Practical Considerations on Design and Installation of Green Roofs: The Waterproofing Challenge

Proceeedings of the RCI 21st International Convention Fishburn – 87
Practical Considerations on Design and
Installation of Green Roofs: The
Waterproofing Challenge
Roof Consultants Institute
Douglas C. Fishburn
Fishburn Building Sciences Group Inc.
Hornby, Ontario, Canada
ABSTRACT
Roof gardens, commonly referred to today as “green roofs,” can be traced
back to the Hanging Gardens of Babylon and have been used in parts of
Europe and Mexico for centuries. It has been reported that they were introduced
to Canada by the Vikings and later the French colonists in
Newfoundland and Nova Scotia, through the use of sod as a roof covering. A
green roof environment does not demand an entirely different roof design philosophy.
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.
SPEAKER
A graduate of O.A.C., now known as the University of Guelph, DOUGLAS FISHBURN has over
40 years of roofing, building envelope, construction, and design experience. He has presented
technical papers at several international building symposiums. Mr. Fishburn contributes technical
articles to professional trade journals, such as Plant Engineer, The Canadian Architect, and
Specifications Canada. He sits on a number of Canadian General Standard Board (CGSB) committees
and the Canadian Standards Association (CSA), which formalizes standards for construction
products. In addition, he has lectured at universities and provides training seminars for
trade organizations, universities, and government bodies. Mr. Fishburn is a past president of the
Ontario Chapter of RCI and has held the position director of the old Region VIII for RCI.
Fishburn – 88 Proceeedings of the RCI 21st International Convention
ABSTRACT
Roof gardens, commonly
referred to today as “green roofs,”
can be traced back to the Hanging
Gardens of Babylon and have
been used in parts of Europe and
Mexico for centuries. It has been
reported that they were introduced
to Canada by the Vikings
and later the French colonists in
Newfoundland and Nova Scotia,
through the use of sod as a roof
covering. A green roof environment
does not demand an entirely
different roof design philosophy.
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.
Over the last 40 or 50 years in
Canada, roof landscaping has
been used over parking decks and
podiums to improve aesthetics
and create market appeal for both
commercial and residential buildings.
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, method of
construction, and 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 system
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
improve long-term performance,
specific considerations must be
paid to load requirements, slope
and drainage, thermal performance,
the design of the details,
the waterproofing membrane,
testing, and the requirements for
maintenance.
This paper focuses on some of
the factors impacting on the durability
of the waterproofing system,
particularly with intensive green
roofs, and suggests methods of
design and construction that can
help achieve long-term watertight
service.
Green roofing can range from
a carpet of flowers to grasslands
to woodlands. The approach to
green roofing has been refined
over the years and is generally
divided into three types: intensive,
semi-extensive, and extensive.
Intensive green roofing is
characterized by its higher weight,
which is due to the depth of growing
medium [150 mm (6 inches) or
more] required to accommodate
larger shrubs and trees. These
systems weigh 290 to 967 kg/m2
(50 to 200lbs/ft2).
Semi-extensive green roofing
is characterized by a depth of
growing medium of approximately
(150 mm/6 inches). The weight of
a semi-extensive system can vary
from 169 to 290 kg/m2 (25 to
50lbs/ft2).
Extensive green roofing is
characterized by its lower weight,
which is due to reduced depth of
growing medium (150mm/6 inches
or less), saturated weights
between 72 and 169 kg/m2 (12 to
25 lbs/ft2), and the use of smaller
plants.
The following are the major
advantages and disadvantages of
green roofing as compared to conventional
roofing applications.
Advantages
• Increased design life expectancy
compared to a
conventional roofing system.
• Decreased roof maintenance.
• Reduced heating and cooling
costs.
• Reduced sound.
• May provide fire protection.
• Reduced water run off.
• Aesthetic appeal.
• Improved air quality.
• Food production.
• May expedite municipal
approval.
• Qualifies for LEEDS and
Energy Star programs.*
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 cost of landscape
maintenance.
• Higher cost of roof
replacement.
Proceeedings of the RCI 21st International Convention Fishburn – 89
Practical Considerations on Design and
Installation of Green Roofs: The
Waterproofing Challenge
• Difficult and costly to find and repair leaks.
