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Paving Systems Over Plaza waterproofing Membranes: The Importance of Membrane-Level Drainage

May 15, 2009

Subsurface drainage in any paving system
is critical to its long-term performance,
particularly in freeze-thaw climates.
“Paving” is a broad term that includes both
continuous (i.e., asphalt or Portland cement
concrete) and unit paving systems (i.e.,
brick, stone, or precast concrete pavers,
installed either in an open-joint configuration
or with filled joints). While each of these
systems performs differently, they all allow
some water to seep through cracks or
joints. Where the paving is installed on
grade, this incidental water typically seeps
down into the ground, aided by gravel layers
and/or perforated pipes where appropriate.
However, when paving is
installed over a waterproofing
membrane, moisture that penetrates
it can become trapped in
the paving and cause drainage
and durability problems. Paving
installed over waterproofing
membranes can be found on
pedestrian or vehicular plaza
decks over below-grade parking
garages or occupied space, and
also on rooftop terraces and bridge decks.
These applications require membrane-level
drainage (see Figures 1A and 1B). Designers
mus make provisions for water that infiltrates
the paving and collects on the membrane
to travel laterally to a drainage outlet.
Plaza drainage systems such as paverpedestals,
prefabricated drainage composites,
and bilevel drain fixtures have long
been available, but we continue to see failures
in paving systems over waterproofing
where membrane-level drainage was not
provided. This article presents examples of
problems the authors have observed on
existing plaza decks, and it reviews principles
for designing successful membranelevel
drainage in new or remedial designs to
prevent premature deterioration of the
Problems Caused by Poor Membrane-Level
Moisture trapped in the paving system
due to poor membrane-level drainage can
create numerous problems, such as the following:
• Freeze-thaw cycles can cause paving
materials, such as the stone shown
in Photo 1, to flake and crumble.
These pavers were installed on a
thin peastone setting bed over
waterproofing membrane, and the
internal drains on this plaza have
only small, slow-flowing seepage
holes at the membrane level. The
concrete paving slab in Photo 2 is
another example of freeze-thaw
Figure 1A (above) and 1B (right) – Two
examples of plaza paving systems with
membrane-level drainage: unit pavers on
pedestals, and concrete paving slab over
drainage composite.
18 • I N T E R FA C E MA R C H 2009
damage caused by poor drainage.
This slab was poured directly on top
of the waterproofing with no
drainage layer, and the plaza drains
are typical roadway storm drains
with no path for water on the membrane
level to get into the drain. It is
important to note that some paving
materials have a greater ability to
resist freeze-thaw cycles than others,
but material selection is outside
the scope of this article.
• Frost heaving is also a concern in
cold climates and can create tripping
hazards, such as
the brick shown in
Photo 3. This closedjoint
(mortar joint)
brick paving system
was installed in a mortar-
setting bed directly
over the waterproofing
(no drainage lay –
er), and the plaza
drains had no seepage
holes to drain water at
the membrane level.
Water that soaks
through the joints
between pavers becomes
trapped in the
setting bed material,
creating frost heaves
in the winter months.
Photo 1 – Freeze-thaw damage to
stone pavers.
Photo 2 – Freeze-thaw damage
to concrete paving slab.
Photo 3 – Frost
heaving of brick
MA R C H 2009 I N T E R FA C E • 1 9
• Efflorescence on paving can be
caused by moisture migrating to the
surface and depositing salts in the
form of white stains. Efflorescence is
particularly common at the bottom
of stairs and other transitions where
water can exit the system (Photo 4),
but it can also occur in stagnant
areas of flat paving where moisture
wicks to and evaporates from its
• Leakage through the waterproofing
(Photo 5) is not directly a paving
problem but is important to note
because leakage is often exacerbated
by drainage problems, and it can
reduce the lifespan of a plaza where
the paving has to be removed to
replace the waterproofing. Many of
the severe plaza leakage problems
that we have seen were in instances
where membrane-level drainage was
not provided under the
paving. Rather than
flowing past, water was
trapped against the
waterproofing, exerting
constant hydrostatic
pressure on any holes
in the waterproofing.
(Wa ter proofing system
selection, flashing de –
tails, and workmanship
also play a major role in
the prevention of leakage,
but these issues
are outside the scope of
this article).
Use Membrane-Level
Drainage to Avoid Paving
The three design
features described be –
low can help provide
good membrane-level drainage and avoid
premature deterioration of the paving.
Provide positive slope-to-drain at the
waterproofing membrane level. An inverted
diamond pattern (four-way slope to an
internal drain – Figure 2) is generally the
most efficient layout. The slope on the
waterproofing should be a minimum of onequarter
in per ft (approximately 2%). Lesser
20 • I N T E R FA C E MA R C H 2009
Photo 4 – Efflorescence on brick
pavers at the base of stairs.
