ASTM C926-23a

Standard Specification for Application of Portland Cement-Based Plaster

ABSTRACT
This specification covers the standard requirements for the application of full thickness Portland cement-based plaster for exterior (stucco) and interior work. It also sets forth tables for proportioning of various plaster mixes and plaster thickness. The materials used shall consist of the following: cement which shall be a Portland cement, air-entraining Portland cement, masonry cement, blended hydraulic cement, air-entraining blended hydraulic cement, or plastic cement; Type S hydrated lime; aggregates such as perlite and sand for base and job-mixed finish coats; water to be used in mixing; admixtures; and fibers. Surfaces of solid bases such as masonry, stone, cast-in-place or precast concrete shall be prepared either by sandblasting, wire brushing, acid etching, chipping, or a combination thereof. Details on curing to resist cracking; materials protection and storage; environmental conditions; and application of plaster, finish-coat, and fog-coat, which shall be done by hand or machine up to the specified nominal thickness on metal and solid plaster bases are discussed.
SCOPE
1.1 This specification covers the minimum technical requirements for the application of full thickness portland cement-based plaster for exterior (stucco) and interior work. These requirements do not by default define a unit of work or assign responsibility for contractual purposes, which is the purview of a contract or contracts made between contracting entities.  
1.2 This specification sets forth tables for proportioning of various plaster mixes and plaster thickness.
Note 1: General information is found in Annex A1. Design considerations are found in Annex A2.  
1.3 The text of this specification references notes and footnotes that provide explanatory material. These notes and footnotes (excluding those in tables and figures) shall not be considered as requirements of the specification.  
1.4 Details of construction for a specific assembly to achieve the required fire resistance shall be obtained from reports of fire-resistance tests, engineering evaluations, or listings from recognized fire testing laboratories.  
1.5 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 14-Nov-2023
• ICS: 91.100.10 - Cement. Gypsum. Lime. Mortar
• Technical Committee: C11 - Gypsum and Related Building Materials and Systems
• Drafting Committee: C11.03 - Specifications for the Application of Gypsum and Other Products in Assemblies

Relations

• Replaces: ASTM C926-23 - Standard Specification for Application of Portland Cement-Based Plaster
• Effective Date: 15-Nov-2023
• Refers: ASTM C219-24 - Standard Terminology Relating to Hydraulic and Other Inorganic Cements
• Effective Date: 01-Apr-2024
• Refers: ASTM E90-23 - Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements
• Effective Date: 01-Dec-2023
• Refers: ASTM C219-20a - Standard Terminology Relating to Hydraulic and Other Inorganic Cements
• Effective Date: 01-Aug-2020
• Refers: ASTM C25-19e1 - Standard Test Methods for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime
• Effective Date: 01-May-2019
• Refers: ASTM C25-19 - Standard Test Methods for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime
• Effective Date: 01-May-2019
• Refers: ASTM E90-09(2016) - Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements
• Effective Date: 01-Dec-2016
• Referred By: ASTM E2266-22 - Standard Guide for Design and Construction of Low-Rise Frame Building Wall Systems to Resist Water Intrusion
• Effective Date: 15-Nov-2023
• Referred By: ASTM C1861-23a - Standard Specification for Lathing and Furring Accessories, and Fasteners, for Interior and Exterior Portland Cement-Based Plaster
• Effective Date: 15-Nov-2023
• Referred By: ASTM C1788-20 - Standard Specification for Non Metallic Plaster Bases (Lath) Used With Portland Cement Based Plaster in Vertical Wall Applications
• Effective Date: 15-Nov-2023
• Referred By: ASTM C1328/C1328M-23 - Standard Specification for Plastic (Stucco) Cement
• Effective Date: 15-Nov-2023
• Referred By: ASTM C1780-23a - Standard Practice for Installation Methods for Cement-based Adhered Masonry Veneer
• Effective Date: 15-Nov-2023
• Referred By: ASTM C1324-20a - Standard Test Method for Examination and Analysis of Hardened Masonry Mortar
• Effective Date: 15-Nov-2023
• Referred By: ASTM C1764-20 - Standard Test Methods for Non Metallic Plaster Bases (Lath) Used With Portland Cement Based Plaster in Vertical Wall Applications
• Effective Date: 15-Nov-2023
• Referred By: ASTM C1063-23 - Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster
• Effective Date: 15-Nov-2023

Overview

ASTM C926-23a is the Standard Specification for Application of Portland Cement-Based Plaster, issued by ASTM International. This standard establishes comprehensive requirements for the application of Portland cement-based plaster, commonly known as stucco, for both exterior and interior surfaces. The specification covers criteria for materials, mixing proportions, base surface preparation, application processes, plaster thickness, curing, and environmental conditions.

The standard ensures that stucco and cement plaster applications achieve durability, fire resistance, and effective sound control in a wide range of building environments. It provides guidance for hand and machine applications on various substrates, including masonry, stone, cast-in-place or precast concrete, and metal or non-metallic lath.

Key Topics

• Material Requirements:
  Outlines acceptable materials, including different types of cement (such as Portland, air-entraining, blended and plastic cement), Type S hydrated lime, aggregates (sand, perlite), water, admixtures, and fibers. Each material must conform to referenced ASTM and ANSI standards.

• Surface Preparation:
  Specifies preparation of solid bases (masonry, stone, concrete) by methods such as sandblasting, wire brushing, acid etching, or chipping, ensuring proper adhesion and plaster keying.

• Plaster Mix Proportions:
  Provides tables for proportioning various plaster mixes and required plaster thickness depending on the substrate and application type. Mixes vary for each coat-scratch, brown, and finish.

• Application Process:
  Addresses hand or machine application techniques, sequence of coats (two or three-coat systems), thickness measurements, methods to avoid cold joints, and procedures for finish and fog coat applications.

• Curing and Environmental Conditions:
  Details curing requirements to reduce cracking and environmental constraints during application and initial setting (e.g., minimum temperature, protection from freezing, and moisture maintenance).

• Fire and Sound Control:
  Stipulates that fire resistance and sound control requirements must be verified with applicable laboratory test reports.

Applications

This ASTM plaster standard is widely used in the construction industry for projects requiring durable, fire-resistant surface finishes. Typical applications include:

• Exterior stucco systems for residential and commercial buildings that require protection against weather, abrasion, and fire.
• Interior cement plaster for areas exposed to heavy use, high moisture, or temperature fluctuations.
• Rehabilitation and renovation projects seeking durable and compatible finishes for a variety of substrates, including historically sensitive masonry works.
• Code compliance in new construction where specific fire resistance or sound transmission performance is mandated by building codes or project specifications.

By following ASTM C926-23a, contractors and specifiers can ensure that plaster applications meet minimum technical standards for performance, safety, and longevity.

Related Standards

ASTM C926-23a references and operates in tandem with several important standards, including:

• ASTM C1063: Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster
• ASTM C897: Specification for Aggregate for Job-Mixed Portland Cement-Based Plasters
• ASTM C150/C150M: Specification for Portland Cement
• ASTM C1328: Specification for Plastic (Stucco) Cement
• ASTM E119: Fire Tests of Building Construction and Materials
• ASTM E90 / E492: Sound Transmission Test Methods
• ANSI A108.1: Specification for Installation of Ceramic Tile

These referenced standards ensure proper material selection, installation, and testing practices, providing a well-rounded framework for quality cement plaster application in line with international best practices.

