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Essential Elements of Durable Exterior Masonry Walls

May 15, 2005

Masonry materials have been used by
mankind for thousands of years. Typically
defined as relatively small units of substantial
material bonded together, masonry is
one of civilization’s oldest construction systems.
It has evolved from the very simple
prehistoric stone and mud wall, to today’s
high-performance, pressure-equalized rain
Throughout its history, masonry has
proven to be durable, easily constructible,
and adaptable. However, modern masonry
exterior walls are prone to many design,
material, and workmanship deficiencies that
can significantly impact their durability.
Until the early 20th century, masonry
buildings in the United States were typically
constructed with solid, load-bearing masonry
walls. These walls were constructed
with several wythes of stone or brick and
were designed to resist structural loads as
well as provide weather resistance for the
building envelope. While these walls were
commonly referred to as “barrier walls,”
solid masonry walls were not impervious to
water penetration. Their ability to resist
water penetration was directly related to
their overall thickness, mass, and ability to
absorb significant amounts of moisture.
Materials used in early solid masonry
walls were very porous. This allowed walls
to absorb substantial amounts of water
penetrating through cracks and other
defects in exterior wall surfaces. Water
would not reach the interior face of the solid
walls until they were completely saturated.
Freeze-thaw protection of the saturated
porous materials was provided by the thermal
mass of the walls. Thick, solid masonry
walls retained heat, significantly limiting
freeze-thaw cycles and thus limiting freezethaw
Solid masonry walls are still being constructed
today, but they are far more susceptible
to water penetration. Increased
labor costs and demand for lighter and
higher building structures required engineers,
architects, and manufacturers to
develop more cost-effective designs in order
to keep masonry a viable building component
option. The primary means of reducing
cost and weight was to reduce the thickness
of the solid walls. This required much higher
quality masonry materials for two reasons.
First, greater strengths were necessary
to carry the same loads that thicker
walls carried. Second, the reduction in thermal
mass of the walls (due to reduced thickness
and the use
of hollow masonry
increased the
need for more
f r e e z e / t h a w –
resistant materials.
The higher
quality masonry
units and mortar
used today are
far more impervious
than the
materials used
in early solid
masonry walls
and absorb far
less moisture.
Therefore, water
penetrating the
surface of an
exterior wall
through cracks
or other defects is forced to continue
through the wall instead of being absorbed
in the wall materials and will reach the interior
surface much quicker (Photo 1).
Due to obvious limitations of solid
masonry walls, cavity walls have been used
extensively for the past 50 to 60 years.
Cavity walls consist of two wythes of masonry
separated by an air space. Typically,
the inner wythe of a cavity wall consists of
concrete masonry units (CMU), while the
outer wythe consists of clay brick masonry.
Another form of cavity wall commonly used
consists of a clay brick exterior wythe in
conjunction with a back-up wall made of
metal studs and an exterior sheathing.
Cavity wall design recognizes that water
penetration into masonry walls is inevitable
and provides the necessary means to manage
the water. Properly designed, detailed,
and constructed cavity walls can prevent
water penetration through the system.
Unfortunately, masonry walls are not
always properly designed, detailed, or constructed.
It is these deficiencies in design,
detailing, and construction of the modern
cavity wall system that significantly reduce
its durability, reliability, and effectiveness.
Photo 1: Water penetrating to interior surface of CMU wythe of solid
masonry wall.
MAY 2005 I N T E R FA C E • 1 3
Durability of
masonry walls begins
with good design.
The two primary
factors that most
affect the durability
of modern masonry
walls (and that
must be thoroughly
understood by the
design professional)
are movement and
moisture control.
Movement Control
Movement in
masonry walls is
typically caused by
changes in moisture
content and temperature.
Clay brick masonry units are their
driest and smallest when they are removed
from the kiln. From that point forward, they
continually absorb moisture and irreversibly
expand in size. Conversely, typical
concrete masonry units are their largest at
the time of casting and irreversibly shrink
with time. The rate of clay masonry expansion
and concrete masonry shrinkage slows
over time. In addition to their initial moisture-
related movements, masonry materials
also expand and contract with changes in
Thermal and moisture expansion of clay
masonry units require both vertical and
horizontal expansion joints in masonry
walls. Long lengths of unrestrained walls
without vertical expansion joints will
increase in length and displace adjacent
walls away from their back-up materials.