Photo 5 – Leakage to interior
spaces below a plaza.
slopes will drain slowly and increase the
likelihood that localized areas of unevenness
in the deck surface will result in ponding
on the membrane (particularly in valleys,
where the slope is already less). All
decks have some natural unevenness due
to construction tolerances and will experience
deflection when loaded.
On a recent waterproofing reconstruction
project, the existing concrete deck (a
two-way slab with 28-ft column bays) in a
large planter area had midspan deflections
ranging from three-quarter to two inches,
which caused water to pond on the membrane.
Positive slope is needed to overcome
the unevenness and deflection. For new
construction, locating the plaza drains near
the midspan of the deck (as opposed to near
the columns – see Figure 2) will allow future
deflection to complement the intended slope
(as the low point at the drain deflects even
lower), rather than working against it. If a
drain cannot be located at the center of
each span, provide sufficient slope to counteract
the anticipated deflection and maintain
good slope-to-drain on the waterproofing
membrane. The structural engineer can
estimate the deflection of the plaza deck.
When reconstructing an existing plaza, a
simple, level survey of the structural deck
can determine existing slopes and locate
low points.
Reconstruction of existing plazas with
little or no slope may require adding tapered
concrete to improve drainage prior to
installing the waterproofing. A structural
engineer must confirm that the existing
structure can safely support the additional
weight of tapered concrete. Improvements
to existing slope may also be limited by the
height of perimeter conditions. When
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MA R C H 2009 I N T E R FA C E • 2 1
Figure 2 – Example drainage plan showing four-way drainage coordinated with the
structural column layout.
designing a drainage layout, coordinate
slope on the paving surface with slope on
the membrane level, and space drainage
outlets (discussed below) closely enough to
accommodate desired slopes without
exceeding the available thickness at the
high point between drains. Coordinate the
drainage layout with curbs, expansion
joints, and other elements that could interfere
with drainage.
Drainage Layer
Most paving materials have such low
permeability that, even with proper slope,
they will block the flow of subsurface water
and become saturated if installed directly
on the membrane. Therefore, a drainage
layer should be provided between the waterproofing
and the paving whenever possible.
The best way to promote free drainage
under the paving is to use a system of
pavers on pedestals (Figure 1A). Pedestals
keep the paving up out of the water that collects
on the membrane and provide an open
space underneath the paving so water can
flow freely to a drain. Paver-on-pedestal
systems also allow easy removal and reinstallation
of the paving for inspection and
maintenance of the waterproofing membrane.
However, pedestals are not applicable
to small-unit pavers (i.e., brick) or continuous
paving. Also, most prefabricated
paver and pedestal products have limited
load-bearing capacity and are intended for
pedestrian traffic only. Custom paving systems
can be designed to accommodate
vehicular traffic using reinforced concrete
“paver slabs” set on concrete piers for
drainage. These systems work similarly to
pedestrian pedestal-paver systems, but the
custom paver slabs are generally thicker
and require special lifting equipment.
On decks where other paving finishes or
traffic-bearing properties are desired, a
geosynthetic drainage core (Figure 1B)
placed under the paving can provide more
uniform support for the paving and still
allow drainage. Drainage cores consist of
plastic that is molded into a dimpled sheet
or woven into an open grid (Photo 6) and are
used to support overlying materials while
maintaining a path for drainage between
dimples or strands. Drainage cores are
more easily clogged than a paver-onpedestal
system because the drainage area
is generally smaller; and soil, concrete, or
mortar placed over them can run or be
washed into the drainage area where the filter
fabric is not continuous or properly
installed. Designs using a geosynthetic
drainage core should consider the following:
• Flow capacity is published for most
drainage cores. A product with
capacity exceeding the expected flow
rate should be selected. For low-permeability
paving systems where
most of the water drains off the surface,
the expected flow rate at the
membrane level is very small.
However, thicker drainage cores
with higher drainage capacity are
still preferred, because the larger
open spaces for drainage are less
susceptible to clogging. Drainage
cores up to 1¼ inch thick in one
layer are available.
• Filter fabric is needed to keep debris
from clogging the drainage core.
Prefabricated products known as
drainage composites include a filter
fabric already laminated to the
drainage core, but additional fabric
will be needed to wrap the edges and
cut ends of the drainage panels.
Careful detailing and installation is
needed to prevent debris from washing
through the filter fabric at joints
22 • I N T E R FA C E MA R C H 2009
Photo 6 – Various examples of geosynthetic drainage cores.
or terminations and potentially clogging
the drainage composite.