• Longer reconstruction periods impact the
aesthetics of intensive systems, since mature
trees and shrubs are typically replaced with
immature ones.
Regardless of the finish, roofs can be classified
as either high-slope (which rely upon their ability to
shed water), or low-slope (which rely upon waterproofing
to provide water-tightness). Low-slope roofs
are divided into two types: conventional, where the
roof membrane is placed above the roof insulation;
or protected, where the roof membrane is placed
below the insulation. In green roofing applications,
the system must employ water-tight technology,
regardless of slope.
While conventional roofs can employ green roof
technology, typically intensive green roofs incorporate
protected roof membrane designs.
Protected roof membrane
In a protected roof membrane design, the roof
membrane is placed under the insulation. With this
configuration, regardless of the roof finish (gardens,
pavers, or gravel), the roof membrane is shielded
from the temperature extremes of the environment
and protected from traffic during and
following construction.
In a protected roof membrane
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 typically provides the function of
waterproofing. When green roofs utilize
a conventional roofing system, they
have many of the features and benefits
of protected roof membrane assemblies.
An example of a conventional roof is
shown in Figure 2.
Roof Deck and Load Requirements
The roof deck must be designed to
carry the anticipated structural loads,
including the temporary loads imposed
Fishburn – 90 Proceeedings of the RCI 21st International Convention
Figure 1
Figure 2
by construction equipment and
stockpiling of materials. While a
number of roof decks types can be
utilized in both intensive and
extensive green roofing, the use of
concrete is preferred in the construction
of intensive green roofs
due to its high strength, lower
cost, and ease of providing slope
to drain.
When concrete decks are left
exposed and used as staging
areas before the installation of the
waterproofing membrane, consideration
should be given to using
additives in the concrete mix to
reduce water absorption, and
epoxy-coated rebar to improve
the deck’s ability to provide
long-term service without
extensive repair. Provided the
deck has been designed to
carry the anticipated dead and
live loads, lightweight decks,
such as wood plank and/or
steel, can be employed in the
design of green roofs.
The use of lightweight
materials in the design of
green roofing increases their
potential for use in both new
and existing buildings. Building
up the planting area with polystyrene
insulation in lieu of a full
depth of soil, using drainage mats
in lieu of a heavy layer of gravel,
or using planting medium and
plant varieties that can grow in a
minimum depth of soil will contribute
to reduced weight.
One solution to address gravity
or live loads is to use planter
boxes for larger shrubs and locate
them over columns or at the roof
perimeter. Examples are shown in
Figures 3 and 4.
Concrete-topped insulation,
rubber walkway pavers, stepping
stones, or wood or plastic walkways
in traffic zones are all
designed to reduce the load on the
roof assembly. 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 as compared
to conventional roofing systems
can be a major limitation from
both a cost and functional point of
view. An example of stepping
stones used to reduce weight is
shown in Figure 5.
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.
While existing protected membrane
roofs may be viewed as
good candidates for green roofing,
the load capacity needs to be
carefully considered. Many protected
membrane 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
floatation, which would result in
damage to the roof membrane.
The weight of the gravel ballast
was typically 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 loose-laid and a water-
Proceeedings of the RCI 21st International Convention Fishburn – 91
Figure 3. Courtesy of the Bank of
Canada, Ottawa, Ontario, Canada.
Figure 4. Courtesy of the Minto
Hotel, Ottawa, Ontario,
Canada.
Figure 5. Courtesy of the Minto
Hotel, Ottawa, Ontario,
Canada.
permeable fabric installed over
the insulation. The ballast was
installed at a weight of 48.8
kg/m2. The insulation was expected
to float under ponding water
conditions.
If the roof periodically ponds
water (which can occur if positive
slope has not been provided or
control flow drains are used), the
water-permeable fabric was
expected to keep the insulation
boards in alignment like a raft
floating on water.
Designers and contractors
must proceed with caution when
substituting the gravel ballast on
lightweight protected roof membrane
assemblies and installing
an extensive green roof cover, particularly
when the roof has been
constructed with 100 mm of insulation.
The unsaturated weight of
growing medium or less dense
soils may be insufficient to prevent
floatation, and failure of the
landscaping will result.
