system failure.
Roof drains must be installed below roof
level and be corrosion resistant. Access to
drains must be provided to allow for inspection
and maintenance. A means of improving
drainage and drying, the subsurface
system at the insulation and membrane
level needs to be implemented. This can be
completed by underscoring the insulation,
providing drainage mats, and installing vent
pipes.
Designs should include continuous
drainage and venting systems adjacent to
the flashings and drains. Extruded polystyrene
insulation provides good service in
green roofs, due to its physical characteristics
and moistureresistant
properties.
Where possible, insulation should be
installed in one layer.
The waterproofing membrane serves
more than one function. Membranes that
are solidly adhered to the deck will limit the
spread of water, should a leak occur. Multilayer
systems provide the benefit of redundancy.
The installation of a cap sheet on
hotrubber
or builtup
roof membranes will
improve the durability of the membrane and
longterm
service. A rootresistant
membrane
and/or root barrier that facilitate topside
venting of the insulation is required.
Special considerations need to be implemented
when using singleply
membranes;
the thicker the membrane, the better.
Double welds or cover strips will improve
the seam’s ability to provide longterm
service.
Large areas should be compartmentalized
to reduce the spread of water, should a
leak occur.
Flashings typically represent 70 percent
of all problems. Flashings should be
designed of sufficient height and durability
to survive in the environment to which they
are exposed. Designs must include continuity
of the moisture/air/vapor barrier and
thermal barrier. The flashings should be
designed to provide accessibility for inspection
and maintenance. The continuity of the
waterproofing membrane needs to be tested
during initial installation.
Systems are available to monitor inservice
performance. Inservice
inspection and
maintenance are important and should be
completed on a regular basis. Failure to do
so will increase the risk of leaks, will shorten
the life expectancy of the system, and
may nullify longterm
warranties.
Green roofs need increased inspection
and maintenance to ensure drains and
drainage paths are kept open and working.
In summary, design of green roofs must
focus on providing longterm
service, making
provisions for structural load, thermal
efficiency, and moisture control. Designs
must provide for inspection and maintenance.
Aesthetics should not override function.
With attention to these factors, the benefits
of roof gardens can be realized, and their
contribution toward the improvement of the
urban environment will be long lived.
BIBLIOGRAPHY
Bass, B., Callaghan, C., Kuhn, M.E.,
and Peck, S.W., “Greenbucks from
Green Roofs: Forging a New Industry
in Canada,” Status Report on Benefits,
Barriers, and Opportunities for
Green Roof and Vertical Garden
Technology Diffusion, Canadian
Mortgage Housing Corporation,
1999.
Fishburn, Douglas C., “Improving the
Performance of the Protected Membrane
Roofing System (Part 1),”
Construction Canada, 1997.
Fishburn, Douglas C., “Improving the
Performance of the Protected Membrane
Roofing System (Part 2),”
Construction Canada, 1997.
Fishburn, Douglas C., “Roof Gardens:
The Waterproofing Challenge,”
Interface, 2001, Vol. 19, No. 11, pp.
2530.
Garden Roofs and Garden Roof Assembly,
Hydrotech Membrane Corporation,
1997.
Peck, Steven, and Kuhn, Monica, Design
Guidelines for Green Roofs, Ontario
Association of Architects.
REFERENCES
1 LEED is a program sponsored by the
United States Green Roof Building
Council. The Leadership in Energy
and Environmental Design (LEED)
certification program offers local tax
incentive credits to buildings for
using sustainable technology and
practices. In August 2003, Canada
became a LEED licensee. The program
is operated by the Canadian
Green Building Council.
2 Marco Schmidt, EnergySaving
Strategies Through the Greening of
Buildings, Technical University of
Berlin, 2000.
3 Fishburn, Douglas C., “Improving
the Performance of the Protected
Membrane Roofing System (Part 1),”
Construction Canada, 1997.
Fishburn, Douglas C., “Improving
the Performance of the Protected
Membrane Roofing System (Part 2),”
Construction Canada, 1997.
4 Fishburn, Douglas C., “Protected
Membrane Flashings Designed to
Work,” Third International Symposium
on Roofing Technology, 1991.
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