This displacement typically results in
cracking at building corners and at discontinuities
along the lengths of walls. Long
lengths of restrained walls without vertical
expansion joints will build up compressive
stresses until the wall buckles.
Horizontal expansion joints must
accommodate vertical expansion of masonry
walls. These joints are usually placed
immediately beneath shelf angles, which
are typically supported by the building
frame at each floor line. If clay masonry is
installed tightly to the bottom of the shelf
angles, the wall will likely buckle since vertical
expansion will be restrained. Placing
shelf angles at every other or every third
floor will increase the accumulated movements
in the exterior wythe and exacerbate
the effects of improperly constructed horizontal
expansion joints. In addition, placement
of shelf angles at every other floor can
create problems at window lintels and intermediate
floor lines if details are not provided
to accommodate wall movement.
Shrinkage of concrete masonry requires
adequately spaced vertical control joints to
minimize cracks along the length of the
wall. Similar horizontal control joints are
necessary at the top of non-loadbearing
walls to accommodate shortening without
opening up gaps between the top of the
walls and structural or architectural elements
Location of control and expansion joints
and sizing of expansion joints are critical to
minimize cracking and displacement of
masonry. Expansion joints that are too narrow
will close and begin to cause cracking
and displacement as if the joints were never
present (Photo 2). Improperly spaced or
located control and expansion joints will do
the same.
Many problems associated with movement
of masonry walls are due to the use of
clay and concrete masonry units in the
same wall system. The expansion of clay
and shrinkage of concrete result in differential
movement between the two materials.
Improperly incorporating these two materials
together in the same or separate wythes
will typically cause unwanted cracking,
bowing, and displacement.
Even if these materials are separated,
as in cavity walls, the differential movement
still needs to be accommodated. The exterior
wythe of brick is required to be tied to the
interior CMU wythe at regular vertical and
horizontal spacing, as specified in Section of the ACI 530 Building Code
Requirements for Masonry
Structures. This is
usually accomplished
with individual ties or
continuous horizontal
reinforcement. In either
case, flexibility is necessary
to allow horizontal
Photo 2: Sealant compressed out of improperly sized vertical
expansion joint.
Photo 3: Narrow wall cavity difficult to keep clean during construction.
14 • I N T E R FA C E MAY 2005
and vertical differential movements between
the two wythes. For instance, continuous
truss-type reinforcing will not allow horizontal
movement between two wythes due
to its inherent rigidity in the horizontal
direction. Ladder-type reinforcing must be
used if continuous horizontal reinforcement
is specified.
Another type of movement that needs to
be addressed in the design of masonry walls
is lateral deflection. Exterior wythes of cavity
or veneer walls are usually not designed
to resist wind loads. Wind loads are typically
transferred to interior wythes or stud
walls through the ties. Those back-up elements
must provide enough stiffness to prevent
excessive deflection, and thus cracking,
of the exterior masonry. This should be
a significant consideration when designing
masonry cavity wall systems with a metal
stud back-up, since metal stud back-up
systems are typically much less rigid than
masonry back-up systems. Although ACI
530 does not provide any specific limitations
on deflection of backup materials, the
Brick Industry Association recommends
that the lateral deflection of metal stud
back-up systems be limited to L/600, where
L is the unsupported length of the stud. For
a 10-foot, unsupported wall height, the limiting
deflection would be 0.2 inches.
Moisture Control and Water Management
The primary purpose of exterior masonry
walls is to protect the interior of buildings
from the environment. If water migrates to
the interior of the building, the exterior walls
have failed to perform their intended function.
As previously indicated, modern solid
masonry walls have little tolerance for deficiencies.
Defects in exterior wall surfaces
will lead to nearly instant water penetration.
If not handled by the internal water management
system, such water penetration
can manifest as leaks inside the building.
On the other hand, properly designed,
detailed, and constructed cavity walls can
accommodate some exterior wythe defects
without allowing water to penetrate to interior
surfaces. For these walls to function as
intended, they must be designed with a
minimum 2-inch wide cavity. This dimen-
Photo 4: Freeze-thaw deteriorated mortar joints.
MAY 2005 I N T E R FA C E • 1 5
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sion is considered the minimum width necessary,
as recommended by the Brick
Industry Association (BIA), to prevent the
cavity from being bridged by mortar or other
materials and allow water to cross over to
the interior wythe. Narrower cavities are
typically more difficult to keep clear during
construction (Photo 3). However, it should
be noted that mortar bridging can still
occur with a wider cavity if the masons do
not exercise care to keep the cavity clear
during construction.