The Canadian national and
provincial building codes are not
static; they change to reflect
increased or decreased snow
loads. In recent years, the snow
loads in Toronto have been
reduced. This reduction may, in
some cases, allow additional load
capacity for the installation of
green roofs.
Conventional roofs, such as
modified membrane or built-up
roofs, are typically much lighter
than protected membrane roof
assemblies and may offer more
flexibility in the design of green
roofing in regards to gravity loads.
The weight of a built-up roof
membrane can be reduced to
approximately that of a modified
roof membrane by substituting
the bitumen and gravel surfacing
with a ply of modified membrane
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 outlined
elsewhere in this paper.
Slope and Drainage
Landscape roofing increases
the control of stormwater management
since it absorbs rainwater
and either releases it more
slowly into the storm drainage
system or releases the water
through evaporation. While the
installation of landscape roofing
may eliminate the requirements
for control flow drains, municipalities
may dictate otherwise.
The use of controlled flow
drains should be avoided since
they can back up water into the
landscaping and increase the
effects of wetting and drying of the
roof membrane.
Roof decks should be designed
to shed water effectively. A slope
of 2% or a 1/4-inch fall per linear
foot should be considered the
minimum requirement. On slopes
in excess of 8%, a restraint system
may be required in order to
keep the roofing system and landscaping
in place.
A restraint system must
transfer the gravity load to the
structure, but must also be designed
to accommodate drainage.
If insufficient slope is provided,
long-term creep deflection of the
structure or oversights in construction
can collect silt that
washes out of the growing medium
and collect 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 mm x 1200 mm and should
slope gradually from general roof
level to a minimum 19 mm at the
roof drain.
Excessive slope 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 at the bottom are preferred
in order to allow 100% drainage at
the membrane level. A buildup of
silt around roof drains will
encourage vegetation growth and
Fishburn – 92 Proceeedings of the RCI 21st International Convention
Figure 7
restrict drainage paths. Figure 6
illustrates an example of a roof
drain for a conventional extensive
green roof.
While most landscape architects
are well aware of the requirements
for irrigation and drainage
to maintain the plant material,
often little consideration is paid to
the impact of water on the performance
and durability of the
waterproofing system.
While membranes used in
these types of applications have
watertight characteristics, membranes
will have a tendency to
absorb moisture over time when
immersed in water.
Typically an increase in the
membrane’s moisture content 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 Canada. Wet
soil will become frozen during
long periods of freezing temperatures,
which will prevent topside
drying. A buildup of snow on the
roof will retard surface drainage;
however, increased water run off
may occur when the soil becomes
frozen and snow cover is minimal.
Good slope and evaporation
will minimize the moisture content
of extensive green roofs when
installed over conventional roof
systems. The installation of a
drainage and venting system in
intensive green roofs will improve
both summer and winter 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 that are connected to
drainage pipes at the insulation
level. The drainage pipes are vented
to the roof surface to promote
drying. The system is shown in
both sectional and isometric views
in Figures 7 and 8.
The plant material or landscape
furniture (such as benches
or planters) can camouflage the
vent pipes. Three types of
drainage pipes are available:
round, square, or triangle. 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 the corrosive
nature of some fertilizers,
the use of drains
that are made of corrosive-
resistant material,
such as copper or stainless
steel, should be considered.
If corrosive-resistant
drains are not available,
preference should
be given to fitting drains
with stainless steel 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 7.
These wells also serve as shafts
for ventilation. Roof drain strainers
should be hinged to facilitate
cleaning.
If cast-iron 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 avoid excessive
damage and 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 wash out can result
in retained moisture around
drains, providing an ideal environment
for vegetation growth.
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
Proceeedings of the RCI 21st International Convention Fishburn – 93
Figure 8
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
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 moisture-resistant
properties. Type 4 extruded polystyrene
is one such insulation.
Use of a high density material
should be considered when the
roof is to be subjected to
increased loads such as roof
planters, waterfall features, or
heavy traffic during or following
construction.
It is recommended that the
insulation be loose-laid. 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. On roofs with
slopes approaching 8%, care must
be used to prevent insulation slippage
(as outlined in the “Slope
and Drainage” section). When calculating
heating and cooling
loads, the growing medium can
add to the system’s thermal performance
and may be substituted
for part of the insulation.