Keywords: Portland cement plaster, stucco application, cement-based plaster standard, plaster thickness, substrate preparation, ASTM C926, building plaster fire resistance, stucco mix proportions, curing cement plaster, exterior stucco, interior cement plaster.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C926 − 23a
Standard Specification for
Application of Portland Cement-Based Plaster
This standard is issued under the fixed designation C926; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* 2. Referenced Documents
2.1 ASTM Standards:
1.1 This specification covers the minimum technical re-
C11 Terminology Relating to Gypsum and Related Building
quirements for the application of full thickness portland
Materials and Systems
cement-based plaster for exterior (stucco) and interior work.
C25 Test Methods for Chemical Analysis of Limestone,
These requirements do not by default define a unit of work or
Quicklime, and Hydrated Lime
assign responsibility for contractual purposes, which is the
C35 Specification for Inorganic Aggregates for Use in Gyp-
purview of a contract or contracts made between contracting
sum Plaster
entities.
C91/C91M Specification for Masonry Cement
1.2 This specification sets forth tables for proportioning of
C150/C150M Specification for Portland Cement
various plaster mixes and plaster thickness.
C206 Specification for Finishing Hydrated Lime
C207 Specification for Hydrated Lime for Masonry Pur-
NOTE 1—General information is found in Annex A1. Design consider-
poses
ations are found in Annex A2.
C219 Terminology Relating to Hydraulic and Other Inor-
1.3 The text of this specification references notes and
ganic Cements
footnotes that provide explanatory material. These notes and
C260/C260M Specification for Air-Entraining Admixtures
footnotes (excluding those in tables and figures) shall not be
for Concrete
considered as requirements of the specification.
C578 Specification for Rigid, Cellular Polystyrene Thermal
Insulation
1.4 Details of construction for a specific assembly to
C595/C595M Specification for Blended Hydraulic Cements
achieve the required fire resistance shall be obtained from
C631 Specification for Bonding Compounds for Interior
reports of fire-resistance tests, engineering evaluations, or
Gypsum Plastering
listings from recognized fire testing laboratories.
C897 Specification for Aggregate for Job-Mixed Portland
1.5 The values stated in inch-pound units are to be regarded
Cement-Based Plasters
as standard. The values given in parentheses are mathematical
C932 Specification for Surface-Applied Bonding Com-
conversions to SI units that are provided for information only
pounds for Exterior Plastering
and are not considered standard.
C1063 Specification for Installation of Lathing and Furring
to Receive Interior and Exterior Portland Cement-Based
1.6 This international standard was developed in accor-
Plaster
dance with internationally recognized principles on standard-
C1116/C1116M Specification for Fiber-Reinforced Concrete
ization established in the Decision on Principles for the
C1328 Specification for Plastic (Stucco) Cement
Development of International Standards, Guides and Recom-
C1787 Specification for Installation of Non Metallic Plaster
mendations issued by the World Trade Organization Technical
Bases (Lath) Used with Portland Cement Based Plaster in
Barriers to Trade (TBT) Committee.
Vertical Wall Applications
E90 Test Method for Laboratory Measurement of Airborne
Sound Transmission Loss of Building Partitions and
This specification is under the jurisdiction of ASTM Committee C11 on
Elements
Gypsum and Related Building Materials and Systems and is the direct responsibility
of Subcommittee C11.03 on Specifications for the Application of Gypsum and Other
Products in Assemblies. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 15, 2023. Published November 2023. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 1981. Last previous edition approved in 2023 as C926 – 23. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/C0926-23A. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C926 − 23a
E119 Test Methods for Fire Tests of Building Construction 3.2.11.2 bedding coat, n—a plaster coat that receives aggre-
and Materials gate or other decorative material impinged into its surface
E492 Test Method for Laboratory Measurement of Impact before it sets.
Sound Transmission Through Floor-Ceiling Assemblies
3.2.11.3 brown coat, n—in three-coat work, the second coat,
Using the Tapping Machine
applied over the scratch coat. In two-coat work, brown coat
2.2 ANSI Standard:
refers to the double-up basecoat. In either use, the brown coat
A108.1 Specification for Installation of Ceramic Tile
is the coat directly beneath the finish coat.
3.2.11.4 dash-bond coat, n—a thick wet mixture of portland
3. Terminology
cement and water, with or without aggregate, dashed onto the
3.1 Definitions:
surface of a plaster base such as smooth monolithic concrete or
3.1.1 Terms shall be defined as in Terminology C11 and
concrete block surfaces to improve the mechanical key for
Terminology C219, except as modified herein.
subsequent plaster coats.
3.1.2 contract documents, n—a series of several individual
3.2.11.5 double-up coat, n—the brown-coat plaster applied
items that generally include drawings and specifications. Either
to the scratch coat plaster before the scratch-coat plaster has
or both of these documents may exist for any particular project.
set.
3.2 Definitions of Terms Specific to This Standard:
3.2.11.6 finish coat, n—the final layer of plaster applied over
3.2.1 accelerator, n—an admixture that will shorten the
basecoat plaster.
setting time of plaster.
3.2.11.7 fog coat, n—a light coat of cement and water, with
3.2.2 acid etching, n—the cleansing and controlled erosion
or without aggregate or color pigment, applied by machine
of a solid surface, using an acid wash.
spray to improve color consistency.
3.2.3 admixture, n—a material other than water, aggregate,
3.2.11.8 scratch coat, n—the first coat of plaster applied to
or basic cementitious material added to the batch before or
a plaster base.
during job mixing.
3.2.11.9 skim coat, n—a thin finish coat applied to an
3.2.4 air entrainment, n—the use of an air-entraining ad-
existing plaster surface or other substrate to improve appear-
mixture or air-entraining cementitious material in a plaster mix
ance.
to yield a controlled quantity of minute (typically between
10 μm and 1000 μm in diameter) disconnected air bubbles in
3.2.11.10 three-coat work, n—application of plaster in three
the plaster (see entrapped air). successive coats with time between coats for setting or drying,
or both.
3.2.5 back wrap, n—a means of terminating a polymer-
modified, fabric reinforced cementitious base coat by wrapping
3.2.12 cold joint (“joining” or “jointing”), n—the juncture
the reinforcing mesh, which has been affixed to the substrate
of fresh plaster application adjacent to set plaster, in the same
onto the outboard surface of the foam plastic core to provide
plane.
continuity of the reinforced base coat and protection for the
3.2.13 critical lighting, n—natural or artificial illumination
foam plastic core.
of a surface that makes planar variations in finished plaster
3.2.6 backplaster, n—plaster applied to the face of metal
work visible.
lath opposite a previously applied plaster.
3.2.14 curing, v—the act or processes of producing a mois-
3.2.7 barrier wall, n—type of wall system that is intended to
ture environment favorable to cement hydration, resulting in
block or interrupt the movement of water to the interior.
the setting or hardening of the plaster.
3.2.8 bond, n—the state of adhesion between plaster coats or
3.2.15 drainage wall, n—a wall system in which the clad-
between plaster and plaster base.
ding provides a substantial barrier to water intrusion, and
which also incorporates a concealed water-resistive barrier
3.2.9 bonding compound or agent, n—compounds surface
applied or integrally mixed with plaster to improve the quality over which drainage will occur.
of bond between plaster and plaster base or between plaster
3.2.16 entrapped air, n—unintentional air voids in the plas-
coats.
ter generally larger than 1 mm.
