Common Sources of Distress in Stucco Facades

Common Sources of Distress
in Stucco Façades
Lee Cope, PE, and Michael Horst, PE
Wiss, Janney, Elstner & Associates, Inc.
2915 Premiere Parkway, Suite 100, Duluth, Georgia 30097
Phone: 770-923-9822 • Fax: 770-232-9044 • E-mail: lcope@wje.com, mhorst@wje.com
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Abstract
In the past several years, the authors have evaluated several buildings throughout the
United States that were clad with Portland cement plaster (stucco) anchored to the wall
using metal lath. Water leakage and distress in the form of cracking and delaminations were
frequent concerns with these buildings. Several conditions were identified as the causes of
these issues, including poor installation of weather-resistive barriers and flashings, poor
installation of lath and stucco accessories, improper stucco mixes, improper sand gradation,
improper placement of the stucco, and improper curing. Evaluation of these conditions
revealed a significant failure on the part of both designers and contractors in understanding
and applying the requirements for stucco and lath installation provided by ASTM C926,
Standard Specification for Application of Portland Cement-Based Plaster, and ASTM C1063,
Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior
Portland Cement-Based Plaster. Therefore, this paper will discuss the typical conditions
responsible for the defects observed in the field and clarify the requirements presented in
the ASTM standards. This paper will also discuss common long-term repairs that can be
performed to address these issues.
Speakers
Lee Cope, PE — Wiss, Janney, Elstner & Associates, Inc.
Lee Cope is a licensed professional engineer with extensive experience in detailing and
proper installation of Portland cement plaster (stucco) façade systems, air barriers, window
systems, waterproofing, and the interfaces of various envelope components. He has evaluated
a variety of structures relating to the causes of problems and/or distress of buildings’
exterior façades/curtain wall systems and building envelope systems. He is a voting member
of ASTM Committee C11 on Gypsum and Related Building Materials and Systems.
Michael Horst, PE — Wiss, Janney, Elstner & Associates, Inc.
Mic hael Horst is a licensed professional engineer specializing in the evaluation and
repair of exterior wall components and assemblies. He has extensive experience in the
investigation and repair of water leakage and distress in Portland cement plaster (stucco)
cladding systems. Horst is a member of ASTM Committee E6 on Performance of Buildings
and a voting member of ASTM Subcommittee E6.58 on Exterior Insulation and Finish
Systems (EIFS).
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INTRODUCTION
Over the past 15 years, the authors
have investigated defects in exterior
Portland cement plaster (stucco) cladding
on hundreds of buildings throughout the
southeastern United States. The investigations
have been performed for a variety of
reasons, including resolving issues noted
during construction, storm damage claims,
water leakage, and litigation arising from
recent construction. The construction of the
buildings under investigation has included
stucco applied to solid bases, such as
concrete or concrete masonry units (i.e.,
directly applied stucco) and stucco applied
to metal plaster bases (metal lath). An adequate
understanding of the properties of the
various components in stucco cladding, the
interaction among the stucco cladding and
other building elements, and how the cladding
resists rainwater penetration are critical
to the initial design and installation of a
stucco cladding system, proper diagnosis of
any stucco defects that may arise, and the
development of long-term repairs to address
these deficiencies.
The focus of this paper is on those defects
noted in buildings with exterior stucco cladding
applied to metal lath. In metal lath
supported systems, the stucco encapsulates
a metal lath, which is attached to the
building structure. Attachment of the metal
lath to the building structure in these systems
is critical to the integrity of the stucco
cladding. The attachment methods must
be able to transfer vertical loads—primarily
the weight of the stucco cladding system—
and horizontal loads—primarily from wind
forces—into the building structure without
resulting in damage to the stucco. Failure to
adequately attach the metal lath can result
in cracks, displacement, or detachment of
the stucco from the building. The metal lath
should also divide the stucco into discrete
areas to enable initial drying shrinkage
and subsequent expansion and contraction
in response to temperature changes and
other environmental factors to occur without
damaging the stucco. Continuity of the
lath within these discrete areas is important
to reduce the risk of isolated cracks
forming. The initial scratch coat must fully
encapsulate the metal lath to provide an
adequate bond and protect the metal lath
from deterioration. The remaining stucco
coats, the brown coat and finish coat, must
be adhered to each other and to the scratch
coat to prevent delamination of the stucco
between coats. In addition, the stucco components—
including cementitious materials,
aggregate (sand), and water—must be
properly proportioned, mixed, applied, and
cured for metal lath-supported systems to
function properly and to endure.
For water leakage issues in stucco systems,
it is important to understand how
the stucco cladding resists water penetration.
The primary method of resisting water
penetration through stucco cladding is at
the exterior face of the assembly. However,
water should be expected to penetrate
the stucco, either by absorption through
the stucco, through cracks or separations
within the stucco system, or failed sealants
at stucco terminations. Therefore, stucco
systems supported by metal lath provide
a drainage system consisting of a weatherresistive
barrier (WRB) and flashings to
collect and drain incidental water that
penetrates the exterior surface back to the
exterior of the building, as well as prevent
the water from reaching the interior of the
building. Reducing the avenues that enable
water to penetrate the stucco through proper
design and installation of these drainage
components is critical to the success of
stucco applied to metal lath.