While the drainage of green
roofs has not been extensively
analyzed, it is believed that
approximately 80% of all water
drains should be above the insulation
and 20% at the membrane
level. Drainage under the insulation
is a slow process. It is retarded
by the offset of insulation
boards one to another, water tension,
irregularities in the deck
over which the membrane is
installed, and irregularities in the
roof membrane that result in variations
in the membrane thickness,
particularly when liquidapplied
systems are installed.
Side laps in the membrane can
dam water when the membrane is
laid across the slope, such as in
the case of single-
ply systems.
In order to
improve drainage
and provide a
direct path of
water to the roof
drains 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. 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 9.
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 landscaping
at the drains will offset any floatation
at the low points of the roof.
This will also increase the
heat loss adjacent to the roof
drains, which 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 10.
When the roof membrane is
installed above the insulation, the
insulation should be installed
with a high-density coverboard in
order to improve the roof membrane
resistance to puncture from
traffic.
Fishburn – 94 Proceeedings of the RCI 21st International Convention
Figure 10
Figure 9
Waterproof Membrane
The incorporation of a root
barrier was not generally included
in most early landscape roofing
designs; however, the protection
of the waterproofing membrane
from root penetration is now
receiving general acceptance.
Some membranes, such as
polyvinyl chloride (PVC), provide a
natural root barrier. Those incorporating
organic material such as
asphalt-based products are susceptible
to micro-organic 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 penetration can be
provided with sheet root
barriers. Other membranes,
such as modified
membranes, can be manufactured
with foil films
or be chemically altered
to avert root penetration.
To prevent roots from
congesting drainage
routes leading to drying
at the insulation level in
protected membrane
roofs, a root barrier
should be used above the insulation
as shown in Figure 11.
The level of protection against
root penetration must be assessed
with each project, since some
plant varieties have more aggressive
and deeper root systems.
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
soil pipes, and carried up flashings
at parapets, walls,
and curbs. During the
design stage, chemically
altered membranes must
be verified to be compatible
with other components,
such as metal
flashings built into the
membrane layer. Figures
11 and 12 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
green roof standard over a three
to five-year period. While testing
for root membrane penetrations is
now under review by ASTM, to
date there is no Canadian test
standard.
While most single-ply membranes,
such as loose-laid ethylene
propylene-diene membrane
(EPDM) and polyvinyl chloride
(PVC), have been used for landscaped
roofs, great care in the
design and application must be
employed. These systems do not
have the benefit of redundancy,
Proceeedings of the RCI 21st International Convention Fishburn – 95
Figure 11
Figure 12
and have a tendency to be more
easily damaged by construction
traffic.
Because single-ply membranes
characteristically do not
have the mass of multi-ply 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 the underside. If
a single ply system is used on a
concrete surface, such as in a
protected roof assembly, it should
be installed with a moisture-resistant
underlay vent such as polyester
felt.
Thicker membranes have
improved physical characteristics
and may carry longer manufacturer’s
warranties. 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 will
increase long-term performance.
Single-ply membranes can be
solidly glued to the substrate or
installed with water cut-off mastic
to limit the spread of water under
the membrane should a leak
occur, however, field experience
has shown that single-ply membranes
have only limited success
unless increased care is provided.
Multiple layer systems that are
solidly bonded to the deck (such
as hot rubber, built-up membrane
using kettle-modified SEBS mopping
asphalts, or prefabricated
modified membranes), offer good
water resistance.
They also provide the redundancy
of a multi-ply system and
(in the case of leaks) will limit
moisture ingress to localized
areas since they are bonded to the
deck.
Additional layers of membrane
can be added if needed to build up
low points and eliminate 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 use.
Increasing the number of plies
will have a minor impact on cost,
but can have a major impact on
long-term performance and roof
service life.
When constructing a built-up
membrane, the use of glass or
polyester felts is recommended. In
addition, the installation of a cap
sheet such as 250 gm/m2 polyester
cap sheet over a bituminous
built-up or hot rubber membrane
will improve the system’s crack
bridging ability, tensile strength,
and puncture resistance.
The use of a granule-surfaced
cap sheet as a surface to the
membrane also provides a resistant
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,
light-colored membranes are preferred.
Because asphalt-based 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 13 shows a
chemically resistant, modified cap
sheet being installed over a builtup
membrane.
