Designer Choices and Responsibilities in ASTM C1063 and ASTM C926 Regarding Portland Cement Plaster Wall Claddings

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Designer Choices and Responsibilities
in ASTM C1063 and ASTM C926 Regarding
Portland Cement Plaster Wall Claddings
Jeff Bowlsby, CCS, CCCA
Simpson Gumpertz & Heger Inc.
100 Pine Street, Suite 1600, San Francisco, CA 94111
Phone: 415-652-4518 • E-mail: jabowlsby@sgh.com
and
Lee Cope, PE
Wiss, Janney, Elstner Associates, Inc.
2915 Premiere Parkway, Suite 100, Duluth, GA 30097
Phone: 770-923-9822 • Fax: 770-232-9044 • E-mail: lcope@wje.com
Abstract
The speakers will discuss designers’ responsibilities for the effective design of Portland
cement plaster (stucco) wall claddings as specified by ASTM stucco industry standards.
Standards ASTM C1063 and ASTM C926 were carefully researched by two design professionals
who contributed to their development. All stucco is not the same, and the range and
variety of choices that designers must evaluate and select from in terms of material and
detailing have implications for function, durability, and creative expression. These implications,
minimum design requirements, best design practices, conclusions, and recommendations
will be explored.
Speakers
Jeff Bowlsby, CCS, CCCA – Simpson Gumpertz & Heger, Inc., San Francisco, CA
Jeff Bowlsby is a specialized exterior wall and stucco consultant. His nationwide
practice includes new building construction and rehabilitation projects, property condition
assessments, and forensic evaluations. He founded and leads the ASTM C11 stucco work
group that develops all ASTM stucco-related industry standards referenced in building codes
and construction contracts. Bowlsby has been published in national industry professional
journals and is the author of the stucco information resource StuccoMetrics.com.
Lee Cope, PE – Wiss, Janney, Elstner Associates, Inc., Duluth, GA
Lee Cope has extensive experience in detailing and installation of Portland cement
plaster façade systems, air barriers, window systems, waterproofing, and the interfaces of
envelope components. He has evaluated structures relating to the cause and/or distress of
buildings’ exterior façade/curtainwall systems and building envelope systems. He is a voting
member of ASTM Committee C11 on Gypsum and Related Building Materials and Systems.
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INTRODUCTION/GENERAL
Brief History of the Stucco
Industry Standards
The earliest stucco standards were
voluntary and intended for the purpose
of documenting best practices. The first
stucco industry standard, from 1920, was
American Concrete Institute (ACI) Standard
No. 25, Standard Recommended Practice for
Portland Cement Stucco. It included both
lathing and plastering in one standard. This
standard has significant valid content even
today, nearly a century later, because the
basic characteristics of Portland cementbased
plaster have not really changed
that much over time. A single standard
continued until 1971, when the standard
was bifurcated into American National
Standards Institute (ANSI) Standard A42.2
for Plastering and A42.3 for Lathing.
Compliance with stucco industry
standards was voluntary through these
decades, but the standards were often specified
as a benchmark for quality in architects’
construction documents. Caretaking
of the ANSI stucco standards transitioned
to the current ASTM International (ASTM)
organization development process in the
early 1980s. Today’s ASTM stucco industry
standards have changed in character to
become minimum building code requirements,
not mere voluntary guidelines and,
at the same time, not best practices. The
current International Building Code (IBC),
as adopted and enforced by many building
department jurisdictions nationwide, incorporates
ASTM C1063 and C926 minimum
stucco cladding requirements by reference,
meaning they are codified as minimum
building code requirements.
ASTM standards are divided into six
types, including test method, specification,
classification, practice, guide, and terminology.
The two primary stucco industry
standards that are the basis of this paper—
ASTM C1063-16a, Standard Specification
for Installation of Lathing and Furring
to Receive Interior and Exterior Portland
Designer Choices and Responsibilities
in ASTM C1063 and ASTM C926 Regarding
Portland Cement Plaster Wall Claddings
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Cement-Based Plaster, and ASTM C926-
16a, Standard Specification for Application
of Portland Cement-Based Plaster—are both
specifications. The requirements of these
minimum standard specifications are stated
in mandatory language.
ASTM standards are continuously
revised and updated with new versions
resulting from the ASTM standard development
process. The 2016a versions of C1063
and C926 are referenced for this paper as
the most current versions, though they are
not yet codified. Both standards incorporate
material standard specifications by
referencing other ASTM standards, such as
ASTM C150 for Portland cement or ASTM
C847 for metal lath. These second-tier reference
standards are therefore also minimum
building code requirements.
Currently, all ASTM standards as they
relate to the stucco industry are specifications,
with the sole exception of ASTM
C11, Standard Terminology Relating to
Gypsum and Related Building Materials and
Systems, which defines stucco industry
terminology. It is important to note that no
stucco cladding system standard for test
method currently exists, although one is
currently in development for evaluating the
bond strength of stucco cladding.
Each ASTM stucco standard is organized
internally into sections describing
the Scope of the standard, a list of
Referenced Documents, Terminology,
Delivery and Storage of Materials, Materials,
Installation or Application Requirements,
and Keywords. An Annex of Mandatory
Information for General Information and
Design Considerations and an Appendix
of Nonmandatory General Information are
also included within the standards.