Two documents, both of which have been
incorporated by reference into the model
building codes, have been developed by
ASTM International, Inc. (ASTM) to provide
designers and installers with the minimum
requirements for metal lath and exterior
stucco cladding. These documents are ASTM
C926, Standard Specification for Application
of Portland Cement-Based Plaster, and
ASTM C1063, Standard Specification for
Installation of Lathing and Furring to Receive
Interior and Exterior Portland Cement-Based
Plaster. Unfortunately, ASTM C926 and
ASTM C1063 are fairly complex standards,
and the authors’ evaluations of defects in
stucco-clad buildings have revealed significant
failures of the various parties involved,
including design architects, waterproofing
consultants, contractors, and subcontractors,
in understanding and applying the
requirements for stucco and lath installation
provided by these documents. The
common failures noted include poor design
and installation of flashings and WRBs;
inadequate or incorrect layout and installation
of control/expansion joints and other
stucco accessories; and improper stucco
mixes, sand gradation, placement of the
stucco, and curing. These conditions have
led to significant deficiencies in the completed
stucco cladding, including cracking,
separations, delaminations, and water
leakage.
In this paper, the authors will describe
several of the defects commonly noted
in exterior stucco cladding. The primary
cause(s) of these defects and the relationship
of these causes with ASTM C926 and
ASTM C1063 will be explored. Failures of
the exterior stucco cladding and concealed
drainage systems leading to water leakage
into the building will also be discussed. In
addition, the authors will provide several
long-term repair options to address defects
in the stucco cladding and failures of the
drainage systems.
CRACKS, SEPARATION S,
AND DELAMINATION S
Cracks form in stucco when forces
or stresses within the stucco exceed its
tensile strength. Although these stresses
can be the result of external forces such
as building displacement, wind, seismic
movements, impact, etc., our experience is
that cracks are more frequently caused by
deficiencies in the design and installation
Common Sources of Distress
in Stucco Façades
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of metal lath and stucco stemming from an
inadequate or inaccurate understanding of
ASTM C926 and ASTM C1063.
For discussion purposes, we have divided
the types of cracks commonly observed
into two separate categories: extensive
cracking and isolated cracking.
Extensive cracking can be defined as
map or pattern cracks that occur throughout
an individual
stucco panel or
are widespread
throughout the
stucco façade.
These cracks may
be fine crazing
cracks as illustrated
in Figure 1 or wider pattern cracks
as shown in Figure 2.
Isolated cracking includes
individual cracks or widely
spaced cracks within an individual
stucco panel. These cracks
can be hairline width or very
wide and may extend across an
entire stucco panel or only a
limited portion of the panel. In
addition, isolated cracks may
occur on multiple stucco panels
throughout a building. Figure 3
illustrates an example of an isolated
crack.
From a distance, separations
often appear similar to isolated
cracks. However, whereas cracks
are fractures within the stucco,
with stucco on each side
of the crack, separations consist
of gaps between the stucco
and stucco accessories, such as
expansion and control joints and
casing beads. Separations often appear
alongside isolated cracks and frequently
have similar causes.
Delaminations occur when the stucco
separates into individual layers or coats,
or develops a separation within a layer.
Delaminations can be difficult to detect in
the early stages, as the defects are generally
in a plane parallel with the exterior face of
the building. However, over time, delaminations
often lead to extensive cracking.
The following sections identify common
causes of excessive and isolated cracking,
separations, and delaminations and
discuss the relationships between these
conditions and ASTM C926 and ASTM
C1063. Since delaminations often lead to
excessive cracking and have similar causes,
delaminations and excessive cracking are
discussed together. Similarly, separations
are presented in a subsequent section along
with isolated cracking.
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Figure 1 – View of excessive cracking with fine cracks.
Figure 2 – View of excessive
cracking with wide cracks.
Figure 3 – View of an isolated crack.
Excessive Cracking and Delaminations
The most common conditions resulting
in delaminations and excessive cracking
identified through our field investigations
of metal-lath-supported stucco systems
include the following:
• Incorrect stucco mix proportions
• Improper gradation of sand
• Poor curing
• Delamination between stucco coats
• Corroded lath
Incorrect Stucco Mix Proportions
Stucco consists of cementitious materials—
frequently Portland cement and lime
mixed with aggregate (usually sand) and
water. Correctly proportioning the various
components and properly mixing the components
are critical to the long-term success
of the stucco cladding. Incorrectly proportioned
stucco mixes that frequently lead
to excessive cracking include mixes with
high cement contents (undersanded), low
water-to-cement ratios, or high water-tocement
ratios. Stucco prepared from these
poorly proportioned mixes is often weak and
can suffer from early drying shrinkage and
high porosity.
For a traditional stucco mix, Tables 2
and 3 in ASTM C926 provide mix proportions
for the base coats (both scratch coat
and brown coat) and finish coat. The tables
are given by volume and specify the proportions
of cement, lime, and sand that shall
be included for a specific stucco mix. Based
on discussions with designers and installers
over the years, it has been our experience
that the tables are often interpreted
improperly, resulting in undersanded or
cement-rich stucco mixes. Table 2 specifies
that the volume of sand be between 2½ and
4 for the first coat and 3 to 5 for the second
coat. Table 3 specifies that the volume of
sand for the finish coat be between 1½ and
3. However, the table also includes “Volume
of Aggregate per Sum of Separate Volumes
of Cementitious Materials.” This requires
adding the separate volumes for cementitious
materials (cement plus lime) and then
multiplying the sum of these materials by
the volume of sand (2½ to 4 for the first
coat, 3 to 5 for the second coat, and 1½ to
3 for the finish coat) to obtain the total volume
of the aggregate to be included in the
mix. For example, to obtain a “CL” plaster
mix with 1 part Portland cement, 1 part
lime, and 3 parts sand for the first coat, and
3 parts sand for the second coat—the actual
amount of sand to be used in the mix should
be calculated as follows: (1 part Portland
cement + 1 part lime) X (3 parts sand) = 6
parts sand. Frequently, this requirement is
misinterpreted and installers and designers
often fail to multiply the volume of sand
by the sum of the cementitious materials
when mixing stucco. Mixtures of this type
have relatively high cement contents, which
are typically prone to increased shrinkage,
resulting in a higher probability of the formation
of excessive cracking.