When incorporating ponds
and waterfalls into landscape
roofing designs, an additional,
independent waterproofing system
should be installed. Large
areas of landscaping should be
separated into smaller sections by
the installation of area dividers.
The separation allows for precise
moisture control according to the
requirements of any given section,
and enables a wider variety of
flora to be successfully established,
which can add to overall
aesthetics. The required drainage
slope while minimizing the impact
of elevations at wall junctions can
also be achieved. In addition, this
approach will reduce the cost of
repairs should the need arise,
since the leaks would be contained
within smaller areas.
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 protected
roof assemblies due to
restricted topside venting can
increase the moisture content of
the membrane and insulation,
even if type 4 polystyrene insulation
is used. Good drainage and
topside venting are prerequisites
if long-term performance is to be
achieved. More study is required
on the negative impact of root barriers
and drainage mats on topside
venting.
Fishburn – 96 Proceeedings of the RCI 21st International Convention
Figure 13
Flashing
Flashings typically represent
70% of all waterproofing problems.
The detailing of flashings on
roofs or podiums often poses
increased challenges. In addition
to landscaping, these areas often
house rooftop mechanical equipment,
and may incorporate conduits
for electrical and mechanical
services, lightning protection,
railings, fall arrest systems, and
davit arms for window-washing
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
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 provided
to separate the flashings
from plantings or patio areas.
The use of curbs can allow
deeper depths of growing medium
without substantially increasing
flashings heights, examples of
which are shown in Figures 14
and 15.
F.L.L. standards as used in
Europe typically provide a vertical
separation (border) between the
curbs and flashings. This separation
allows for phased construction
and prevents conflict between
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 is typically
shown at 500mm.
When a border is provided at
exterior parapet walls, the height
of the parapet and the size of the
aggregate used must be sufficient
to prevent the roof and roof gardens
from being dislodged. Wind
issues have not typically been a
problem when constructing roof
gardens at or near grade level;
however, their use on taller
structures is a concern.
Building code and Factory
Mutual requirements must
be considered early in the
design stage.
While L.L.C. recommendations
in regard to positioning
the gravel bed adjacent to
flashings have merit, typically
when designing parking
decks in North America, the
landscaping has been typically
carried up to the flashings with
good success.
Given specific requirements of
the design, the width of the gravel
bed could be reduced, providing a
vertical and horizontal drainage
plane, and should be provided
adjacent to all flashings to
encourage drainage away from
these critical points. The drainage
plane at flashings can also be
used to vent moisture out of the
system and minimize the impact
of local environment on the membrane
and flashings.
The drainage plane can be
provided by a combination of
insulation, protection board, and
crushed aggregate or river stone.
It is also recommended that root
barriers, drainage mats, and
insulation be installed parallel to
the roof perimeter. This will facilitate
ease of finish and access,
should a leak occur.
Where possible, membrane
flashings should be carried over
and turned down the outside face
Proceeedings of the RCI 21st International Convention Fishburn – 97
Figure 14
Figure 15. Courtesy of the
Minto Hotel, Ottawa,
Ontario, Canada.
of the building. The top of
all flashings should be
sloped to drain to the
building interior. Depending
on the type of
membrane flashing, between
4 to 8% slope is
recommended.
In order to improve
long-term 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 performance,
helps to eliminate condensation
traps, and
reduces the need and frequency
of maintenance.
In order to meet minimal
height standards, membrane
flashing can be carried up the
walls and hidden behind siding or
pavers as shown in Figure 16.
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,
high-grade pre-finished metal,
copper, or stainless sheet counter
flashings are also recommended
for the same reasons.
Aluminium flashings are not
recommended due to poor performance
when exposed to some fertilizers,
and due to their high
thermal coefficient of expansion.
While this article primarily
reviews the application of landscape
roofing on protected membrane
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 protected
roofing systems (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 similar
to protected membrane
roofs as shown earlier in
this paper.
A comparison of a typical
conventional roof design
at a parapet wall is shown
in Figure 17. An alternate
approach as suggested in
this paper is shown in
Figures 18 and 19.
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.
Fishburn – 98 Proceeedings of the RCI 21st International Convention
Figure 16
Figure 17 – From a report on environmental
benefits and cost of landscape
roofing technology for the city
of Toronto. Typical Sopranature
green roof assembly on conventional
roof (adapted from Soprema Inc.).