3.2.10 cementitious material, n—a material that, when
3.2.17 factory prepared (“mill-mixed” or “ready mixed”),
mixed with water and with or without aggregate, provides the
adj—pertaining to material combinations that have been for-
plasticity and the cohesive and adhesive properties necessary
mulated and dry-blended by the manufacturer, requiring only
for placement and the formation of a rigid mass.
the addition of and mixing with water to produce plaster.
3.2.11 coat, n—a thickness of plaster applied in a single
3.2.18 fiber, natural or synthetic, n—an elongated fiber or
operation.
strand admixture added to plaster mix to improve cohesiveness
3.2.11.1 basecoat, n—all plaster applied before the applica-
or pumpability, or both.
tion of the finish coat.
3.2.19 floating, v—act of compacting and leveling brown-
coat plaster to a reasonably true surface plane using a float tool
or the act of bringing the aggregate to the surface of finish-coat
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036. plaster.
C926 − 23a
3.2.20 key (also mechanical key), n—plaster that physically 5. Materials
surrounds, penetrates, or deforms to lock onto the perforations
5.1 Materials shall conform to the requirements of the
or irregularities of the plaster base or previous coat of plaster.
referenced specifications and standards and to the requirements
3.2.21 lath, n—a mechanically attached base material used
specified herein.
to support plaster including metal plaster bases (expanded
5.2 Cement:
metal lath, woven wire plaster base, and welded wire lath) and
5.2.1 Portland Cement—Specification C150/C150M, Type
non-metallic plaster bases.
I, II, and III, as specified. White where specified.
3.2.22 lathing, n—plural for lath.
5.2.2 Air-entraining Portland Cement—Specification C150/
C150M, type as specified. White where specified.
3.2.23 lathing, v—a process of installing lath.
5.2.3 Masonry Cement—Specification C91/C91M, Types N,
3.2.24 metal plaster base, n—expanded metal lath, or
S, and M. White where specified.
welded or woven wire lath.
5.2.4 Blended Hydraulic Cement—Specification C595/
3.2.25 plaster, n—portland cement-based cementitious mix-
C595M, Type IP, IS(<70), IL, and IT(S<70), as specified.
ture (see stucco).
5.2.5 Air-entraining Blended Hydraulic Cement—
Specification C595/C595M, Type IP-(A), IS(<70)-(A), IL-(A),
3.2.26 polymer modified cementitious base coat, n—a base
and IT (S<70)-(A), as specified.
coat containing portland cement modified with chemical ad-
5.2.6 Plastic Cement—Plastic Cement shall meet the re-
mixtures (typically polymer latexes) to improve characteristics
quirements of Specification C1328, Standard Specification for
of the finished product, such as workability, plasticity, water
Plastic (Stucco) Cement.
resistance, and adhesion.
NOTE 2—Plastic cements are not available nationally.
3.2.27 required, adj—pertaining to a mandatory obligation
imposed by a force outside of this specification, such as a
5.3 Type “S” Hydrated Lime—A hydrated lime that contains
building code, project specification, contract, or purchase
not more than 8 % unhydrated oxides when tested in accor-
order.
dance with Test Methods C25. See Specifications C206 and
C207 for a complete description of a Type “S” hydrated lime.
3.2.28 rustication (also “break”), n—an interruption or
change in plane of a plastered surface.
5.4 Aggregates:
5.4.1 Sand for Base Coats—Specification C897. Aggregate
3.2.29 scoring (also known as “scratching”), n—the groov-
failing to meet gradation limits in Specification C897 shall be
ing of the surface of an unset plaster coat to provide a key for
permitted to be used, provided the plaster made with this sand
a subsequent coat.
has an acceptable demonstrated performance record in similar
3.2.30 set, n—the chemical and physical change in plaster as
construction and climate conditions.
it goes from a plastic, workable state to a rigid state.
5.4.2 Perlite—Specification C35.
3.2.31 solid plaster bases, n—substrates that do not require
5.4.3 Sand for Job-mixed Finish Coats—Specification
lath, including cast in place and precast concrete, concrete and
C897.
stone masonry, clay brick, and tile.
5.5 Water—Water used in mixing, application, and finishing
3.2.32 stucco, n—portland cement-based plaster used on
of plaster shall be clean, fresh, suitable for domestic water
exterior locations.
consumption, and free of such amounts of mineral or organic
3.2.33 stucco finish, n—a factory-prepared, dry blend of substances as would affect the set, the plaster, or any metal in
materials for finish coat applications. the system.
3.2.34 temper, v—to mix or restore unset plaster with water
5.6 Admixtures—See 3.2.3 and A2.5.
to a workable consistency.
5.7 Fibers—Specification C1116/C1116M on alkali-
3.2.35 texture, n—any surface appearance as contrasted to a
resistant fibers, glass fibers, nylon, polypropylene, or carbon
smooth surface.
fibers.
5.8 Product Marking—Packaged materials shall be clearly
4. Delivery and Storage of Materials
marked or labeled to indicate product, brand name, the
4.1 Delivery: manufacturer, and the weight of the material contained therein.
Similar information shall be provided in the shipping advices
4.1.1 Packaged materials shall be delivered in factory-
accompanying the shipment of bulk materials.
sealed, unopened, and unbroken packages, containers, or
bundles.
6. Requirements for Bases to Receive Portland Cement-
4.1.2 Bulk materials shall be delivered in clean transport
Based Plaster
vessels, free of contaminates.
6.1 Lathing accessories, furring accessories, and accessory
4.2 Storage:
fasteners used to receive plaster shall be installed in confor-
4.2.1 Weather-sensitive materials shall be kept in a dry
mance with Specification C1063, except as otherwise specified.
condition until ready for use. (See A2.4.)
4.2.2 Bulk materials shall be stored to prevent subsequent
NOTE 3—Plaster bases and lathing accessories shall be free of delete-
contamination and segregation. rious amounts of rust, oil, or other foreign matter, which could cause bond
C926 − 23a
failure or unsightly discoloration.
6.2.2.1 Sandblasting, wire brushing, acid etching, or chip-
ping or a combination thereof,
6.1.1 Metallic lath and lath fasteners used to receive plaster
6.2.2.2 Application of a dash-bond coat applied forcefully
shall be installed in conformance with Specification C1063,
against the surface, left untroweled, undisturbed, and moist
except as otherwise specified.
cured for at least 24 h, or
6.1.2 Non-metallic lath and lath fasteners shall be installed
6.2.2.3 Application of a bonding compound suitable for
in conformance with Specification C1787, except as otherwise
exterior or interior exposure solid surfaces in accordance with
specified.
the manufacturer’s written directions.
6.2 Surfaces of solid bases to receive plaster, such as
6.2.3 Where bond cannot be obtained by one or more of the
masonry, stone, cast-in-place or precast concrete shall be
methods in 6.2.2, a furred or self-furring plaster base shall be
straight and true within ⁄4 in. (6 mm) in 10 ft (3 m) and shall
installed in accordance with Specification C1063 or C1787 as
be free of form oil or other elements, which would interfere
appropriate. Where plaster base is used in areas where bond
with bonding. Conditions where the surfaces are out of
cannot be obtained by one or more of the methods in 6.2.2,
tolerance shall be corrected prior to the application of the
accessories shall be installed in accordance with Specification
plaster. Ferrous-containing form ties or other obstructions shall
C1063 or C1787 as appropriate.
be removed or receded a minimum ⁄8 in. (3 mm) below the
surface of the solid base and treated with a corrosion-resistant
7. Application
coating. Non-ferrous protuberances shall be permitted to be
trimmed back even with the surface of the solid base. 7.1 Plaster Proportions:
7.1.1 All portland cement plasters shall be mixed and
proportioned in accordance with the following tables and
TABLE 1 Plaster Bases—Permissible Mixes
accompanying requirements, using measuring devices of
NOTE 1—See Table 2 for plaster mix symbols.
known volume with successive batches proportioned alike.
Mixes for Plaster Coats
7.1.2 Plaster mix used shall be as designated and referenced