It should be noted that a significant
amount of current stucco installation on
buildings uses preblended bag mixes rather
than site mixing in accordance with the
tables provided by ASTM C926. Assuming
that adequate quality control measures
are followed by the manufacturer, these
preblended bag mixes provide the correct
proportions of lime and cement—and, in
some cases, sand—leaving only water and/
or sand to be added at the project site.
Instructions on the bagged mix typically
specify the amount of sand and water to be
added. Therefore, it is important to follow
the manufacturer’s instructions during the
mix process.
In some cases, we have determined that
the proportions of sand and cementitious
components in the cured stucco were incorrect,
even though the mix design provided
for correct proportions. Further investigation
revealed that inadequate mixing of the
components resulted in the proportional differences.
Guidelines for measuring the components
and mixing the stucco are provided
in ASTM C926. These requirements include
using measuring devices of known volume
and mixing the constituents in a mechanical
mixer. In addition, Appendix XI of ASTM
C926, which is stipulated as nonmandatory
information, recommends that the stucco be
mixed for three to five minutes after all components
are added to the mixer. Following
these requirements will help ensure that the
stucco is properly blended.
ASTM C926 does not stipulate the specific
volume of water to be used in the
stucco mix. Instead, the stucco contractor
is specifically charged with determining the
amount of water to be included. Several factors
are identified in Appendix XI, including
the suction of the base or the previous coat,
water content of the aggregate, drying conditions,
and finishing operations. Because
of these factors, including curing, the final
water-to-cement ratio is very difficult to
control. However, in our experience, the
final effective water-to-cement ratio should
be less than 0.55.
To help ensure that the stucco to be
used on a project is properly mixed and is
of good quality, petrographic and chemical
analysis in accordance with ASTM C1324,
Standard Test Method for Examination and
Analysis of Hardened Mortar, can be performed
during the mock-up phase of a
project. The results of the analysis can be
used to determine the final mix proportions.
Improper Gradation of Sand
Another common issue that leads to
excessive cracking of installed stucco is
the use of sand that fails to comply with
the requirements of ASTM C926, which
specifies that the sand be in accordance
with ASTM C897, Standard Specification for
Aggregate for Job-Mixed Portland Cement-
Based Plasters. In our experience, it is
often difficult for contractors to obtain sand
that matches the gradation requirements
specified in ASTM C897. Therefore, sand
that does not meet the ASTM C897 requirements
is often substituted. In most cases,
the substituted sand is much finer than
that required by ASTM C897. Using fine
sand often results in the installer using too
much water due to the increase in total surface
area of the sand, resulting in a stucco
mix that has a high water-to-cement ratio.
A mix with a high water-to-cement ratio
would result in a weak stucco mix, which
could result in excessive cracking.
Poor Curing
Good curing practices, which maintain
sufficient moisture in the stucco mix to permit
continuous hydration of the cementitious
materials, can help prevent or reduce
the risk of excessive shrinkage cracking.
The premature loss of water from the stucco
caused by high temperatures and exposure
to wind and sun can result in early hydration
and excessive cracking if good curing
practices are not employed.
It should be noted that stucco can actually
dry out faster in cool weather when
exposed to direct sunlight and wind than it
will on hot days when not exposed to direct
sunlight and wind. ASTM C926 requires
curing of the stucco with three different
methods:
(1) Moist curing by applying a fine
fog spray of water as frequently as
required, generally twice a day in the
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morning and evening. Care must be
exercised to avoid erosion damage
to Portland cement-based plaster
surfaces. Except for severe drying
conditions, the wetting of the finish
coat should be avoided; that is, wet
the base coat prior to the application
of the finish coat.
(2) Plastic film, when taped or weighted
down around the perimeter of the
plastered area, can provide a vapor
barrier to retain moisture between
the membrane and plaster. Care
must be exercised in placing the
film. If placed too soon, the film may
damage surface texture; if placed too
late, the moisture may have already
escaped.
(3) Canvas, cloth, or sheet material
barriers can be erected to deflect
sunlight and wind, both of which
will reduce the rate of evaporation. If
the humidity is very low, this option
alone may not provide adequate protection.
The amount of water or the length of
time the curing should be performed is
not defined in ASTM C926; however, the
Portland Cement Association (PCA) recommends
maintaining 80% relative humidity
for at least 24 hours and, in some cases, up
to seven days. The method that has proven
to be most successful in curing stucco,
in our experience, is to tent the stucco to
prevent exposure to sunlight and wind, and
maintain at least 70% relative humidity
inside the tent for seven days; or to cover
the stucco with wet burlap and maintain
the moisture in the burlap for seven days.
After the initial seven days of curing, we
also recommend that the stucco continue
to cure for an additional 23 days to help
ensure all of the initial shrinkage has taken
place prior to placing the finish over the
stucco. While this is not often performed,
most coating manufacturers recommend
this as well. Allowing the stucco to fully
cure prior to placing the finish will allow the
contractor to repair cracks and deficiencies
in the stucco prior to placing the finish.
Delamination Between Stucco Coats
If the stucco is not properly placed,
properly scratched, or properly prewetted
between coats, a poor bond can develop
between the various stucco layers, leading
to delaminations between the stucco coats.
Delaminations between stucco coats can
lead to excessive cracking. Requirements
for applying each stucco coat are provided
in ASTM C926.