ASTM stucco industry standards sometimes
include more than a single choice
of acceptable materials and methods for a
Portland cement-based plaster wall cladding
system, which is an indication of their
versatility. The designer, if only broadly
specifying that the stucco be in compliance
with ASTM C1063 and C926, allows the
contractor a wide range of available choices
that meet the minimum requirements.
For situations where more design control
is required, designers must evaluate and
specify which components and methods are
required: which materials, how many coats,
what finish and texture, etc., to achieve the
intended results.
Introduction
ASTM C1063 and C926 are specifications—
minimum industry standard
requirements—not guidelines. A designer,
as a best practice, can and should specify
higher requirements where the specific conditions
and requirements of a given project
warrant. Higher expectations for quality
of materials and workmanship than minimum
standards require should inform and
direct the designer to specify higher quality
materials and workmanship. Specific environmental
conditions, such as coastal-zone
corrosivity, should require specifications for
materials that are sufficiently corrosionresistant.
Higher expectations for minimizing
cracking or maximizing water management
should inform and direct design decisions
and specifications for all aspects of a
stucco wall cladding system.
This paper is directed at the designer of
Portland cement-based stucco wall cladding
systems. The term “designer” is intentionally
broad, and the standards are currently
under revision to use the term “design
authority” to describe the entity responsible
for design decisions, which will also
be used going forward in this paper. The
design authority can be one of many entities,
depending on the given context—from
a traditional design professional, to the
contractor, the building owner, an authority
having jurisdiction (AHJ), or even a product
manufacturer—and may function in a
design authority role for all or a portion of
the project. An objective of this paper is to
assist the design authority in determining
needs and expectations for stucco wall cladding
systems, and help in evaluating stucco
wall cladding choices, specifications, and
installation requirements resulting in stucco
that meets or exceeds the requirements.
The design professional stucco consultants
who researched and coauthored
this paper focused on the design authority
requirements related to the two primary
aspects of Portland cement-based exterior
stucco wall cladding: lathing and plastering.
At the same time both interesting and
potentially confusing, C1063 is primarily
about lathing, and C926, primarily about
plastering. These two topics are intermingled
throughout both standards. While
specific text from the ASTM standards
cannot be quoted in respect of copyrights,
references are given for the readers’ further
information and review.
METAL LATH INSTALLATION
Metal lath and lathing accessories as a
base to receive Portland cement plastering is
almost solely used for stucco wall cladding
systems placed over a water-resistive barrier
(WRB) and framed substrate, which may
or may not include continuous sheathing,
continuous insulation, or a defined drainage
cavity. While circumstances exist that
merit direct-applied, continuously bonded
stucco wall cladding onto a cementitious
substrate that may require lathing and lath
accessories, these are specialized conditions.
The design authority is required to
design lathing and lath accessories, as this
paper further elaborates.
Framed Structural Support Substrates
With a Maximum Deflection of L/360.
Until recently, the structural design
sections of the building code allowed L/240
for “brittle finishes,” presenting a dilemma
for some design authorities, but recent
changes to the IBC reflect the long-standing
stucco industry requirement for L/360 as
the maximum allowable substrate support
deflection. Minimizing deflection of the substrate
support for the relatively thin section
of a stucco wall cladding is critical to minimize
the potential for mid-span cracking,
which may occur if the framing can bend
to a greater extent than the cured Portland
cement plaster membrane can resist without
cracking. The L/360 deflection criteria
apply to both wall stud framing and
ceiling/soffit framing conditions—indeed
any framed substrate support for Portland
cement-based wall cladding. Reference:
ASTM C926 A2.1.6.
Sufficient Slope for Drainage at Surfaces
Related to Wall Cladding Surfaces, Such
as at Recessed Windowsills and Wall
Parapets
Stucco wall cladding systems are not
inherently waterproof, and all climates and
conditions where they are used are exposed
to water accumulation in the form of rain,
snow, or ice. Sufficient slope that uses
gravity to assist with drainage is necessary
in minimizing water intrusion and is a
basic requirement to facilitate drainage and
minimize concealed, water-related damage.
However, slope must be accommodated in
the design of stucco wall cladding systems
and their substrate support and not left to
chance. Consider also that manufacturers
of polymer-finish coat materials have
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specific minimum slope (often 6:12 pitch) and maximum planar
dimension requirements (often 6-12 inches maximum) that affect
the durability of polymer-finish coat material and are not specifically
expressed in the ASTM stucco standards. The recessed
windowsill detail depicted in Figure 1 may be sufficient for a
cement-finish coat stucco, but not sufficient for a polymer-finish
coat stucco. Reference: ASTM C926-A2.1.1.
Flashings at Openings, Perimeters, and Terminations
Stucco shall not be considered “waterproof” and requires the
design authority to describe the requirements for furnishing and
application of flashings to prevent water from getting behind the
plaster and to provide drainage from behind the stucco cladding.
Flashings are required to consist of corrosion-resistant materials
and shall be installed at openings, perimeters, and terminations
of the stucco wall cladding system. Reference: ASTM
C926-A2.1.2.
A foundation weep screed shall be installed at the bottom of
all steel- and wood-framed exterior walls, and the requirements
regarding the position of the foundation weep screed and its
installation requirements are as follows:
• The bottom edge of the weep screed shall be not less than
1 inch below the formed joint between the foundation and
the framing.