• Coordinate the use of drainage cores
with support requirements for the
paving system (both during installation
and in service). While drainage
cores are available with overall compressive
strengths high enough for
most applications, the drainage core
will not lie perfectly flat until loaded,
resulting in uneven support. For
example, a thin, sand-setting bed for
unit pavers may be difficult to compact
when installed over drainage
cores. The drainage core can also
act as a slip plane, preventing the
transfer of in-plane shear loads from
the paving to the structural deck.
For example, traffic-bearing asphalt
paving installed over drainage cores
may be more prone to rutting due to
the slip plane created by the
drainage layer.
• Continuity of drainage path is
imperative. The drainage cores must
extend all the way to the drainage
outlet. Depending on the drainage
layout and paving design, this may
require the drainage cores to be continuous
underneath curbs and
other features that would otherwise
block drainage.
• Gravel can also be used as a
drainage layer in plaza paving systems,
but is generally less desirable
than either pedestal systems or
drainage cores, because it can have
slower drainage capacity and adds
more weight to the structure.
Drainage Outlet
Drainage outlets at the low points of the
waterproofing membrane are needed to
receive and carry away water that collects
in the drainage layer; these outlets are in
addition to the outlets at the surface of the
paving. Drainage at both levels can be
achieved via “bilevel” drains or by separate
systems of surface and subsurface drains.
In some cases, water on the membrane level
drainage can be drained off the edge of the
foundation wall, but this requires providing
a drainage system at the foundation wall to
receive this runoff and exposes the foundation
wall to additional water and potential
leakage. Designers of internal drains should
consider the following:
• Many drain assemblies promoted as
“bilevel” plaza drains have only a
small number of tiny weep openings
to collect water on the membrane
level. These openings are prone to
clogging with debris or minerals that
seep out of the paving and cannot be
relied upon to provide membranelevel
drainage over the long term.
When relying solely on these weep
openings for bilevel drainage, use
drains that contain a large number
of substantial-sized openings or
modify the drain to enlarge or supplement
the openings provided by
the manufacturer.
• Many plaza drains are available with
stainless steel perforated extensions,
which increase the number
and size of the openings to receive
water that has infiltrated the paving
(Figure 3). These extensions are generally
not traffic-bearing by themselves;
traffic-bearing applications
require a separate, heavy-duty
frame and drain grate or a manhole
cover embedded in the paving and
spanning over the membrane-level
drain to protect it from traffic loads.
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MA R C H 2009 I N T E R FA C E • 2 3
• Trench drains
are generally not
effective for bilevel
drainage applications
as waterproofing
below paving;
most do not have
m e m b r a n e –
clamping hardware
or subsurface
weep slots.
Those that have
these fea tures
generally have
only small, clogprone
weeps that
need to be
enlarged and/or sup plemented.
Membrane-level trench drains also
require coordination with the structural
design, because they require
forming a continuous slot in the
deck to receive the trench drain.
Due to these difficulties, unit drains
are generally preferred for plaza
paving and waterproofing systems.
Where trench drains must be used
for surface drainage, a secondary
system of subsurface drains may be
needed to drain away water at the
membrane level.
Many problems with paving systems
installed over waterproofing membranes
can be avoided with a basic understanding
of drainage issues and careful detailing. The
following strategies will help improve the
long-term performance of both the paving
and the waterproofing:
• Slope the deck a minimum of ¼ in
per ft at the waterproofing membrane
level using a four-way drain –
age pattern that is coordinated with
the structural supports and anticipated
• Maintain a free-flowing, continuous
drainage layer at the membrane
level, coordinated with the support
requirements for the paving system.
• Select drain hardware that provides
an adequate drainage outlet on the
waterproofing membrane level and
is not prone to blockage.
Greg Doelp, PE, is a principal at Simpson Gumpertz & Heger
Inc. (SGH) and has 24 years of experience as a consulting
engineer. He specializes in investigating and designing plaza
waterproofing, below-grade waterproofing, and roofing systems.
His projects have also included analyzing building
moisture problems and leakage problems as well as repairing
and renovating structures. Mr. Doelp is a member of RCI and
the American Society of Civil Engineers. He can be reached at and at 781-907-9217.
Greg Doelp, PE
Phil Moser, LEED AP, is on the building technology staff at
SGH and specializes in investigating and designing plaza
waterproofing, below-grade waterproofing, and roofing systems.
His projects have also included evaluating and designing
repairs to window systems and to masonry and concrete
façades. Mr. Moser can be reached at and
at 781-907-9281.
Phil Moser, LEED AP
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Figure 3