The first or scratch coat must be applied
so that the metal lath is fully embedded in
the stucco and the stucco forms full keys
through the lath. The thickness of the
stucco in the scratch coat over the metal
lath must be thick enough to enable the
entire surface to be scored. The scratch coat
is required to be scored in one direction only
(horizontally for wall surfaces) as soon as it
becomes firm, as indicated in ASTM C926.
For the second or brown coat, sufficient
pressure to ensure tight contact with the
previous coat is required by ASTM C926.
The brown coat can be installed as soon
as the scratch coat is rigid enough to support
it without damaging the continuity of
the scratch coat or the keys between the
scratch coat and metal lath. The brown
coat surface has to be in a true, even plane,
and any defects must be filled with plaster,
after which the surface must be uniformly
floated. The purpose of floating the brown
coat is to increase the density of the coat
and to provide a better bonding surface for
the finish coat. Similar to the application of
the brown coat, the finish coat is required to
be applied with enough pressure to ensure
tight contact with the brown coat.
ASTM C926 permits the second and
third coats to be applied as soon as the
underlying layer becomes rigid enough to
support the application of the next layer
without damage. This eliminates the need
to prewet the prior coat before the application
of the next coat. If the application of
the second coat or finish is delayed after
the underlying stucco has reached its initial
set, the underlying layer of stucco should be
dampened by prewetting the surface.
We have observed three primary defects
in the application of the stucco coats that
violate the requirements of ASTM C926
and often cause delaminations to occur.
Delaminations frequently occur when the
scratch coat or subsequent coats are not
applied with enough pressure to—in the
case of the scratch coat—form full keys
through the lath or—in the case of the brown
and finish coats—ensure tight contact
between the coats. Small voids or discontinuities
form between the coats or within
the coats due to this lack of pressure. Over
time, these conditions can cause largerscale
delaminations between the stucco
coats. Failure to score the entire surface of
the scratch coat is another common condition
that results in delaminations. The third
defect we have commonly observed is the
failure to prewet the underlying layer when
applying a subsequent coat after initial set
of the support layer.
Corroded Metal Lath
Corroded metal lath can also result in
excessive cracking. Corroded metal lath is
typically the result of poor keying between
the first layer of stucco and the lath,
improper stucco mix, poor maintenance,
aged stucco, and water traveling through
unsealed cracks and separations in the
stucco.
One of the reasons ASTM C926 requires
the first coat of stucco to encapsulate the
metal lath is to protect the lath from corrosion.
Embedment of the metal lath in the
stucco provides protection in a number of
ways. First, if the stucco is relatively impervious,
the stucco reduces the exposure of
the metal lath to oxygen and water. Both
elements are necessary for the corrosion
reaction to occur, and the absence of either
element brings the corrosion to a halt. The
second (and probably most significant) way
in which stucco can protect embedded
metal lath is by creating an environment
that is favorable to the formation of a metal
oxide surface coating on the lath, thereby
“passivating” it from further corrosion, similar
to the manner in which concrete protects
embedded reinforcing steel. The alkalinity
of the stucco is considered the most
important quality of the matrix in creating a
passivating environment. The passive metal
oxide layer can be broken down by reactive
ions (such as chlorides), which decrease
the alkalinity of the stucco. Carbonation of
the stucco can also create an environment
that does not favor passivation. Relatively
impermeable stucco not only reduces exposure
of the steel to oxygen and water, but
also slows the intrusion of chloride ions and
the propagation of the carbonation reaction,
relative to the more-pervious stucco.
Isolated Cracking and Separations
Isolated cracking and separations are
primarily the result of improper design and
installation of metal lath, or poor practices
in the application of the stucco. Based
on our investigations, the most common
defects resulting in isolated cracking and
separations include the following:
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• Improper design and placement of
control joints
• Overfastening of metal lath
• Incorrect lapping of metal lath segments
• Poor integration of stucco accessories
Improper Design and Placement
of Control Joints
The failure to follow the design and
installation requirements provided in ASTM
C926 and ASTM C1063 for control and
expansion joints is perhaps the most frequent
cause of isolated cracking observed
in our investigations. The purpose of control
joints, as defined in ASTM C1063, is
to minimize stresses in the stucco induced
by drying shrinkage and minor movement
of the building or stucco. In addition,
ASTM C1063 indicates that control joints
are required to accommodate both expansion
and contraction. If control joints are
not designed or installed in a manner that
allows the joint to accomplish this purpose,
isolated cracking frequently occurs.
Control joints must be installed to separate
the stucco into areas of less than
144 square feet on walls, as required by
ASTM C1063. The distance between control
joints is further limited to 18 feet. We have
observed that adherence to these two criteria
has become more common in recent years,
resulting in fewer isolated cracks occurring
due to oversized stucco panels. However,
ASTM C1063 also limits the length-to-width
ratio of the stucco areas delineated by control
joints. The maximum lengthto-
width ratio of any stucco panel
is 2½ to 1. The stucco panels
adjacent to windows, particularly
along the jambs, frequently
exceed this ratio. Often, these
relatively tall, narrow stucco panels
contain isolated cracks. The
cracks generally form at locations
that would enable the stucco
panel to meet the required ratio.
For example, panels that are 9
ft. tall and 18 in. wide commonly
form two horizontal, isolated
cracks. These cracks typically
break the panels into segments
that are 3 ft. tall by 18 in. wide,
for a length-to-width ratio of 2 to
1. Figure 4 illustrates an example
where the stucco panels exceed
the 2½ to 1 ratio.
One of the common arguments regarding
improper installation of control joints
relates to the party responsible for determining
their location. Annex A1 of ASTM C926
provides clear direction that the designer
is responsible for illustrating the type,
depth, and location of all accessories (which
include control joints) in the contract documents.