• The nose of the weep screed shall be no less than 4 inches
above grade, and no less than 2
inches above paved surfaces.
• The WRB and lath shall entirely
cover and lap over the vertical
flange of the weep screed and terminate
at the top edge of the nose.
Reference: ASTM C1063-7.11.5.
• Where the drainage plane (WRB)
is interrupted by a floor, supporting
structure (such as at a projecting
balcony or soffit corner),
or foundation, or when a drainage
assembly is constructed above a
barrier wall assembly, the design
authority is required to specify
an effective means of drainage
(drainage screed, flashing, etc.) to
drain away moisture that may get
behind the stucco at the bottom of
exterior drainage walls (Figure 2).
Reference: ASTM C926, A2.2.1.
Specially configured, custom-fabricated
flashings that are soldered or
sealed watertight and provided by other
suppliers/contractors—such as window
head flashings and sill pan flashings (not
considered to be a lath accessory)—must
be integrated with the WRB to manage
incidental water infiltrating behind the
stucco wall cladding system.
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Figure 1 – Drainage slope at recessed windowsill with cementfinish
coat.
Figure 2 – Soffit drainage flashing at soffit corner condition.
Lath and Lath Accessories Used With
Portland Cement Plaster
To the extent that a design authority
designs the stucco wall cladding system,
its individual components, and stucco wall
cladding system details, as well as specifies
stucco materials, the design needs to
conform to or exceed the minimum requirements
of ASTM C1063 and C926. This
should be an easy-to-understand concept,
but in practice, variations from this minimum
requirement frequently occur.
Common examples are the design omission
of casing beads and sealant at stucco
perimeter interfaces with dissimilar materials
such as windows (C1063-7.11.3 and
C926-A2.1.3), allowing lath fasteners set
into sheathing only and not framing members
(C1063 6.7.2, 7.8.1 et al.), and allowing
continuous lath at control joints (C1063-
7.10.1.5). Each of these conditions can detrimentally
affect the performance and aesthetics
of stucco wall claddings.
The opposite is true for any aspect of the
stucco wall cladding system that is intended to exceed the minimum requirements of
the stucco industry standards as a best practice. Special requirements for materials,
methods, aspects, or evaluation criteria must be designed by the design authority.
Examples might include a project-specific requirement for stainless steel lath, lath
accessories, and fasteners as may be appropriate for a corrosive environment. The
design authority needs to design details of the stucco wall cladding system deemed
important to achieving the designer’s intent and expectations. (See Figure 3.) Reference:
ASTM C926-6.1.
Requirements for Lath Accessories, Including Their Type,
Depth, Location, and Orientation
Required lath accessories must be selected and identified, including their ground
dimension requirements and location requirements within the stucco wall cladding
assembly, as
typically indicated in
detail drawings. Lath
accessory “type” can
refer to things such
as whether a corner
bead is square-nose,
plastic-nose, or bullnose;
and a material
specification, such
as whether it is of
expanded sheet metal
or plastic. “Depth”
refers to the critical
dimensional requirements
of lath accessories,
such as the
ground dimension or
width of a vent screed
or reveal screed. The
design authority must
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Figure 3 – Stucco separation from window (a dissimilar material) using casing
bead and sealant.
Figure 4 – Wall elevation drawing
indicating jointing locations.
Figure 5 – Vertical (parallel
to framing members) control
joint construction detail.
locate required lath accessories, including
drainage screeds, stucco perimeter termination
requirements, expansion joints, control
joints, etc., and depict their installation
requirements in the contract documents,
such as any required framing members or
blocking for fasteners. The requirement for
“orientation” can mean two things: 1) any
distinction in requirements for a lath accessory
that is oriented vertically or horizontally
on a wall; and 2) how a lath accessory is
installed relative to other stucco wall cladding
components, such as determining if the
solid flange of a weep screed is overlapped by
one or both layers of the WRB. See Figures
4 and 5. Reference: ASTM C926-A1.6.2,
A2.3.1, A2.3.1.2, X1.1.6; and ASTM C1063-
Section 7, A6.2.1.
A Complete and Coordinated Stucco
Wall Cladding System
The design authority should recognize
that a complete and coordinated stucco wall
cladding system includes ancillary requirements
such as for acceptable substrate
conditions and for sealant at perimeter
terminations and penetrations. The opening
sentence of ASTM C1063 states that the
standard describes the minimum technical
requirements for lathing and furring
for stucco wall cladding systems, but that
the requirements of the standard do not
describe a unit of work for contracting purposes.
Specifiers will realize that certain
requirements for a complete stucco wall
cladding system—even though indicated in
a stucco industry standard—are provided
by craftsmen other than lathers and plasterers.
For example, wood-based sheathing
substrates must be gapped at panel edges
(C1063, Table 3, Footnote A). While it is a
minimum requirement for stucco, providing
the gaps is customarily the purview of
carpenters. Required perimeter sealants
(C926-A2.1.3) can be installed by other
trades and are typically left to the contractor
to assign responsibility and coordinate.
Reference: ASTM C1063-1.1
Concrete Substrates and High-Wind
or Uplift Conditions
Furring, furring attachments, and
hanger attachments into concrete substrates
and any aspect of the stucco wall
cladding installation (such as at suspended
exterior building soffits) subject to high
-wind or uplift forces, may require design
(including an engineering evaluation) to
determine specific requirements to resist
the forces imposed on or by the stucco
wall cladding system. Vertical wire hangers
are used primarily for supporting the
framework for suspended stucco soffits or
ceilings, and embedded concrete inserts
(hanger attachments) or similar devices
must accommodate the full strength of
the hanger. References: ASTM C1063-7.2.3,
7.7.1, Table 2, Footnotes 9 and 10.