Another requirement for cavity walls is a
system to remove water from the wall.
Flashing and weeps at the base of the wall
and at all penetrations are typically used for
this purpose. The flashing must be continuous,
terminated properly at the interior
wythe, extended past the face of the exterior
wythe, and terminated at its ends with
end dams. The joint above the flashing is
where water exits the wall system. Weeps
should be placed at no more than 24 inches
apart at this level, and the joint should
be filled with mortar instead of sealant.
Another method to improve the moisture
resistance of masonry cavity walls with
a CMU back-up is to coat the outside face of
the back-up with a dampproofing material.
While dampproofing does not provide any
substantial protection against water intrusion,
it will minimize the moisture absorption
of the CMU when bridging occurs. Even
if the cavity is kept completely clear, the
masonry ties can bridge the gap and allow
the water that runs down the inside face of
the exterior wythe to reach the outside face
of the interior wythe. Application of dampproofing
requires careful consideration to
ensure that moisture vapor transmission
characteristics of the wall assembly are not
adversely impacted.
In cavity wall construction with metal
stud and sheathing back-up, a weatherresistive
barrier (WRB) should be provided
to minimize water penetration through the
Lastly, proper design details should be
conveyed to the masonry contractor in the
design documents. In the authors’ opinion,
it is not uncommon to see a set of drawings
for complicated masonry construction without
adequate detailing of the water management
system within the walls. During the
design phase, the designers should consider
all installation conditions and develop an
appropriate detail for each condition.
Materials used in the construction of
masonry walls have a major impact on their
sustainability. The primary component
materials include the masonry units, mortar,
flashing, and metal supports.
Masonry units must have the appropriate
physical properties to withstand the service
conditions in which they will be placed.
Poor freeze/thaw-resistant brick will quickly
deteriorate in severe weathering regions.
Excessive coefficients of thermal and moisture
expansion will likely cause expansion
of walls to exceed that anticipated by the
designer, causing cracking and displacement
the walls.
Similarly, mortar must have appropriate
physical properties for the intended service
conditions. Strength, workability, and
freeze/thaw resistance are all important
properties to consider when specifying mortar
(Photo 4). The mortar must also be compatible
with the masonry units to ensure
proper bond.
Durable flashing materials are necessary
for durable masonry walls. Flashing
that can be easily punctured, extrudes
under the weight of the brick wall, is difficult
to seal at its seams, and is UV degradable,
will increase the likelihood that water will
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16 • I N T E R FA C E MAY 2005
penetrate beyond the flashing to the interior
surfaces of the masonry wall system.
Metal supports include ties and shelf
angles. These components are subject to far
more water in masonry walls today than
they were in early masonry walls, primarily
due to their design but also due to workmanship.
As a result, metal components in
masonry walls will corrode more quickly.
Selection of metal components should
consider the expected service life of the wall
system. Most masonry wall systems are
expected to last far more than 50 years. Yet
the use of corrosive metals within the walls
will significantly lower their life expectancy.
Corrosion of metals in masonry walls can
lead to many problems, including cracking
and bowing. Correction of such deficiencies
requires costly and extensive rehabilitation.
As such, durable metals should be used in
conjunction with masonry wall construction.
At a minimum, ties and bolts should
be made of galvanized steel. In many cases,
the use of stainless steel ties and bolts can
be justified when considering the life cycle
costs of the system. In the authors’ opinion,
shelf angles and lintels used in modern
masonry construction should be hot-dipped
galvanized after fabrication. If galvanizing
cannot be justified, shelf angles and lintels
should be coated with high performance,
corrosion-inhibiting coating systems to
achieve a service life that is compatible with
the expected service life of the wall system.
In most cases, life cycle costs for these
items are very small in comparison with the
cost to replace them part of the way through
the design life of the wall system.
The final step to achieving sustainable
masonry walls is assuring good workmanship
during installation. Properly designed
and detailed walls with carefully specified
materials will still be subject to accelerated
deterioration if
good workmanship
is not provided
during installation.
The various
components of
masonry walls
are as follows:
• The air space
between the
exterior and
interior masonry
must be
kept clean.
P a r t i a l –
height mortar
nets are
not sufficient
they still allow
to bridge
between the
two wythes at the top of the mortar
net. The use of full-height mortar
nets or drainage boards is gaining
more popularity to ensure an open
cavity for drainage.