The requirement for the designer
to provide control joint locations is even
more clearly stipulated in the Appendix to
ASTM C926. This appendix states that
the intent of the standard is “to have the
type, location, depth, and orientation of
control and expansion joints both stated in
the project specifications as well as shown
and detailed on the contract drawings.”
The contractor and subcontractor are not
completely absolved of responsibility in this
area. However, Annex A1 also requires the
contractor or subcontractor to evaluate the
surfaces to receive stucco, notify the proper
authorities, and correct any unsatisfactory
conditions.
Another common issue observed in our
investigations related to installation of control
joints is the failure to cut the metal lath
at the control joint locations. Some groups
have promulgated the concept that extending
the metal lath continuously through the
control joints is appropriate.
The first argument against this concept
is that the engineering mechanics involved
do not support continuous metal lath. The
stucco, if properly installed, is keyed into
the metal lath, which restricts the ability of
the stucco to freely shrink unless the lath is
-cut. If the metal lath is not cut, the stucco
will likely either shrink away from the control
joint, developing a separation between
the control joint accessory and the stucco;
or cracks will develop between control joints
because the accessories can’t function as
intended. Our research into this condition
has not revealed any documentation based
on engineering mechanics that would support
forgoing the cutting of the metal lath at
control joints.
A second argument is that ASTM
C1063, which has been adopted by reference
into the model building codes, requires
the metal lath to be discontinuous at the
control joints and tied to each side of the
control joint accessory. Although isolated
cracks do not always occur at locations
where the metal lath is continuous through
the control joints, we have investigated
numerous cracks that were the result of
the inability of the stucco to accommodate
movements and/or drying shrinkage.
Overfastening of the Metal Lath
ASTM C1063 requires the metal lath to
be fastened to the building framing members
by screws, staples, or nails, depending
on the framing material. The primary
purpose of the requirement is to ensure
that the metal lath is attached to an adequate
support. However, the requirement to
attach the metal lath to framing members
also has the effect of limiting the number
of fasteners installed in a given area. We
have observed isolated cracking caused by
metal lath installations containing several
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Figure 4 – View of stucco panels exceeding the 2½-to-1 length-to-width ratio requirement.
fasteners installed in a relatively small area.
The profuse fastener installation limits the
ability of the metal lath to move laterally,
thereby causing excessive stresses in the
stucco as a result of drying, shrinkage, or
other movements. In addition, overfastening
the stucco prevents the stucco from forming
a full key with the metal lath and fully
embedding the lath, the ramifications of
which were discussed previously.
Overfastening can also refer to installing
fasteners too tightly. Staples used in
lath installation are
required to be crowned
and driven flush with
the metal lath per
ASTM C1063. For
screws, the standard
requires the screw to
“pass through, but not
deform” the metal lath. Overdriving the fasteners
also restricts the ability of the metal
lath to accommodate movements without
damaging the stucco. The use of guides
on drill drivers and adequate location of
structural members can reduce the risk of
overdriving fasteners. Figure 5 illustrates an
example of an overdriven screw.
Incorrect Lapping of Metal
Lath Segments
According to ASTM C1063, the side laps
of metal lath segments must be attached to
framing members or wire-tied together. The
failure to follow these criteria frequently
causes isolated cracking. If the segments
are not connected at the side laps, adjacent
segments can move independently, particularly
if the stucco scratch coat is not applied
with enough force to form full keys with the
lath. These independent movements result
in isolated cracks in the stucco.
Another common issue occurs in stucco
installed with paper-backed metal lath.
Metal lath with backing is specifically
required by ASTM C1063 to be installed
such that metal abuts metal at the laps.
If the paper is placed between the metal
laths at the laps as illustrated in Figure 6,
the stucco cannot achieve full keys through
both sections of lath at the laps. This installation
also permits differential movement
between adjacent portions of the stucco,
which causes isolated cracks to occur, as
illustrated in Figure 7. Sometimes, isolated
cracks will form at about 3 ft. on center
vertically (i.e., at each lap) when these conditions
occur.
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Figure 5 – Example of an overdriven screw.
Figure 6 – View of incorrect lapping
of paper-backed lath.
Figure 7 – View of isolated crack related to
incorrect lapping of paper-backed lath.
Poor Integration of Stucco Accessories
Poor integration of the stucco accessories
with the field metal lath often causes
separations to occur between the stucco
and the accessory. Several controls are provided in ASTM C1063 to ensure that the
accessories are integrated with the field metal lath. Where paper-backed metal lath is
employed, the standard forbids placing the paper backing between the lath and the
accessory flanges. In addition, the accessory flanges are required to be fully embedded
in the stucco. Basically, the accessories should be integrated with the field metal lath
and treated similarly to laps in the lath discussed previously. Failure to comply with
these constraints enables the stucco to shrink away from the accessory, resulting in a
separation between the stucco and accessory. Figures 8 through 12 illustrate examples
of improper connection between the metal lath and stucco accessories.
2 9 t h RC I I n t e r n a t i o n a l C o n v e n t i o n a n d T r a d e S h ow • Ma rc h 2 0 – 2 5 , 2 0 1 4 C o p e a n d Ho rs t • 2 3 1
Figure 8 – View of improper integration
between metal lath and stucco accessory.
Figure 11 – Improper connection between
stucco accessory and metal lath.
Figure 12 – View of an improper lap between
paper-backed metal lath and a stucco accessory.
Figure 9 – View of improper
connection between stucco
accessory and metal lath.
Figure 10 – Improper
connection between
stucco accessory
and metal lath.