Special Stucco Assembly Requirements
for Fire Resistivity, Sound Control,
and Shear Walls
The design authority is required to depict
the “details of construction” for specialized
fire-resistive, acoustical control, and
shear wall assemblies that utilize stucco.
Expanding the fuction of stucco may bring
special requirements not normally a part of
typical stucco wall cladding systems. The
design authority needs to explicitly describe
all special requirements, including special
materials, lath fastener requirements, and
field testing, in the contract documents.
For example, some tested assemblies for
fire-resistivity require a full 1-in. cement plaster
thickness, specific lath or lath fasteners,
sheathing requirements, and stud cavity-fill
materials, such as mineral wool. While C1063
and C926 do not describe these specific
requirements for tested assemblies, they are
indicated in other stucco industry or building
code documents. Industry-recognized laboratory
testing reports describing requirements
necessary to achieve desired fire-resistivity
and acoustical performance parameters
should be reviewed and indicated as the basis
of design, and their requirements incorporated
into the contract documents. References:
ASTM C1063-1.2, 1.3, IBC Table 2306.3(3).
Stucco Submittals
The common process
of specifying and
providing submittals
for a building construction
project is routine
on many projects, but
this constructionphase
quality control
procedure is not
required or even mentioned
in ASTM stucco
standards. Submittals
are essentially a communication
device used
to convey the contractor’s
understanding of the design authority
requirements to the design authority and
owner. This is part of the quality assurance
process to communicate that the contractor
plans to provide what the design authority
intended in the construction documents,
and to resolve conflicts. Stucco wall
cladding system submittals often include
product data or actual samples of all components
used, such as the WRB, lath, lath
accessories, plastering materials, and a
manufacturer’s standard colors brochure
for finish coat color selection. A physical
sample of the stucco wall cladding system
(or even just the finish coat), as well as a
site-constructed mock-up wall (Figure 6)
may also be specified as submittals, each
of which will confirm the completeness
and accuracy of the design, the workmanship
quality, and installation coordination.
These submittals will also enhance communication
amongst the project team to avoid
potential misunderstandings in the final
installed stucco wall cladding. To require
stucco submittals, the design authority
must specify the list of items to be submitted
for review and acceptance.
Stucco Shop Drawings
Design and construction professionals
know the benefits of shop drawings to
a building construction project, but this
standard construction phase quality control
procedure is not required or even mentioned
in ASTM stucco standards. Shop
drawings are also a submittal to the design
authority for review and acceptance, and
are considered a best practice in the stucco
industry. A stucco wall cladding system
is often the single most prominent and,
therefore, important wall cladding mate-
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Figure 6 – Stucco mock-up wall.
rial on a building, projecting the building’s
aesthetic and architectural image, as well
as providing a critical weather protection
system for the building. If not designed or
installed satisfactorily in form and function,
the stucco wall cladding system may turn
out to be the building’s biggest aesthetic or
functional disaster.
A stucco wall cladding system consists
of many individual materials and components
that must coordinate with other
building components, such as doors, windows,
roofing, and paving. Different materials
and components of the stucco wall cladding
system may be provided or installed by
different manufacturers, suppliers, trades,
and craftsmen, so their coordination and
compatibility is paramount to achieving a
stucco wall cladding that looks and performs
to meet standard requirements. Other
specialized construction systems, such as
cabinetry, curtainwalls, sheet metal flashings,
roofing, and waterproofing systems,
utilize the shop drawing process prepared
by the contractor or industry specialists to
coordinate all the variables and potential
risks. The shop drawings submittal process
is an ideal solution for stucco wall-cladding
systems as a step towards ensuring that
the design authority and building owner’s
expectations are well defined and satisfied
in the final installation. Shop drawings
expand and further detail the design and
specification of the stucco wall cladding
system and its components. They serve as
a reference for comprehensive evaluation
which helps resolve conflicts and conditions
before they become problems during or after
installation. To require stucco shop drawings,
the design authority must specify that
stucco wall cladding system shop drawings
be submitted for review and acceptance.
PLASTERING
Stucco can be applied to a metal plaster
base (as discussed previously) or directly
applied to a solid base such as masonry,
precast concrete, cast-in-place concrete,
or concrete masonry units (CMUs). Early
signs of distress, in the form of delamination,
cracking, and water intrusion, are
often the result of poor surface preparation,
improper mixing, defective installation,
inadequate curing, or a combination
of these. While the minimum industry
standards for application of stucco—including
surface preparation, mixing procedures,
and curing requirements—are outlined in
ASTM C926, the authors often observe significant
inconsistences in the application
of these requirements by the various parties
involved, including design architects,
waterproofing consultants, contractors, and
subcontractors.
Based on the authors’ experiences,
these inconsistencies are frequently caused
by inadequate understanding of the ASTM
standards and often lead to significant deficiencies
in the completed stucco cladding.