• Vertical and horizontal expansion
joints must be kept clean to allow
them to function as intended (Photo
5). Obstructions over only a portion
of these joints can still cause cracking
and displacement of adjacent
• Flashing must be carefully installed
to ensure that it will be watertight.
Open seams, punctures, and other
damage to the flashing must be
avoided, end dams must be
installed, and flashing must not be
trimmed back too close to the face of
the wall (Photo 6). Since most
through-wall flashing materials
degrade rapidly when exposed to
UV, stainless steel drip edges are
typically incorporated into the flashing
system to extend beyond the face
Photo 5: Mortar improperly placed in horizontal mortar joint
and brick not properly supported on shelf angle.
Test your knowledge of roofing with
the following questions, developed by
Donald E. Bush Sr., RRC, FRCI,
chairman of the RRC Examination
Development Committee.
These wind-related questions are
based on the information contained
in RCI’s Wind and Drainage class.
1. At normal temperature
and pressure,
what is the
density of air?
2. Velocity pressure,
q, is a function of
wind speed, v. If
wind speed is 100
mph, what is the
velocity pressure?
3. What is Bernoulli’s
4. The protective
effect of a parapet
at least 3 feet high
is to reduce the
corner pressure.
When should this
protective effect
be considered?
Answers on page 18
MAY 2005 I N T E R FA C E • 1 7
of the masonry.
• Materials must be supplied and
installed as specified by the design
professional. Use of poor quality
mortar materials, poor quality
brick, and non-corrosion-resistant
metal cannot be allowed. In
addition, steps should be taken to
avoid exposing freshly placed
masonry materials to hot or cold
conditions. ACI 530.1 provides
excellent guidelines for hot and
cold weather masonry construction.
• Shelf angles must be installed so
they provide continuous support
around corners and are properly
anchored. Their connections to the
building frame should be constructed
to minimize deflections when
subjected to the weight of the
masonry above. Shelf angles must
also be protected from corrosion.
Masonry can be as durable as any other
building material. We know this because of
all of the buildings with masonry components
that are still performing centuries
after construction. However, without the
essential elements of good design, material
specifications, and workmanship, masonry
building components will deteriorate far
faster than they should.
For the long-term durability and effectiveness
of exterior masonry walls, it is critical
that the design process consider wall
movement, moisture penetration, and water
management. Masonry materials specified
and used must be suitable for their intended
environment. Finally, the walls must be
constructed as designed and specified.
Ensuring these essential elements in exterior
masonry projects will result in serviceable
walls for the anticipated life of the
This article was recently published as part of
the Proceedings of RCI’s 2004 Symposium
on Building Envelope Technology, Nov. 4-5,
2004 in New Orleans, Louisiana.
Joshua J. Summers is a principal structural engineer at
Building Technology Consultants, PC. Mr. Summers has evaluated
and developed repair designs for numerous masonry
building components. These projects have included both solid
and cavity wall construction with brick, CMU, terra cotta,
limestone, and clay tile materials.
Joshua J. Summers, SE, PE
Kami Farahmandpour is the principal of Building Technology
Consultants, PC. His expertise is concentrated in the evaluation
and repair of building envelopes, including various types
of exterior walls, waterproofing systems, and roofs. Among
his many professional activities, he served as the president of
the Chicago Area Chapter of RCI and the chair of RCI’s
Building Envelope Committee and was recently awarded the
Richard Horowitz Award. He is also co-author of a Practical
Guide to Weatherproofing of Exterior Walls, currently being
developed for the Sealants and Waterproofing Restoration Institute.
Kami Farahmandpour, PE, RRC, RWC, CCS, CCCA
Photo 6: Improperly sealed flashing seam.
Answers from page 17:
1. Nearly 0.0765 pounds
per ft3.
2. q = 0.00256(100)2
= 25.6 psf
3. Bernoulli’s Equation is
the application of the
Law of Conservation to a
fluid (liquid or gas) in
motion. When applied to
air, this equation states
that the pressure at any
point in the air [which is
in steady state (nonturbulent)
motion], is constant.
This pressure has
two components: (a) a
static pressure, which is
the ambient atmospheric
pressure, and (b) velocity
pressure, which is
the kenetic energy of air
(total air pressure equals
atmospheric pressure
plus velocity pressure).
4. Only when the parapet
has been designed to
withstand peak wind
18 • I N T E R FA C E MAY 2005