Incorrectly installed
screws in control
joint accessory
Another common deficiency related to
the integration of the accessories into the
stucco is inadequate placement or keying
of the stucco at accessories. ASTM C926
requires that the stucco be applied with
sufficient pressure to fully embed the metal
lath and form full keys with the lath. We
have identified many instances in which
the stucco was not fully keyed with the
accessory, enabling the stucco to separate
from the accessory. Figure 13 illustrates an
example where the scratch coat is poorly
keyed into a stucco accessory. Figures 14
and 15 illustrate examples of a separation
between the stucco and a stucco accessory.
Water Leakage
As discussed previously, stucco cladding
assemblies supported by metal lath are
drainage systems. As described in Annex
A2 of ASTM C926, proper installation and
curing can greatly increase the water resistance
of stucco cladding. However, ASTM
C926 goes on to state that stucco should
not be considered waterproof. Water can
penetrate the stucco in a variety of ways,
including absorption through the stucco,
via cracks within the stucco or separations
between the stucco and stucco accessories,
and through failed or missing sealants at
stucco terminations or penetrations. As
with any drainage
system, WRBs
and flashings are
required to prevent
water that
penetrates the
stucco from passing
into the interior
of the building.
For water to
infiltrate into the building, either the WRB
or flashings must have failed or been incorrectly
installed.
Neither ASTM C926 nor
ASTM C1063 provides extensive
requirements for installation of
flashings and WRBs other than
indicating that these components
are required for stucco
supported by metal lath. In particular,
ASTM C926 requires
the stucco designer to specify
and describe flashings for openings,
terminations, and perimeters
of the stucco system. In
addition, both ASTM C926 and
ASTM C1063 discuss installation
of weep screeds at the bases
of walls and the integration of
the WRB with the weep screed.
Sloping of horizontal surfaces
and appropriate methods of construction
at the interface of vertical
and horizontal surfaces are
also discussed in the standards.
The following paragraphs discuss
several defects identified during
our field investigations of water
leakage in stucco systems supported
by metal lath.
Flashings
Generally, designers specify throughwall
flashings at heads of windows, as
required by ASTM C926. This flashing typically
terminates above the window jambs
and does not extend beyond the jambs, as
illustrated in Figure 16. Formed end dams
are not normally installed in this instance
due to the propensity for cracking at the
end dam locations, amongst other concerns.
These conditions create several difficulties
regarding integration of the flashings
with the WRB and the windows and ensur-
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Figure 13 – View of improper key
between stucco and stucco accessory.
Figure 15 – View of a typical separation between
stone and a stucco accessory.
Figure 14 – View of a typical separation
between stucco and a stucco accessory.
ing the integrity of these systems. Due to
these difficulties, small holes often occur at
the interfaces of these materials, enabling
water to bypass the flashings and WRB and
penetrate the building. One way to resolve
these concerns is to install sealant on the
through-wall flashing when accessory casing
bead (J-mold) is installed above the
flashing in an effort to create an end dam.
However, in our experience, the placement
of the sealant is often not watertight or
fails prematurely, and water continues to
bypass these sealant end dams. Our recommendation
is to install continuous flashing
around the building at the elevation of the
window heads. The authors have successfully
employed this method in major stucco
repair and reclad projects on a variety of
facilities.
Another concern relating to flashings
occurs at the base of walls, where flashings
or weep screeds are required. Through-wall
flashings or “other effective means to drain
away any water” are required by ASTM C926
to be provided at the bottom of any stucco
wall supported by a floor or foundation. We
have investigated numerous buildings that
lack appropriate flashings, particularly at
balcony locations
or other interruptions
above
grade. The lack of
flashings at these
locations enables
water that penetrates the stucco to enter
the building and often causes significant
issues.
Weather-Resistive Barriers
As discussed previously, ASTM C926
and C1063 provide very little guidance with
regards to WRB installation. The fact that
the standards remain fairly silent on the
installation of WRBs should not minimize
the importance of the WRB in stucco construction.
As discussed previously, water
can penetrate stucco through a variety of
means. Once water gets behind the stucco,
any defect in the WRB provides a potential
avenue for water leakage into the building.
The primary defect we have observed
in WRBs that has led to water leakage into
buildings has been unsealed screw holes
through the WRB as illustrated in Figures
17 and 18. This typically occurs because the
locations of the framing members are not
readily identifiable after installation of the
WRB, and the lath installer installs screws
through the exterior sheathing that don’t
penetrate framing members as required by
the ASTM standards. Usually, the installer
either leaves the screw in the wall, providing
an avenue for water to follow the screw
penetration, or removes the screw, leaving a
hole through the wall for water to penetrate.
Rather than trying to guess the location of
the framing after the WRB is installed, we
recommend marking the framing locations
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Figure 16 – View of a common flashing detail at a window head.
Figure 18 – View of unsealed screw holes
viewed from the inside of the wall.
Figure 17 – View of unsealed screw holes.
on the WRB using chalk lines. This will help
the metal lath installer to accurately install
screws through the framing members. If a
screw misses the framing, we recommend
that the metal lath and WRB be cut to seal
the hole in the sheathing. Once the hole in
the sheathing is sealed, the WRB can be
sealed and the metal lath can be wire-tied
back together.
Other common WRB issues unique to
stucco construction include degradation
of a sheet WRB by the stucco and stucco
adhering to the sheet WRB. Many investigations
have revealed that the stucco adhered
to the WRB, decreasing or eliminating the
drainage plane behind the stucco. In some
cases, the stucco had damaged the WRB,
resulting in holes in the WRB. To avoid
these problems, we recommend installation
of an outer, intervening layer between the
stucco and WRB. This layer can comprise a
paper backing on the metal lath or a second
layer of WRB and is generally considered
a sacrificial layer. All flashings should be
incorporated into the inner WRB rather
than the outer layer. The model building
codes typically require similar installations
for stucco cladding with wood-based
sheathing; however, we recommend this
double layer for all framed construction.