Property of Base
First (Scratch) Second (Brown)
to Table 1.
Low absorption, such C C, CL, M, or CM
7.1.3 Base-coat proportions shall be as shown in Table 2 for
as dense, smooth clay CM or MS CM, MS, or M
the mix specified from Table 1.
tile, brick, or concrete P P
CL CL
7.1.3.1 Measurement of Materials—The method of measur-
High Absorption, such
M M
ing materials for the plaster shall be such that the specified
as concrete masonry,
CM or MS CM, MS, or M
absorptive brick, or tile
proportions are controlled and accurately maintained. The
P P
C C, CL, M, CM, or MS weights per cubic foot of the materials are considered to be as
CL CL
follows:
CM or MS CM, MS, or M
Lath
3 3
M M Material Weight, lb/ft (kg/m )
Portland cement 94 (1505)
CP CP or P
P P Blended cement Weight printed on bag
Masonry or plastic cement Weight printed on bag
Hydrated Lime 40 (640)
Lime Putty 80 (1280)
Sand, Damp and Loose (7.1.3.2) 80 (1280) of dry sand
6.2.1 Solid surfaces shall have the suction (ability to absorb
7.1.3.2 For purposes of this specification, a weight of 80 lb
water) or surface roughness, or both, to provide the bond
required for the plaster. (1280 kg) of oven-dried sand shall be used. This is, in most
cases, equivalent to one cubic foot of loose, damp sand.
6.2.2 Smooth or nonabsorbent solid surfaces, such as cast-
in-place or precast concrete, shall be prepared to receive 7.1.4 Finish-coat proportions for job-mixed finish coats
portland cement plaster by one of the following methods: shall be as specified in Table 3.
A B
TABLE 2 Base-coat Proportions, Parts by Volume
Cementitious Materials Volume of Aggregate per Sum of
Separate
Plaster Mix
Portland Cement
Masonry Cement
Volumes of Cementitious Materials
Plastic
Symbols
or Blended Lime
Cement
C
N M or S 1st Coat 2nd Coat
Cement
3 1
C 1 . . . . . . . . . 0– ⁄4 2 ⁄2 –4 3–5
3 1 1
CL 1 . . . . . . . . . ⁄4 –1 ⁄2 2 ⁄2 –4 3–5
M . . . . . . 1 . . . . . . 2 ⁄2 –4 3–5
CM 1 . . . 1 . . . . . . 2 ⁄2 –4 3–5
MS . . . . . . . . . 1 . . . 2 ⁄2 –4 3–5
P . . . 1 . . . . . . . . . 2 ⁄2 –4 3–5
CP 1 1 . . . . . . . . . 2 ⁄2 –4 3–5
A
The mix proportions for plaster scratch and brown coats to receive ceramic tile shall be in accordance with the applicable requirements of ANSI A108.1 series applicable
to specified method of setting time.
B
Variations in lime, sand, and perlite contents are allowed due to variation in local sands and insulation and weight requirements. A higher lime content generally supports
a higher aggregate content without loss of workability. The workability of the plaster mix governs the amounts of lime, sand, or perlite.
C
The same or greater sand proportion shall be used in the second coat than is used in the first coat.
C926 − 23a
TABLE 3 Job-mixed Finish Coat Proportions, Parts by Volume
Volume of Aggregate
Cementitious Materials
per Sum of Separate
A A
Plaster Mix Symbols Masonry Cement Volumes of
Portland Cement or
Plastic Cement Lime
Cementitious
Blended Cement
B
N M or S
Materials
3 1 1
F 1 . . . . . . . . . ⁄4 –1 ⁄2 1 ⁄2 –3
1 1
FL 1 . . . . . . . . . 1 ⁄2 –2 1 ⁄2 –3
FM . . . . . . 1 . . . . . . 1 ⁄2 –3
FCM 1 . . . 1 . . . . . . 1 ⁄2 –3
FMS . . . . . . . . . 1 . . . 1 ⁄2 –3
FP . . . 1 . . . . . . . . . 1 ⁄2 –3
A
Additional portland cement is not required when Type S or M masonry cement is used.
B
In areas not subject to impact, perlite aggregate shall be permitted to be used over base-coat plaster containing perlite aggregate.
7.1.5 Factory-Prepared Finish Coats—See 3.2.17. 7.3.1 Portland cement plaster shall be applied by hand
7.1.6 Dash-bond coat proportions shall be 1 volume part trowel or machine to the nominal thickness specified in Table
portland cement and not more than two volume parts of
4. The nominal values expressed in Table 4 represent neither a
aggregate mixed to a consistency that will permit application as maximum nor minimum value. They consider the inherent
specified in 7.3.5.
variation of thickness due to the nature of the application
7.1.7 Admixtures shall be proportioned, mixed, and applied process, and the allowable variation of the substrate and the
in accordance with the printed directions of the manufacturer. finished plane of the plaster.
(See A2.5.)
7.3.2 Plaster nominal thickness shall be measured from the
7.1.8 Where specified, natural or synthetic fibers shall be
back plane of the metal plaster base, exclusive of ribs or
free of contaminants and used only in the base coat(s). The
dimples, or from the face of the solid backing with or without
quantities per batch shall be in accordance with the published
metal plaster base, to the outer surface exclusive of texture
directions of the fiber manufacturer.
variations.
7.2 Mixing: 7.3.3 Portland cement-based plaster shall be applied on
7.2.1 All plaster shall be prepared in a mechanical mixer, furred lath when the surface of solid backing material consists
using sufficient water to produce a workable consistency and of gypsum board, gypsum plaster, wood, or rigid foam board-
uniform color. (See X1.1.) type products.
7.2.2 Base-coat plasters that have stiffened because of
NOTE 5—On horizontal ceiling supports or roof soffits protected by a
evaporation of water shall be permitted to be tempered one
drip edge or designated drainage screed, gypsum board products shall be
time only to restore the required consistency. Plaster not used
permitted to be used as backing for metal base to receive portland cement
within 1 ⁄2 h from start of initial mixing shall be discarded.
plaster.
NOTE 4—Severe hot, dry climate conditions accelerate the stiffening of
7.3.4 Separation shall be provided where plaster abuts
plaster and require reduction of this limit. The use of cold waters will slow
dissimilar construction materials or openings. (See A2.1.3.)
the stiffening process.
7.3.5 Each plaster coat shall be applied to an entire wall or
7.2.3 Finish-coat plaster shall not be tempered.
ceiling panel without interruption to avoid cold joints and
7.3 General Application: abrupt changes in the uniform appearance of succeeding coats.
A
TABLE 4 Nominal Plaster Thickness for Three- and Two-coat Work, in. (mm)
Vertical Horizontal
B B
BASE 1st Coat 2nd Coat 3rd Coat Total 1st Coat 2nd Coat 3rd Coat Total
Interior/Exterior
C,D
Three-coat work:
D D
3 3 1 7 1 1 1 5
Lath ⁄8 (10) ⁄8 (10) ⁄8 (3) ⁄8 (22) ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16)
Solid plaster base:
D
1 1 1 5
Unit masonry ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16) Use two-coat work
D
1 1 1 5 3
Cast-in-place or precast ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16) ⁄8 (10), max
concrete
D D
1 1 1 7 1 1 1 7
Lath over solid base ⁄2 (13) ⁄4 (6) ⁄8 (3) ⁄8 (22) ⁄2 (13) ⁄4 (6) ⁄8 (3) ⁄8 (22)
Two-coat work:
Solid plaster base:
3 1 1 3
Unit masonry ⁄8 (10) ⁄8 (3) ⁄2 (13) ⁄8 (10)
1 1 3 3
Cast-in-place or ⁄4 (6) ⁄8 (3) ⁄8 (10) ⁄8 (10)
pre-cast concrete
A
Exclusive of texture variations.
B
For solid plaster partitions, additional coats shall be applied to meet the finished thickness specified.
C
For exposed aggregate finishes, the second (brown) coat shall become the “bedding” coat and shall be of sufficient thickness to receive and hold the aggregate.
D
The basecoat thickness of three-coat work is the sum of the first and second coats combined.
C926 − 23a
Wet plaster shall abut set plaster at naturally occurring inter- 7.5.1.1 Where masonry or concrete surfaces vary in plane,
ruptions in the plane of the plaster, such as corner angles, plaster thickness required to produce level surfaces shall not be
rustications, openings, expansion joints, and control joints required to be uniform.
where this is possible. Joinings, where necessary, shall be cut 7.5.2 Three-coat Application on Solid Bases:
square and straight and not less than 6 in. (152 mm) away from
7.5.2.1 The first (scratch) coat shall be applied with suffi-
a joining in the preceding coat. cient material and pressure to ensure tight contact and complete
coverage of the solid base, to the nominal thickness shown in
7.3.6 Lath shall be covered with three-coat work. The
Table 4. As soon as the first (scratch) coat becomes firm, the
combined total nominal thickness shall be as shown in Table 4.
entire surface shall be scored in one direction only. The vertical