Deficiencies can include cracking, separations,
delaminations, and water leakage,
and can result in premature maintenance
costs and often costly litigation. Therefore,
the following section will discuss the design
authority’s responsibilities as identified in
ASTM C926, as well as provide recommendations
that a design authority may want
to consider in his or her design to help
ensure successful installation, a successful
finished product, and a satisfied client. The
topics discussed in this section include:
• Surface preparation
• Mix design
• Mixing and application
• Curing and crack acceptance criteria
• Provisions for drainage behind exterior
stucco
Surface Preparation for Solid Bases
Throughout the coastal United States,
directly applied stucco is a common cladding
system. Unfortunately, the bonding
requirements and substrate preparation
are rarely specified by the design authority
responsible for these cladding systems.
Similarly, treatment of the stucco at dissimilar
substrate materials, and requirements
for addressing substrate conditions
that exceed the permitted plane of ¼ inch
in 10 feet, are often not identified by the
design authority. Rather, the design authority
frequently only references ASTM C926 in
the specifications, enabling the contractor
or applicator to apply the minimum requirements
identified in the specification, which
often results in stucco delaminations and
cracking of the stucco. ASTM C926 provides
a variety of design authority responsibilities
regarding surface preparation for
bond, treatment of dissimilar materials,
and requirements to correct surfaces that
are out of tolerance prior to placement of
stucco. The following summarizes these
responsibilities and also provides recommendations
for the design authority to consider
to help ensure bond is achieved.
Design Requirements Regarding Bond
Section 6 of ASTM C926 provides minimum
requirements for surface preparation
to solid bases prior to placement of stucco.
These requirements are as follows:
• Solid surfaces to receive stucco shall
be free of form oil or other elements
that may interfere with bond
between the stucco and the substrate.
• Solid surfaces shall have the ability to
absorb moisture, have surface roughness,
or both, in order to provide the
bond required for the plaster.
• Smoothing of nonabsorbent surfaces
(i.e., cast-in-place concrete or precast
concrete) shall be prepared in
one of the following methods:
— Sandblasting, wire-brushing,
acid-etching, or chipping, or a
combination
— Application of a dash-bond coat
applied forcefully against the
surface, left untroweled, undisturbed,
and moist-cured for at
least 24 hours
— Application of a bonding compound
suitable for exterior-exposure
solid surfaces in accordance
with the manufacturer’s
written directions
• Where bond cannot be obtained by
one of the methods listed above, a
furred or self-furring metal plaster
base shall be installed per ASTM
C1063. At these locations, accessories
shall also be installed per ASTM
C1063.
While many of these requirements could
be considered means and methods and
are therefore the purview of the contractor,
Section A1.6.3 (Annex A1. General
Information – Mandatory Information),
requires the design authority to describe
“the physical characteristics of solid-surface
bases to receive plaster, including
measures to promote bond” in the proper
section of the contract documents. This
section also states that form release agents
shall either be compatible with the stucco
or shall be completely removed from the
substrate prior to the application of the
stucco. Furthermore, Section A2.1.5 (Annex
A2. Design Considerations – Mandatory
Information), obliges the specifier to indicate
in the appropriate specification section
that solid bases to receive stucco shall not
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be treated with “bond breakers, parting
compounds to prevent adhesion, form oil, or
other material that will inhibit the bond of
the stucco to the base.”
Therefore, in accordance with ASTM
C926, it is the design authority’s responsibility
to specify what is required to achieve
bond. If the specifier does not include a
description of “measures to promote bond,”
he or she is failing to meet the requirements
of ASTM C926. Since ASTM C926 is incorporated
by reference in the model building
codes, the design authority has failed to
meet the minimum design requirement set
forth by the code. As a result, inadequate
surface preparation is often provided, which
frequently results in stucco delaminations.
In addition to these requirements, it
is recommended that the design authority
also specify the required minimal bond
for directly applied stucco. While ASTM
C926 does not provide quantitative bond
requirements for directly applied stucco,
the authors have measured bond strengths
from 0 to 5 psi, to well over 100 psi. The
lower bond strengths are often recorded on
cast-in-place or precast concrete, as a CMU
generally provides an acceptable substrate
with little to no surface prep needed. It is
the author’s opinion that bond strengths
of less than 30 psi are likely indicative
of poor surface preparation on surfaces
such as cast-in-place or precast concrete,
and/or poor stucco installation. However,
when cast-in-place concrete is clean, free of
form oil, and when the surface profile has
been prepared to comply with International
Concrete Repair Institute (ICRI) CSP-7,
bond strengths over 50 psi can be achieved.
Requiring construction of mock-ups in
the project specifications and performing
bond tests to record the bond strength on
these mock-ups can help verify the appropriate
surface prep needed to achieve the
desired bond.
Design Requirements for Treatment
of Dissimilar Base Materials
Cracking is often observed in stucco
that is directly applied over dissimilar base
materials, such as the joint between the
CMU infill walls and the cast-in-place concrete
frame. This cracking is often the result
of or a combination of improper detailing,
improper installation of structural connections
between the CMU and cast-in-place
concrete, and incomplete placement of mortar
between the CMU and cast-in-place
frame. Because of these possible problems,
ASTM C926 recognizes the potential risk of
distress in the form of cracking along dissimilar
materials.
In Section A2.3.3 (Annex A2, Design
Considerations–Mandatory Information),
the standard requires the design authority
to specify one of the following methods be
installed to treat the joint where dissimilar
base materials abut and are to receive a
continuous coat of stucco. The methods are
as follows:
• Either a two-piece expansion joint, a
casing bead placed back-to-back, or
a premanufactured control-expansion
joint should be used (Figure 7).