Some warranties
also give the manufacturer
the right to claim
against third parties to
recover the cost to investigate
and repair 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 leaks in walls
that eventually find their
way to the building interior.
While the use of
modular systems makes
the green roof forklift
accessible and reduces
the time and cost of
investigating and repairing
leaks, testing of the
system to ensure it is
defect and leak free
should be incorporated
into all landscape roofing
construction.
Compared on a per
square meter 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
in-service problems,
some form of testing
is recommended.
Although visual inspection
during construction provides useful
information, testing can also
include water testing, infrared,
nuclear, capacitance, electric field
vector 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 is
intended to highlight some of the
systems available.
Positive bonding of the waterproofing
membrane offers advantages
over loose-laid systems;
however, some single-ply systems,
such as PVC, can be installed
with double welds that allow them
to be pressure treated with air to
confirm their continuity.
When water testing is
employed, the test is completed
on the exposed waterproofing
membrane by dividing the roof
into zones, temporarily capping
the roof drains, and flooding the
roof surface with water to a depth
of approximately 100 mm.
The water is left over a 24- or
48-hour period. The water-tightness
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. Flood tests should
include a spray test of flashings to
ensure that seals are intact.
The use of electronic field vector
mapping to test the continuity
of the membrane is relatively new
and has proven to be beneficial in
Proceeedings of the RCI 21st International Convention Fishburn – 99
Figure 18
Figure 19
not only detecting leaks but also
defects in the waterproof membrane.
The grid wire used with this
system can be left in place to
allow for future in-service monitoring.
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
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, 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.
Because the membrane is not
accessible for inspection without
the removal of the landscaping or
plant material, maintenance of
the system generally includes
keeping the surface and roof
drains clean of debris, ensuring
that caulking seals are maintained,
metal flashings are kept in
place, and deteriorated or damaged
membrane and flashings are
repaired in a timely manner.
While there is usually great
care in the design and installation
of a green roof, based on the
author’s experience, this same
care is not often afforded to roof
modifications (such as the installations
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, needs 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
building commissioning process.
A maintenance inspection is
recommended in both the spring
and late fall, as well as prior to the
lapse of the contract or manufacturer
warranties. Most roofing
trade organizations, such as the
Canadian Roofing Contractors’
Association, provide information
on the frequency and type of
maintenance required for roofs in
general. Specific maintenance
information is available and forms
part of most extended warranty
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 4 to 10 times
the cost of original construction.
While other types of roof
assemblies can be used, roofs of
protected membrane 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 be sloped to
achieve positive roof drainage. A
minimum slope of 2% is recommended.
As mentioned, slopes
above 8% need to be given specific
consideration to avoid slippage.
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
channels adjacent to the flashings
and drains. Type 4 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. Multi-layer systems provide
the benefit of redundancy.
The installation of a cap sheet on
hot rubber or built-up roof membranes
will improve the membrane’s
durability and long-term
service. A root-resistant membrane
is recommended over a root
barrier to better facilitate topside
venting.
Special considerations need to
be implemented when using single-
ply 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% of all problems. Flashings
should be designed of sufficient
height and be durable enough 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 accessible for inspection
and maintenance. The continuity
of the waterproofing membrane
needs to be tested during
initial installation.
Fishburn – 100 Proceeedings of the RCI 21st International Convention
Systems are available to monitor
in-service 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, shorten the life
expectancy of the system, and
may nullify long-term warranties.
In summary, design of green
roofs must focus on providing
long-term 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.
REFERENCES
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.
November 2001, Vol. 19,
No. 11, pp. 25-30.
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.
Peck, Steven, and Kuhn, Monica,
“Design Guidelines for
Green Roofs.” Ontario Association
of Architects
Garden Roofs and Garden
Roof Assembly. Hydrotech
Membrane Corporation.
1997.
*LEEDS is a program sponsored
by the United States Green
Roof Building Council. 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 Leeds licensee. The program is
operated by the Canadian Green
Building Council. Energy Star is a
program of the Environmental
Protection Agency, USA (EPA) that
provides energy efficiency performance
ratings.
Proceeedings of the RCI 21st International Convention Fishburn – 101