A dash-bond coat shall not replace one of the specified number
surfaces shall be scored horizontally.
of coats.
7.5.2.2 The second (brown) coat shall be applied using the
7.3.7 Two-coat work shall be used only over solid bases
same procedures specified in 7.4.2 and 7.4.2.1, bringing the
meeting the requirements of 6.2. The combined total nominal
surface to a true, even plane with a rod or straightedge, filling
thickness shall be as shown in Table 4. A dash-bond coat shall
any defects in plane with plaster and darbying. The surface
not replace one of the specified number of coats.
shall be floated uniformly to provide a surface receptive to the
7.3.8 Backplaster where required, shall be applied only after
application of the third (finish) coat.
the coat on the opposite side has set sufficiently to resist
7.5.2.3 The third (finish) coat shall be applied as specified in
breaking or cracking the plaster keys.
7.4.3.
7.3.9 Each coat shall be permitted to set before the next coat
7.5.3 Two-coat Application on Solid Plaster Bases:
is applied. (See X1.5.2.)
7.5.3.1 The first coat shall be applied to the nominal
7.3.10 Plaster coats that have become dry shall be evenly
thickness shown in Table 4 using sufficient material and
dampened with water prior to applying subsequent coats to
pressure to ensure tight contact and complete coverage of the
obtain uniform suction. There shall be no visible water on the
solid base. The applied material shall be brought to a true, even
surface when plaster is applied.
plane with a rod or straightedge. Dry rodding the finish surface
of the applied material shall be permitted.
7.4 Plaster Application on Metal and Non-metallic Plaster
Bases: 7.5.3.2 The second (finish) coat shall be applied as specified
in 7.4.3.
7.4.1 The first (scratch) coat shall be applied with sufficient
material and pressure to form full keys through, and to embed
7.6 Finish-coat Application:
the metal base, and with sufficient thickness of material over
7.6.1 Job-mixed or factory-prepared finish coats shall be
the metal to allow for scoring the surface.
applied, by machine or by hand, as specified in 7.4.3.
7.4.1.1 As soon as the first (scratch) coat becomes firm, the
7.6.2 The use of excessive water during the application and
entire surface shall be scored in one direction only. The vertical
finishing of finish-coat plaster shall be avoided.
surfaces shall be scored horizontally.
7.7 Fog-coat Application—Job-mixed or factory-prepared
7.4.1.2 The first (scratch) coat shall become sufficiently
fog coats shall be applied in accordance with the directions of
rigid to support the application of the second (brown) coat
the manufacturer.
without damage to the monolithic continuity of the first
7.8 Curing and Time Between Coats:
(scratch) coat or its key.
7.8.1 Provide sufficient moisture in the plaster mix or by
7.4.2 The second (brown) coat shall be applied with suffi-
moist or fog curing to permit continuous hydration of the
cient material and pressure to ensure tight contact with the first
cementitious materials. The most effective procedure for curing
(scratch) coat and to bring the combined thickness of the base
and time between coats will depend on climatic and job
coat to the nominal thickness shown in Table 4.
conditions. (See X1.5.2.)
7.4.2.1 The surface of the second (brown) coat shall be
7.8.2 Sufficient time between coats shall be allowed to
brought to a true, even plane with a rod or straightedge, filling
permit each coat to cure or develop enough rigidity to resist
surface defects in plane with plaster. Dry rodding the surface of
cracking or other physical damage when the next coat is
the brown coat shall be permitted.
applied. (See X1.5.2.)
7.4.2.2 The surface shall be floated uniformly to promote
7.9 Environmental Conditions:
densification of the coat and to provide a surface receptive to
7.9.1 Portland cement-based plaster shall not be applied to
bonding of the finish coat.
frozen base or to a base containing frost. Plaster mixes shall not
7.4.3 The third (finish) coat shall be applied with sufficient
contain frozen ingredients. Plaster coats shall be protected
material and pressure to ensure tight contact with, and com-
from freezing for a period of not less than 24 h after set has
plete coverage of the base coat and to the nominal thickness
occurred.
shown in Table 4 and 7.5.1.1.
7.9.2 Portland cement plaster shall be protected from un-
7.5 Plaster Application on Solid Plaster Bases:
even and excessive evaporation during dry weather and from
strong blasts of dry air.
7.5.1 High-suction bases shall be evenly dampened with
clean water prior to the application of plaster. Do not dampen 7.9.3 Plaster Application—When artificial heat is required,
low-suction solid bases, such as dense concrete or smooth heaters shall be located to prevent a concentration of heat on
brick. uncured plaster. Heaters shall be vented to the outside to
C926 − 23a
prevent toxic fumes and other products of combustion from 7.9.3.2 For exteriors, plaster shall be applied when the
adhering to or penetrating plaster bases and plaster. Adequate ambient temperature is higher than 40 °F (4.4 °C), unless the
ventilation shall be maintained in all areas, particularly in work area is enclosed and heat is provided as described in
interior areas with little or no natural air movement. 7.9.3.
7.9.3.1 Interior environment shall be maintained at a tem-
8. Keywords
perature above 40 °F not less than 48 h prior to and during
application of portland cement-based plaster. Interior tempera- 8.1 bond; brown coat; cementitious; exterior plaster; fog
ture shall be maintained above 40 °F until normal occupancy. coat; portland cement; scratch coat; stucco
ANNEXES
(Mandatory Information)
A1. GENERAL INFORMATION
A1.1 Where a specific degree of fire resistance is required ined to determine if the applicable requirements of Specifica-
for plastered assemblies and constructions, details of construc- tion C1063 have been met unless otherwise required by the
tion shall be in accordance with official reports of fire tests contract specifications.
conducted by recognized testing laboratories, in accordance
A1.5.2 Accessories shall be installed prior to the application
with Test Methods E119.
of plaster; therefore, their type, depth, location, and orientation
shall be included in the contract documents. Control joint and
A1.2 Where a specific degree of sound control is required
expansion joint locations and their integration with the sub-
for plastered assemblies and constructions, details of construc-
strate and water resistive barrier shall be graphically depicted
tion shall be in accordance with official reports of tests
in the contract documents. Where masonry or concrete surfaces
conducted by recognized testing laboratories, in accordance
vary in plane, plaster thickness required to produce
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C926 − 23 C926 − 23a
Standard Specification for
Application of Portland Cement-Based Plaster
This standard is issued under the fixed designation C926; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This specification covers the minimum technical requirements for the application of full thickness portland cement-based
plaster for exterior (stucco) and interior work. These requirements do not by default define a unit of work or assign responsibility
for contractual purposes, which is the purview of a contract or contracts made between contracting entities.
1.2 This specification sets forth tables for proportioning of various plaster mixes and plaster thickness.
NOTE 1—General information will be is found in Annex A1. Design considerations will be are found in Annex A2.
1.3 The text of this specification references notes and footnotes that provide explanatory material. These notes and footnotes
(excluding those in tables and figures) shall not be considered as requirements of the specification.
1.4 Details of construction for a specific assembly to achieve the required fire resistance shall be obtained from reports of
fire-resistance tests, engineering evaluations, or listings from recognized fire testing laboratories.
1.5 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C11 Terminology Relating to Gypsum and Related Building Materials and Systems