• The joint between the two dissimilar
materials shall be covered with
a 6-inch-wide strip of galvanized,
self-furring metal plaster base, with
3 inches extending on either side of
the joint (Figure 8).
• Where one of the bases is a metal
plaster base, the self-furring metal
plaster base shall be extended 4
inches onto the abutting base.
In our experience, the design authority
generally does not like to see “pictureframe”
joints in the stucco where dissimilar
materials occur (joints between CMUs and
cast-in-place columns and joints between
CMUs and cast-in-place slabs). Therefore,
expansion joints, casing beads placed back
to back, or premanufactured control-expan-
3 2 n d 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 1 6 – 2 1 , 2 0 1 7 B owl sb y a n d C o p e • 1 6 7
Figure 7 – Two-piece casing bead back-to-back at a joint
between two dissimilar base materials.
Figure 8 – Six-inch-wide strip of galvanized self-furring metal
plaster base at a joint between two dissimilar base materials.
sion joints are rarely specified or allowed
or illustrated on the contract drawings.
However, it is common to observe installers
place a section of fiberglass reinforcing
mesh or plastic/PVC mesh to “reinforce”
the stucco over these joints. Unfortunately,
cracking is often observed due to the conditions
mentioned above, as well as substandard
installation resulting in a poor key
between the stucco and the mesh.
Based on the requirements of ASTM
C926, the design authority shall specify
how to treat the joint between dissimilar
base materials in directly applied stucco.
It is the authors’ opinion that if cracking is
not acceptable along joints where dissimilar
materials abut, a two-piece expansion joint,
a casing bead placed back to back (with
sealant in between), or a premanufactured
control-expansion joint should be specified.
Design Authority Requirements
for Treatment of Substrates
That Are Out of Tolerance
For stucco that is directly applied to a
high-rise, cast-in-place concrete frame, it is
likely that the stucco will be thicker than
the required nominal thicknesses specified
in ASTM C926, Table 4, and sometimes
substantially so. This condition is likely due
to the less stringent tolerances controlled
by ACI 117, Specification for Tolerances for
Concrete Construction and Materials, compared
to the substrate tolerance required
by ASTM C926.
When constructing a high-rise structure
that consists of a cast-in-place concrete
frame, the tolerances for plumbness
and horizontal variances are detailed by
ACI 117. Section 4.1 of ACI 117 provides
plumbness requirements for cast-in-place
concrete building heights less than or equal
to 83 feet, 4 inches. The permissible out-ofplumb
dimension is the lesser of 0.3 percent
of the total height, or 1 inch. Therefore, for
a building with a 10-ft. floor-to-floor height,
the allowed out-of-plumbness in a building
surface between the top and bottom of the
floor is 0.36 in., or approximately 3/8 in.
Similarly, the maximum allowed tolerance
over two stories is 0.72 in. or approximately
¾ in., and 1 in. for three or more stories.
Section 4.2 of ACI 117 provides tolerances
for horizontal variances in the vertical
edge of an opening in a floor, wall, or
beam. The permissible out-of-plumb dimension
for floor edges, beams, or walls is (+/-)
1 in. Therefore, as an example, for a typical
building with multiple bays, the first bay
is allowed to be out of variance by +1 in.
compared to the specified plan dimension,
and the adjacent bay is allowed to be out of
variance by -1 in., which equates to an offset
in the framed floor edges of 2 in. across
the two bays.
When placing stucco on a high-rise
structure that consists of a cast-in-place
concrete frame with CMU infill walls, the
columns, edges of floors, and block walls
are to be within ¼ in. of the same vertical
plane. This tolerance is based on Section
6.2 of ASTM C 926, which requires the substrate
to be “straight and true within ¼ inch
in 10 feet.” This section also requires that
surfaces that are out of these tolerances be
corrected prior to the placement of stucco.
Annex A 1, G eneral I nformation
(Mandatory Information), Section A1.6.3
limits the tolerances of the substrate to no
more than ¼ inch in 10 feet and requires
that ferrous materials, such as reinforcement,
tie wires, etc., shall be cut back a
minimum of 1/8 in. below the surface and
treated with a corrosion-resistant coating.
In addition, Appendix X1, General
Information (Nonmandatory Information),
Section X1.1.5 states that corrective measures
for surfaces outside of the tolerances
specified in Section 6.2 should “include
sandblasting, chipping, or grinding of the
solid plaster base, application of a repair/
build-out mortar, installation of a self-furring
plaster base, or combination thereof.”
It also states that since these measures
may have structural consequences, the
repair should be considered with all parties
involved, with the ultimate selection left to
the discretion of the design authority.
In our experience, the design authority
rarely provides direction to the contractor
for correcting out-of-tolerance surfaces. As
a result, the authors have observed contractors
using various methods to attempt
to reduce planar irregularities in the finished
stucco. These methods have included
increasing the thickness of the stucco well
beyond the nominal thicknesses specified
in ASTM C926 (up to 3 in. thick), use of a
myriad of build-out materials, and chipping
out concrete to depths that exposed
reinforcing bars. Excessively thick stucco
(1½ to 3 in. or more) is usually placed in
several layers, which increases the possibility
of a delamination occurring between
the various coats. More often than not,
the build-out materials utilized are not
structural repairs and do not include structural
anchors that help secure the buildout
material to the substrate. Rather, the
build-out material is often placed directly
on the substrate with little to no surface
preparation, which often results in poor
bond of the build-out material to the substrate.