LON G-TERM REPAIRS
In order to develop long-term repairs
for stucco defects, it is critical to determine
the root cause or causes of the defects.
Only after the problem has been defined
should design of the repair begin. As mentioned
above, numerous deficiencies in the
design and installation of stucco and/or
metal lath can result in excessive cracking,
isolated cracking, interior water leakage,
or some combination of these three conditions.
We have developed several repairs,
ranging from recladding to coating, which
can function as long-term repairs if they are
appropriate for the defects identified during
an in-depth investigation of the stucco. The
following paragraphs briefly describe some
of these repairs and conditions in which the
repairs may be appropriate.
Full-Scale Removal and Replacement
Full-scale removal and replacement is
generally the most appropriate repair for
stucco that is exhibiting excessive cracking
due to incorrect stucco mix proportions,
improper gradation of sand, poor curing,
delamination between stucco coats, and
corroded metal lath. In addition, depending
on the character of the water leakage
being experienced, full-scale removal and
replacement can be the most fitting repair
for water leakage caused by unsealed holes
through the WRB. These conditions are
typically indicative of major deficiencies in
the design and/or application of the stucco,
as described previously. Therefore, longterm
repairs should include removal and
replacement of the affected stucco.
Depending on the extent of the excessive
cracking, the replacement may encompass
the entire façade (i.e., recladding) or simply
specific affected stucco panels. When
removing isolated panels of stucco, it is
important to not cut through the metal lath
adjacent to the stucco accessory (construction
joint) that separates adjacent stucco
panels. Therefore, it is recommended that
the stucco be cut approximately 4 in. from
the stucco accessory. The stucco over the
4 in. margin should be carefully removed,
eliminating or minimizing damage to the
underlying metal lath. The metal lath in
the field of the stucco panel can be cut;
however, caution should be used not to cut
through the exterior sheathing. Once the
stucco and metal lath are removed in the
field of the stucco panel, repairs should
be made to the exterior sheathing and the
WRB, as necessary. At the edges of the
stucco panel, the existing metal lath adjacent
to the stucco accessory should be bent
out to allow for the new WRB to be installed
in a shingle fashion with the existing WRB.
Laps between the new and existing WRBs
must be at least 2 in., or as required by
the WRB manufacturer and/or applicable
building code. Once the repairs to the exterior
sheathing and WRB are made, new
metal lath and new stucco can be installed.
Excessive cracking caused by incorrect
stucco mix proportions and improper
gradation of sand are material defects in
the stucco cladding. When premature water
loss enabled by poor curing methods results
in wide, excessive cracking, the stucco is
typically weaker and prone to additional
cracking and degradation. Excessive cracking
caused by corroded metal lath typically
occurs when maintenance efforts have not
been performed or have failed. Application
of an elastomeric coating over the stucco
façade will not address the primary causes
of these issues and, therefore, is not recommended
as a long-term repair solution for
these conditions.
Removal and Replacement
of Isolated Stucco Areas
Removal and replacement of isolated
stucco areas is often a suitable, long-term
repair for isolated cracks caused by incorrect
lapping of metal lath segments or
overfastening of isolated areas of stucco.
Depending on the water leakage experienced,
isolated removal and replacement
of stucco can be an appropriate long-term
method to address water leakage caused
by inadequate flashing installation and isolated
damage to the WRB.
To repair a crack in the field of the
stucco due to underlying conditions such as
improperly placed/lapping of paper-backed
metal lath, the stucco should be removed to
expose underlying conditions. The removal
of the stucco should be performed by cutting
the undamaged stucco at least 6 in.
away from either side of the crack. The saw
cut should be at a depth that does not damage
the metal lath. The cut should be made
with a bevel that will allow the new stucco
to be placed under the existing stucco that
will remain. The stucco material within the
cut area is then carefully removed to avoid
damage to the metal lath. Once the stucco
is removed, the metal lath should be cut
to expose the condition causing the crack.
Once the condition is repaired, the metal
lath should be restored by using tie wire
to connect the new lath to the old so as to
cover the damaged area.
When removing and patching small
isolated areas of stucco, the type and
properties of the existing stucco should
be determined so the new stucco will be
as similar to the existing as possible and
avoid any incompatibilities between materials
or differences in strength and thermal
expansion. A stucco mix may be composed
of Portland cement, blended cement, plastic
cement, or masonry cement. The amount
of lime may vary between none to twice
the amount of cement. The amount of the
aggregate may vary between 2½ and 5 times
the sum of the cement and lime, and there
may or may not be an acrylic-type modifier
or bonding agent and fibers in the existing
mix. Petrographic and chemical analyses, in
accordance with ASTM C1324, of samples
of the existing stucco can determine the mix
components and relative content by volume.
Coating the stucco with an acrylic paint
or an elastomeric coating is often part of
isolated replacement of stucco. The decision
to paint or coat the stucco is often an
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aesthetic issue. The aesthetic appearance of
the repaired area is a function of achieving
the same color and texture for the finish
as was used for the adjacent stucco finish
that was not removed. Achieving the same
texture is a function of using the proper mix
to achieve the texture and the installer’s
ability to reproduce the texture when applying
the new finish. Replicating the existing
texture of stucco with a heavy texture is an
art form that requires a very skilled applicator.
If a suitable matching color cannot
be achieved, the entire wall may need to be
coated to provide a uniform appearance.