C25 Test Methods for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime
C35 Specification for Inorganic Aggregates for Use in Gypsum Plaster
C91/C91M Specification for Masonry Cement
C150/C150M Specification for Portland Cement
C206 Specification for Finishing Hydrated Lime
C207 Specification for Hydrated Lime for Masonry Purposes
This specification is under the jurisdiction of ASTM Committee C11 on Gypsum and Related Building Materials and Systems and is the direct responsibility of
Subcommittee C11.03 on Specifications for the Application of Gypsum and Other Products in Assemblies.
Current edition approved March 1, 2023Nov. 15, 2023. Published March 2023November 2023. Originally approved in 1981. Last previous edition approved in 20222023
as C926 – 22c.C926 – 23. DOI: 10.1520/C0926-23.10.1520/C0926-23A.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C926 − 23a
C219 Terminology Relating to Hydraulic and Other Inorganic Cements
C260/C260M Specification for Air-Entraining Admixtures for Concrete
C578 Specification for Rigid, Cellular Polystyrene Thermal Insulation
C595/C595M Specification for Blended Hydraulic Cements
C631 Specification for Bonding Compounds for Interior Gypsum Plastering
C897 Specification for Aggregate for Job-Mixed Portland Cement-Based Plasters
C932 Specification for Surface-Applied Bonding Compounds for Exterior Plastering
C1063 Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster
C1116/C1116M Specification for Fiber-Reinforced Concrete
C1328 Specification for Plastic (Stucco) Cement
C1787 Specification for Installation of Non Metallic Plaster Bases (Lath) Used with Portland Cement Based Plaster in Vertical
Wall Applications
E90 Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements
E119 Test Methods for Fire Tests of Building Construction and Materials
E492 Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the
Tapping Machine
2.2 ANSI Standard:
A108.1 Specification for Installation of Ceramic Tile
3. Terminology
3.1 Definitions:
3.1.1 Terms shall be defined as in Terminology C11 and Terminology C219, except as modified herein.
3.1.2 contract documents, n—a series of several individual items that generally include drawings and specifications. Either or both
of these documents may exist for any particular project.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 accelerator, n—an admixture that will shorten the setting time of plaster.
3.2.2 acid etching, n—the cleansing and controlled erosion of a solid surface, using an acid wash.
3.2.3 admixture, n—a material other than water, aggregate, or basic cementitious material added to the batch before or during job
mixing.
3.2.4 air entrainment, n—the use of an air-entraining admixture or air-entraining cementitious material in a plaster mix to yield
a controlled quantity of minute (typically between 10 μm and 1000 μm in diameter) disconnected air bubbles in the plaster (see
entrapped air).
3.2.5 back wrap, n—a means of terminating a polymer-modified, fabric reinforced cementitious base coat by wrapping the
reinforcing mesh, which has been affixed to the substrate onto the outboard surface of the foam plastic core to provide continuity
of the reinforced base coat and protection for the foam plastic core.
3.2.6 backplaster, n—plaster applied to the face of metal lath opposite a previously applied plaster.
3.2.7 barrier wall, n—type of wall system that is intended to block or interrupt the movement of water to the interior.
3.2.8 bond, n—the state of adhesion between plaster coats or between plaster and plaster base.
3.2.9 bonding compound or agent, n—compounds surface applied or integrally mixed with plaster to improve the quality of bond
between plaster and plaster base or between plaster coats.
3.2.10 cementitious material, n—a material that, when mixed with water and with or without aggregate, provides the plasticity and
the cohesive and adhesive properties necessary for placement and the formation of a rigid mass.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036.
C926 − 23a
3.2.11 coat, n—a thickness of plaster applied in a single operation.
3.2.11.1 basecoat, n—all plaster applied before the application of the finish coat.
3.2.11.2 bedding coat, n—a plaster coat that receives aggregate or other decorative material impinged into its surface before it
sets.
3.2.11.3 brown coat, n—in three-coat work, the second coat, applied over the scratch coat. In two-coat work, brown coat refers
to the double-up basecoat. In either use, the brown coat is the coat directly beneath the finish coat.
3.2.11.4 dash-bond coat, n—a thick wet mixture of portland cement and water, with or without aggregate, dashed onto the
surface of a plaster base such as smooth monolithic concrete or concrete block surfaces to improve the mechanical key for
subsequent plaster coats.
3.2.11.5 double-up coat, n—the brown-coat plaster applied to the scratch coat plaster before the scratch-coat plaster has set.
3.2.11.6 finish coat, n—the final layer of plaster applied over basecoat plaster.
3.2.11.7 fog coat, n—a light coat of cement and water, with or without aggregate or color pigment, applied by machine spray
to improve color consistency.
3.2.11.8 scratch coat, n—the first coat of plaster applied to a plaster base.
3.2.11.9 skim coat, n—a thin finish coat applied to an existing plaster surface or other substrate to improve appearance.
3.2.11.10 three-coat work, n—application of plaster in three successive coats with time between coats for setting or drying, or
both.
3.2.12 cold joint (“joining” or “jointing”), n—the juncture of fresh plaster application adjacent to set plaster, in the same plane.
3.2.13 critical lighting, n—natural or artificial illumination of a surface that makes planar variations in finished plaster work
visible.
3.2.14 curing, v—the act or processes of producing a moisture environment favorable to cement hydration, resulting in the setting
or hardening of the plaster.
3.2.15 drainage wall, n—a wall system in which the cladding provides a substantial barrier to water intrusion, and which also
incorporates a concealed water-resistive barrier over which drainage will occur.
3.2.16 entrapped air, n—unintentional air voids in the plaster generally larger than 1 mm.
3.2.17 factory prepared (“mill-mixed” or “ready mixed”), adj—pertaining to material combinations that have been formulated and
dry-blended by the manufacturer, requiring only the addition of and mixing with water to produce plaster.
3.2.18 fiber, natural or synthetic, n—an elongated fiber or strand admixture added to plaster mix to improve cohesiveness or
pumpability, or both.
3.2.19 floating, v—act of compacting and leveling brown-coat plaster to a reasonably true surface plane using a float tool or the
act of bringing the aggregate to the surface of finish-coat plaster.
3.2.20 key (also mechanical key), n—plaster that physically surrounds, penetrates, or deforms to lock onto the perforations or
irregularities of the plaster base or previous coat of plaster.
3.2.21 lath, n—a mechanically attached base material used to support plaster including metal plaster bases (expanded metal lath,
woven wire plaster base, and welded wire lath) and non-metallic plaster bases.
3.2.22 lathing, n—plural for lath.
3.2.23 lathing, v—a process of installing lath.
3.2.24 metal plaster base, n—expanded metal lath, or welded or woven wire lath.
C926 − 23a
3.2.25 plaster, n—portland cement-based cementitious mixture (see stucco).
3.2.26 polymer modified cementitious base coat, n—Aa base coat containing portland cement modified with chemical admixtures
(typically polymer latexes) to improve characteristics of the finished product, such as workability, plasticity, water resistance, and
adhesion.
3.2.27 required, adj—pertaining to a mandatory obligation imposed by a force outside of this specification, such as a building
code, project specification, contract, or purchase order.
3.2.28 rustication (also “break”), n—an interruption or change in plane of a plastered surface.
3.2.29 scoring (also known as “scratching”), n—the grooving of the surface of an unset plaster coat to provide a key for a
subsequent coat.