Chipping out the substrate to correct
the substrate tolerance frequently results
in poor concrete cover over the reinforcing
bars and/or exposed reinforcing without a
corrosion-resistant coating. These methods
often result in delamination and cracking.
In Appendix X.1, Section X1.1.5, ASTM
C926 recognizes that multiple parties can
be responsible for out-of-tolerance substrates
on solid bases, and indicates that, since
repairing the substrate may have structural
consequences, the ultimate discretion of the
repairs should rest with the design authority.
In addition, Section 6.2 and Appendix
A1 state that the plane tolerances shall be
no more than ¼ inch in 10 feet, and that
the substrate shall be repaired prior to the
application of the plaster.
Therefore, it is the responsibility of the
design authority to specify how substrates
shall be prepared if substrates are out of
the required tolerances. In addition, to
avoid extensively thick stucco, the authors
recommend that the design authority specify
the maximum stucco thickness allowed.
Surface Preparation for
Metal Plaster Bases
Outside of the coastal United States, the
majority of stucco is applied to metal plaster
bases. The minimal code requirements for
installation of the metal plaster bases and
accessories used to receive plaster are provided
in ASTM C1063. The design authority’s
responsibilities as per ASTM C1063
were discussed previously in this paper.
Stucco Mix Design
The requirements for mix designs are
discussed in Section 7, Application, of
ASTM C926. Section 7.1 states that all
stucco shall be mixed and proportioned in
accordance with Tables 1, 2, and 3 of ASTM
C926. Table 1 provides mix design, as mix
symbols, based on the type of plaster base.
Tables 2 and 3 provide proportions of the
various stucco constituents for the base
coat and finish coat, respectively. In the
authors’ experience, the two most common
mixes specified include mix designs with
mix symbols C and CL for the base coats
1 6 8 • B owl sb y a n d C o p e 3 2 n d 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 1 6 – 2 1 , 2 0 1 7
and F and FL for the finish coats. These mix
designs include Portland cement, lime, and
sand. The other mix designs include plastic
cement and masonry cement. These two
cements are not often specified unless they
are matching existing stucco.
Although the design authorities continue
to specify a mix containing Portland
cement, sand, and lime in accordance with
ASTM C926, proprietary preblended, prebagged
mixes are increasingly being utilized
for stucco applications. There are
several advantages to using the prebagged
mixes rather than specifying on-site mixing.
These prebagged mixes are generally
easy to use and often only require the addition
of water. Instructions are also typically
printed on the bag, reducing the risk of
mixing errors. Therefore, quality control is
typically improved when weighing of ingredients
is performed in a controlled environment
rather than on a jobsite. Additionally,
the batches are more consistent (assuming
that the same amount of water is used in
each batch) when they solely require the
addition of water.
The disadvantage is that these bags contain
proprietary blends of ingredients. The
safety data sheets don’t often have enough
information to determine the materials that
may be present in the bag, let alone the proportions
of these ingredients. If the design
authority specifies or accepts the use of particular
products on jobs, they should also be
aware that masonry cement, plastic cement,
fly ash, gypsum, limestone, plasticizers,
waterproofing additives, and many other
ingredients may also be present or used as
substitutions in these prebagged mixes, in
addition to Portland cement, sand, and lime.
The effects of each of these ingredients on
the workability, bond, or shrinkage of the
stucco is unknown until each proprietary
blend is fully tested in the laboratory or
from successful performance in the field.
While these blends may adhere to ASTM
C926, Standard Specification for Application
of Portland Cement-Based Plaster, the design
authority cannot be assured of their suitability
until they are more fully characterized.
While the use of these materials may
be advantageous, the unknown ingredients
are cause for some concern.
Mixing and Application of Stucco
Poor mixing and application of stucco
are common sources of distress in stucco.
Poor mixing due to improper proportioning
and mixing of ingredients can result in
cracking. Poor application can also result
in cracking, as well as delaminations and
separations between the stucco and accessories.
ASTM C926 recognizes these potential
problems and provides requirements
on both mixing and application. These
requirements are primarily the responsibility
of the contractor. However, to help
ensure that the contractor is qualified, the
authors recommend the design authority
require the stucco contractor to submit
qualifications. Typically, these qualification
submittals should include evidence that the
stucco applicator’s company has a minimum
of five years of continuous experience
in similar stucco work. The qualifications
could require the contractor to list a minimum
of five representative projects with
similar scope and size, including the project
name, owner’s name, description of work,
materials used, project supervisor, total
cost of the stucco work and total cost of the
project, and completion date. In addition, it
is recommended that the design authority
require the contractor to submit the mix
design, including proportions of ingredients
and sieve analysis for the aggregate to help
ensure that it meets the requirements of
ASTM C897 as required by ASTM C926.
To help ensure that the stucco to be
used on a project is properly mixed and is of
good quality, the design authority can specify
that petrographic and chemical analysis
in accordance with ASTM C1324, Standard
Test Method for Examination and Analysis
of Hardened Mortar, be performed during
the mock-up phase of a project. The results
of the analysis can be used to determine
the final mix proportions. The construction
of the mock-up and the application of the
stucco can also be reviewed to help ensure
correct installation.