Installation or Replacement
of Stucco Accessories
When isolated cracking or separations
along stucco accessories are caused by
improper design and/or installation of control
joints or poor integration of stucco
accessories, installation of new stucco
accessories or removal and replacement of
stucco accessories have proven to be effective
long-term repair options. This repair
can be effective for water leakage caused
by improperly installed flashings or failure
to install flashings or weep screeds. The
actual removal of the stucco and metal lath,
repairs to any damaged sheathing or WRB,
and installation of new lath and stucco will
be similar to that discussed in the previous
section.
At locations where the designer and
installer failed to provide adequate control
joints—either by excessive spacing or
failure to abide by the permitted length-towidth
ratio established in ASTM C1063—
new control joints will be required. As
stipulated in ASTM C1063 for new stucco
construction, we recommend that the locations,
sizing, and installation requirements
for new control joint accessories be provided
by the designer in the contract documents
for the repairs. Similarly, where flashings
or weep screeds were not installed, removal
of stucco and installation of these accessories
should be appropriately detailed by the
designer for the repairs.
This repair can also be applicable to
situations where cracks form due to failure
to cut the lath at the control joints. In these
conditions, the existing stucco is cut on
a straight line at least 3 in. on each side
of the existing control joint. The contractor
must be careful to restrict the cut to a
depth that does not damage the metal lath
and WRB. The metal lath is then cut at the
control joint location to create a break in
the existing lath and allow the new control
joint to be installed under the existing
lath. The new control joint accessory must
be wire-tied to the existing metal lath at a
spacing not to exceed 7 in. on center, following
which, the new stucco is installed in
accordance with the procedure described in
the previous section. Similar repairs would
be applicable for replacement of flashings or
weep screeds that were incorrectly installed
and for addressing separations along stucco
accessories caused by poor application of
the stucco or failure to adequately integrate
the stucco accessory with the field metal
lath.
As discussed in the previous section, a
final step in these repairs is often application
of an acrylic paint or elastomeric coating.
However, as discussed previously, this
decision is largely an aesthetic one.
Routing and Sealing Cracks
and Separations
Routing and sealing of isolated cracks
in the field of a stucco wall and separations
along a stucco accessory can also be appropriate
repair options to address cracks
caused by inadequate control joints and
separations caused by poor integration of
the stucco accessories. However, the designer
and/or installer of the repair should
make it abundantly clear to the owner that
this repair does not address the underlying
cause of the defect. Therefore, this repair
should be considered a short-term repair.
Over time, as the sealants age and the
stucco continues to move in response to
environmental factors, the sealants should
be expected to fail and the repairs repeated.
In addition, we recommend that mockup
repairs be performed to illustrate the
aesthetics of this repair scenario. These
repairs—particularly routing and sealing of
cracks in the field of the stucco caused by
inadequate control joints—can be considered
unsightly. The mock-ups would enable
the owner to evaluate the aesthetics of the
completed repair. This repair is often used
to address the deleterious conditions while
funding is procured for a larger project that
would address the underlying causes of the
cracks and separations.
Application of an Elastomeric Coating
Application of an elastomeric coating
can be an appropriate repair in a few isolated
conditions. For instance, an elastomeric
coating can be a very good long-term repair
option for addressing excessive fine crazing
caused by poor curing. Note that the craze
cracks in this case should be no more than
hairline width, and no other major defects
should be identified in the stucco. The
elastomeric coating should have at least
300% elongation to allow the coating to stay
watertight as the cracks move due to thermal
temperature changes, and the perm
rating should exceed at least 5.0 to allow
the wall to breathe. If the perm rating of the
coating is too low, moisture can be trapped
in the stucco system and freeze/thaw or
other damage can occur.
However, application of an elastomeric
coating is not the panacea that some consultants
and contractors have made it out
to be. We have performed investigations on
numerous buildings that continue to suffer
from water leakage after application of an
elastomeric coating. These buildings continued
to leak because the repair designer
and/or contractor failed to understand that
the elastomeric coating was an attempt to
create a barrier wall system of the stucco
cladding, which, as we discussed previously,
is intended to perform as a drainage
system. In order to develop a barrier
system from stucco, every avenue for water
penetration—including windows and other
penetrations, terminations, etc.—must be
addressed and formed into a barrier. This is
a very difficult and expensive process and
should only be attempted by very experienced
designers and contractors. In addition,
the owners should be apprised of the
need for an extremely diligent maintenance
program that monitors and addresses degradation
of the various components comprising
this barrier system.
CON CLUSION S
When designed and installed correctly,
stucco supported by metal lath can provide
a durable, aesthetically pleasing covering for
building walls, requiring relatively minimal
regular maintenance to continue functioning
as intended. In order to assist designers
and contractors, ASTM has developed
two standards, ASTM C926 and C1063.
For a project to be successful, all parties
involved in designing or installing stucco
walls should have a thorough understanding
of these documents. Unfortunately,
there is some confusion among both designer
and installers regarding the requirements
promulgated in these documents and the
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ramifications of the failure to comply with
these requirements. This confusion or disregard
often results in significant defects
in stucco installation, including excessive
cracking, delamination, isolated cracking,
separations, and water leakage. Because
these documents have been incorporated
by reference into the model building codes,
failure to comply with the stipulations of
these documents frequently represents a
violation of the building code.
In this paper, we have endeavored to
inform the reader of several defects commonly
noted in our investigations that have
resulted in significant failures or distress
in stucco construction. These defects could
have been avoided simply by understanding
and following the stipulations outlined
in ASTM C926 and C1063. We have also
described some long-term repair options
that can be implemented and the fallacy
of expecting application of a coating to the
exterior face of the stucco to provide a
long-term solution to all of these defects.
For long-term repairs to be successful, the
repairs should address the root cause(s) of
the defects, as determined following a thorough
investigation by a competent person,
rather than merely attempting to address
the symptoms of these defects, such as
cracking or separations.
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