3.2.30 set, n—the chemical and physical change in plaster as it goes from a plastic, workable state to a rigid state.
3.2.31 solid plaster bases, n—substrates that do not require lath, including cast in place and precast concrete, concrete and stone
masonry, clay brick, and tile.
3.2.32 stucco, n—portland cement-based plaster used on exterior locations.
3.2.33 stucco finish, n—a factory-prepared, dry blend of materials for finish coat applications.
3.2.34 temper, v—to mix or restore unset plaster with water to a workable consistency.
3.2.35 texture, n—any surface appearance as contrasted to a smooth surface.
4. Delivery and Storage of Materials
4.1 Delivery:
4.1.1 Packaged materials shall be delivered in factory-sealed, unopened, and unbroken packages, containers, or bundles.
4.1.2 Bulk materials shall be delivered in clean transport vessels, free of contaminates.
4.2 Storage:
4.2.1 Weather-sensitive materials shall be kept in a dry condition until ready for use. (See A2.4.)
4.2.2 Bulk materials shall be stored to prevent subsequent contamination and segregation.
5. Materials
5.1 Materials shall conform to the requirements of the referenced specifications and standards and to the requirements specified
herein.
5.2 Cement:
5.2.1 Portland Cement—Specification C150/C150M, Type I, II, and III, as specified. White where specified.
5.2.2 Air-entraining Portland Cement—Specification C150/C150M, type as specified. White where specified.
5.2.3 Masonry Cement—Specification C91/C91M, Types N, S, and M. White where specified.
C926 − 23a
5.2.4 Blended Hydraulic Cement—Specification C595/C595M, Type IP, IS(<70), IL, and IT(S<70), as specified.
5.2.5 Air-entraining Blended Hydraulic Cement—Specification C595/C595M, Type IP-(A), IS(<70)-(A), IL-(A), and IT
(S<70)-(A), as specified.
5.2.6 Plastic Cement—Plastic Cement shall meet the requirements of Specification C1328, Standard Specification for Plastic
(Stucco) Cement.
NOTE 2—Plastic cements are not available nationally.
5.3 Type “S” Hydrated Lime—A hydrated lime that contains not more than 8 % unhydrated oxides when tested in accordance with
Test Methods C25. See Specifications C206 and C207 for a complete description of a Type “S” hydrated lime.
5.4 Aggregates:
5.4.1 Sand for Base Coats—Specification C897. Aggregate failing to meet gradation limits in Specification C897 shall be
permitted to be used, provided the plaster made with this sand has an acceptable demonstrated performance record in similar
construction and climate conditions.
5.4.2 Perlite—Specification C35.
5.4.3 Sand for Job-mixed Finish Coats—Specification C897.
5.5 Water—Water used in mixing, application, and finishing of plaster shall be clean, fresh, suitable for domestic water
consumption, and free of such amounts of mineral or organic substances as would affect the set, the plaster, or any metal in the
system.
5.6 Admixtures—See 3.2.3 and A2.5.
5.7 Fibers—Specification C1116/C1116M on alkali-resistant fibers, glass fibers, nylon, polypropylene, or carbon fibers.
5.8 Product Marking—Packaged materials shall be clearly marked or labeled to indicate product, brand name, the manufacturer,
and the weight of the material contained therein. Similar information shall be provided in the shipping advices accompanying the
shipment of bulk materials.
6. Requirements for Bases to Receive Portland Cement-Based Plaster
6.1 Lathing accessories, furring accessories, and accessory fasteners used to receive plaster shall be installed in conformance with
Specification C1063, except as otherwise specified.
NOTE 3—Plaster bases and lathing accessories shall be free of deleterious amounts of rust, oil, or other foreign matter, which could cause bond failure
or unsightly discoloration.
6.1.1 Metallic lath and lath fasteners used to receive plaster shall be installed in conformance with Specification C1063, except
as otherwise specified.
6.1.2 Non-metallic lath and lath fasteners shall be installed in conformance with Specification C1787, except as otherwise
specified.
6.2 Surfaces of solid bases to receive plaster, such as masonry, stone, cast-in-place or precast concrete shall be straight and true
within ⁄4 in. (6 mm) in 10 ft (3 m) and shall be free of form oil or other elements, which would interfere with bonding. Conditions
where the surfaces are out of tolerance shall be corrected prior to the application of the plaster. Ferrous-containing form ties or
C926 − 23a
other obstructions shall be removed or receded a minimum ⁄8 in. (3 mm) below the surface of the solid base and treated with a
corrosion-resistant coating. Non-ferrous protuberances shall be permitted to be trimmed back even with the surface of the solid
base.
TABLE 1 Plaster Bases—Permissible Mixes
NOTE 1—See Table 2 for plaster mix symbols.
Mixes for Plaster Coats
Property of Base
First (Scratch) Second (Brown)
Low absorption, such C C, CL, M, or CM
as dense, smooth clay CM or MS CM, MS, or M
tile, brick, or concrete P P
CL CL
High Absorption, such
M M
as concrete masonry,
CM or MS CM, MS, or M
absorptive brick, or tile
P P
C C, CL, M, CM, or MS
CL CL
CM or MS CM, MS, or M
Lath
M M
CP CP or P
P P
6.2.1 Solid surfaces shall have the suction (ability to absorb water) or surface roughness, or both, to provide the bond required
for the plaster.
6.2.2 Smooth or nonabsorbent solid surfaces, such as cast-in-place or precast concrete, shall be prepared to receive portland
cement plaster by one of the following methods:
6.2.2.1 Sandblasting, wire brushing, acid etching, or chipping or a combination thereof,
6.2.2.2 Application of a dash-bond coat applied forcefully against the surface, left untroweled, undisturbed, and moist cured for
at least 24 h, or
6.2.2.3 Application of a bonding compound suitable for exterior or interior exposure solid surfaces in accordance with the
manufacturer’s written directions.
6.2.3 Where bond cannot be obtained by one or more of the methods in 6.2.2, a furred or self-furring plaster base shall be installed
in accordance with Specification C1063 or C1787 as appropriate. Where plaster base is used in areas where bond cannot be
obtained by one or more of the methods in 6.2.2, accessories shall be installed in accordance with Specification C1063 or C1787
as appropriate.
7. Application
7.1 Plaster Proportions:
7.1.1 All portland cement plasters shall be mixed and proportioned in accordance with the following tables and accompanying
requirements, using measuring devices of known volume with successive batches proportioned alike.
7.1.2 Plaster mix used shall be as designated and referenced to Table 1.
7.1.3 Base-coat proportions shall be as shown in Table 2 for the mix specified from Table 1.
7.1.3.1 Measurement of Materials—The method of measuring materials for the plaster shall be such that the specified proportions
are controlled and accurately maintained. The weights per cubic foot of the materials are considered to be as follows:
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A B
TABLE 2 Base-coat Proportions, Parts by Volume
Volume of Aggregate per Sum of
Cementitious Materials
Separate
Plaster Mix
Portland Cement
Masonry Cement
Volumes of Cementitious Materials
Plastic
Symbols
or Blended Lime
Cement
C
N M or S 1st Coat 2nd Coat
Cement
3 1
C 1 . . . . . . . . . 0– ⁄4 2 ⁄2 –4 3–5
3 1 1
CL 1 . . . . . . . . . ⁄4 –1 ⁄2 2 ⁄2 –4 3–5
M . . . . . . 1 . . . . . . 2 ⁄2 –4 3–5
CM 1 . . . 1 . . . . . . 2 ⁄2 –4 3–5
MS . . . . . . . . . 1 . . . 2 ⁄2 –4 3–5
P . . . 1 . . . . . . . . . 2 ⁄2 –4 3–5
CP 1 1 . . . . . . . . . 2 ⁄2 –4 3–5
A
The mix proportions for plaster scratch and brown coats to receive ceramic tile shall be in accordance with the applicable requirements of ANSI A108.1 series applicable
to specified method of setting time.
B
Variations in lime, sand, and perlite contents are allowed due to variation in local sands and insulation and weight requirements. A higher lime content will generally
supportsupports a higher aggregate content without loss of workability. The workability of the plaster mix will govern governs the amounts of lime, sand, or perlite.
C
The same or greater sand proportion shall be used in the second coat than is used in the first coat.
3 3
Material Weight, lb/ft (kg/m )
Portland cement 94 (1505)
Blended cement Weight printed on bag