Curing
Good curing practices that 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.
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. Because environmental
conditions can play a large role in when and
how to cure the stucco, the responsibility of
curing, per ASTM C926, relies heavily on
the contractor. ASTM C926 states that the
method of curing can be one or a combination
of the following:
1. Moist curing by applying a fine
fog spray of water as frequently as
required, generally twice daily in the
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 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 too soon, the film may damage
surface texture; if 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 are
not defined in ASTM C926; however, the
Portland Cement Association (PCA) recommends
maintaining 80 percent relative
humidity for at least 24 hours and, in some
cases, up to seven days. Therefore, to help
ensure that stucco is properly cured, the
authors recommend the design authority
specify the curing method and the minimum
curing time.
In addition to the minimum moist curing
time, since most finishes for stucco
consist of a decorative finish (acrylic paint,
elastomeric coatings, or proprietary finish
coats), the authors also recommend that
the design authority specify the required
time for the stucco to fully cure and/or the
acceptable pH level of the stucco prior to the
placement of the finish. Most manufacturers
require a total of 28 to 30 days of total
curing or a pH level less than 7 (neutral)
prior to the placement of the finish.
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Crack Acceptance Criteria
While ASTM C926 and C1063 provide
the minimum requirements for the design
and construction of a stucco assembly,
they do not provide information regarding
crack acceptance criteria. While it may be
assumed by the owner that no cracking
is allowed, some amount of cracking is
typically present in most stucco applications.
Cracks form in stucco when forces
or stresses within the stucco exceed the
tensile strength of the stucco. Although
these stresses can be the result of external
forces, such as building displacement,
wind, seismic, impact, etc., our experience
is that cracks are more frequently caused
by deficiencies in the design and installation
of metal lath and stucco. These cracks
can be categorized as extensive cracking,
isolated cracking, or separations.
Stipulating crack acceptance criteria
(such as crack viewing distance, critical
lighting, crack types [patterns], locations,
and crack widths) in the project specifications
can help improve the overall quality
of the stucco-related work, as well as
clarify what cracking (if any) is acceptable.
If cracking is not acceptable, repairs to
the cracks can be performed during the
construction phase. Allowing the stucco to
cure until it reaches the acceptable pH to
receive the finish will allow the contractor to
identify and repair areas of cracking prior
to placing the finish.
CONCLUSION
When properly designed and installed,
stucco has proven to be a durable and
an aesthetically pleasing cladding material
that requires minimal regular maintenance.
ASTM C1063 and C926 are specifications
for the installation of lathing and
furring to receive Portland cement-based
plaster and the application of Portland
cement-based plaster. These specifications
are incorporated into the model building
codes by reference, and provide the minimum
requirements for stucco wall cladding
system design and application.
Unfortunately, in our experience, there
is often confusion as to what should be
provided by the design authority versus
what is left to the contractor. This confusion
frequently leads to inadequate information
provided by the design authority
in the project specifications and drawings,
including a failure to design or specify elements
required by the ASTM standards.
In an effort to clarify some of the more
frequently ignored responsibilities of the
design authority, we have reviewed both
ASTM C926 and C1063 and discussed
these responsibilities, as well as provided
additional recommendations for the design
authority to consider. These recommendations
are more demanding requirements
that can improve the minimum standards
presented in the specifications.
REFERENCES
1. Standard Recommended Practice for
Portland Cement Stucco. American
Concrete Institute (ACI) Standard
No. 25, adopted by letter ballot of the
Institute, April 17, 1920.
2. American National Standards Institute.
Specifications for Portland Cement
and Portland Cement-Lime
Plastering, Exterior (Stucco) and Interior,
ANSI A42.2-1971, and Lathing
and Furring for Portland Cement
and Portland Cement-Lime Exterior
(Stucco) and Interior, A42.3-1971.
American Society for Testing
and Materials, 1916 Race Street,
Philadelphia, PA. 19103.
3. ASTM C1063-16a, Standard Specification
for Installation of Lathing and
Furring to Receive Interior and Exterior
Portland Cement-Based Plaster. ASTM
International, 100 Barr Harbor Drive,
PO Box C700, West Conshohocken,
PA 19428-2959.
4. ASTM C926-16a, Standard Specification
for Application of Portland
Cement-Based Plaster. ASTM
International, 100 Barr Harbor Drive,
PO Box C700, West Conshohocken,
PA 19428-2959.
5. Various webpages from www.
StuccoMetrics.com.
6. Lee Cope, PE, and Michael Horst,
PE. “Common Source of Distress in
Stucco Façades.” Proceedings of the
RCI 29th International Convention
and Trade Show, March 20-25,
2014, Anaheim, CA.
7. Jeff Bowlsby, CCS, CCCA. “Cement
Plaster Metrics: Quantifying Stucco
Shrinkage and Other Movements;
Crack Acceptability Criteria for
Evaluating Stucco,” Proceedings
of the RCI Symposium on Building
Envelope Technology, November
2010.
8. 2013 International Building Code,
International Code Council, Inc.,
500 New Jersey Ave. NW, 6th Floor,
Washington DC 20001.
1 7 0 • B owl sb y a n d C o p e 3 2 n d 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 1 6 – 2 1 , 2 0 1 7