Masonry or plastic cement Weight printed on bag
Hydrated Lime 40 (640)
Lime Putty 80 (1280)
Sand, Damp and Loose (7.1.3.2) 80 (1280) of dry sand
7.1.3.2 For purposes of this specification, a weight of 80 lb (1280 kg) of oven-dried sand shall be used. This is, in most cases,
equivalent to one cubic foot of loose, damp sand.
7.1.4 Finish-coat proportions for job-mixed finish coats shall be as specified in Table 3.
7.1.5 Factory-Prepared Finish Coats—See 3.2.17.
7.1.6 Dash-bond coat proportions shall be 1 volume part portland cement and not more than two volume parts of aggregate mixed
to a consistency that will permit application as specified in 7.3.5.
7.1.7 Admixtures shall be proportioned, mixed, and applied in accordance with the printed directions of the manufacturer. (See
A2.5.)
7.1.8 Where specified, natural or synthetic fibers shall be free of contaminants and used only in the base coat(s). The quantities
per batch shall be in accordance with the published directions of the fiber manufacturer.
7.2 Mixing:
7.2.1 All plaster shall be prepared in a mechanical mixer, using sufficient water to produce a workable consistency and uniform
color. (See X1.1.)
TABLE 3 Job-mixed Finish Coat Proportions, Parts by Volume
Volume of Aggregate
Cementitious Materials
per Sum of Separate
A A
Plaster Mix Symbols Masonry Cement Volumes of
Portland Cement or
Plastic Cement Lime
Cementitious
Blended Cement
B
N M or S
Materials
3 1 1
F 1 . . . . . . . . . ⁄4 –1 ⁄2 1 ⁄2 –3
1 1
FL 1 . . . . . . . . . 1 ⁄2 –2 1 ⁄2 –3
FM . . . . . . 1 . . . . . . 1 ⁄2 –3
FCM 1 . . . 1 . . . . . . 1 ⁄2 –3
FMS . . . . . . . . . 1 . . . 1 ⁄2 –3
FP . . . 1 . . . . . . . . . 1 ⁄2 –3
A
Additional portland cement is not required when Type S or M masonry cement is used.
B
In areas not subject to impact, perlite aggregate shall be permitted to be used over base-coat plaster containing perlite aggregate.
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7.2.2 Base-coat plasters that have stiffened because of evaporation of water shall be permitted to be tempered one time only to
restore the required consistency. Plaster not used within 1 ⁄2 h from start of initial mixing shall be discarded.
NOTE 4—Severe hot, dry climate conditions accelerate the stiffening of plaster and require reduction of this limit. The use of cold waters will slow the
stiffening process.
7.2.3 Finish-coat plaster shall not be tempered.
7.3 General Application:
7.3.1 Portland cement plaster shall be applied by hand trowel or machine to the nominal thickness specified in Table 4. The
nominal values expressed in Table 4 represent neither a maximum nor minimum value. They consider the inherent variation of
thickness due to the nature of the application process, and the allowable variation of the substrate and the finished plane of the
plaster.
7.3.2 Plaster nominal thickness shall be measured from the back plane of the metal plaster base, exclusive of ribs or dimples, or
from the face of the solid backing with or without metal plaster base, to the outer surface exclusive of texture variations.
7.3.3 Portland cement-based plaster shall be applied on furred lath when the surface of solid backing material consists of gypsum
board, gypsum plaster, wood, or rigid foam board-type products.
NOTE 5—On horizontal ceiling supports or roof soffits protected by a drip edge or designated drainage screed, gypsum board products shall be permitted
to be used as backing for metal base to receive portland cement plaster.
7.3.4 Separation shall be provided where plaster abuts dissimilar construction materials or openings. (See A2.1.3.)
7.3.5 Each plaster coat shall be applied to an entire wall or ceiling panel without interruption to avoid cold joints and abrupt
changes in the uniform appearance of succeeding coats. Wet plaster shall abut set plaster at naturally occurring interruptions in the
plane of the plaster, such as corner angles, rustications, openings, expansion joints, and control joints where this is possible.
Joinings, where necessary, shall be cut square and straight and not less than 6 in. (152 mm) away from a joining in the preceding
coat.
7.3.6 Lath shall be covered with three-coat work. The combined total nominal thickness shall be as shown in Table 4. A dash-bond
coat shall not replace one of the specified number of coats.
7.3.7 Two-coat work shall be used only over solid bases meeting the requirements of 6.2. The combined total nominal thickness
shall be as shown in Table 4. A dash-bond coat shall not replace one of the specified number of coats.
A
TABLE 4 Nominal Plaster Thickness for Three- and Two-coat Work, in. (mm)
Vertical Horizontal
B B
BASE 1st Coat 2nd Coat 3rd Coat Total 1st Coat 2nd Coat 3rd Coat Total
Interior/Exterior
C,D
Three-coat work:
D D
3 3 1 7 1 1 1 5
Lath ⁄8 (10) ⁄8 (10) ⁄8 (3) ⁄8 (22) ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16)
Solid plaster base:
1 1 D 1 5
Unit masonry ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16) Use two-coat work
D
1 1 1 5 3
Cast-in-place or precast ⁄4 (6) ⁄4 (6) ⁄8 (3) ⁄8 (16) ⁄8 (10), max
concrete
D D
1 1 1 7 1 1 1 7
Lath over solid base ⁄2 (13) ⁄4 (6) ⁄8 (3) ⁄8 (22) ⁄2 (13) ⁄4 (6) ⁄8 (3) ⁄8 (22)
Two-coat work:
Solid plaster base:
3 1 1 3
Unit masonry ⁄8 (10) ⁄8 (3) ⁄2 (13) ⁄8 (10)
1 1 3 3
Cast-in-place or ⁄4 (6) ⁄8 (3) ⁄8 (10) ⁄8 (10)
pre-cast concrete
A
Exclusive of texture variations.
B
For solid plaster partitions, additional coats shall be applied to meet the finished thickness specified.
C
For exposed aggregate finishes, the second (brown) coat shall become the “bedding” coat and shall be of sufficient thickness to receive and hold the aggregate.
D
The basecoat thickness of three-coat work is the sum of the first and second coats combined.
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7.3.8 Backplaster where required, shall be applied only after the coat on the opposite side has set sufficiently to resist breaking
or cracking the plaster keys.
7.3.9 Each coat shall be permitted to set before the next coat is applied. (See X1.5.2.)
7.3.10 Plaster coats that have become dry shall be evenly dampened with water prior to applying subsequent coats to obtain
uniform suction. There shall be no visible water on the surface when plaster is applied.
7.4 Plaster Application on Metal and Non-metallic Plaster Bases:
7.4.1 The first (scratch) coat shall be applied with sufficient material and pressure to form full keys through, and to embed the
metal base, and with sufficient thickness of material over the metal to allow for scoring the surface.
7.4.1.1 As soon as the first (scratch) coat becomes firm, the entire surface shall be scored in one direction only. The vertical
surfaces shall be scored horizontally.
7.4.1.2 The first (scratch) coat shall become sufficiently rigid to support the application of the second (brown) coat without damage
to the monolithic continuity of the first (scratch) coat or its key.
7.4.2 The second (brown) coat shall be applied with sufficient material and pressure to ensure tight contact with the first (scratch)
coat and to bring the combined thickness of the base coat to the nominal thickness shown in Table 4.
7.4.2.1 The surface of the second (brown) coat shall be brought to a true, even plane with a rod or straightedge, filling surface
defects in plane with plaster. Dry rodding the surface of the brown coat shall be permitted.
7.4.2.2 The surface shall be floated uniformly to promote densification of the coat and to provide a surface receptive to bonding
of the finish coat.
7.4.3 The third (finish) coat shall be applied with sufficient material and pressure to ensure tight contact with, and complete
coverage of the base coat and to the nominal thickness shown in Table 4 and 7.5.1.1.
7.5 Plaster Application on Solid Plaster Bases:
7.5.1 High-suction bases shall be evenly dampened with clean water prior to the application of plaster. Do not dampen low-suction
solid bases, such as dense concrete or smooth brick.
7.5.1.1 Where masonry or concrete surfaces vary in plane, plaster thickness required to produce level surfaces shall not be required
to be uniform.
7.5.2 Three-coat Application on Solid Bases:
7.5.2.1 The first (scratch) coat shall be applied with sufficient material and pressure to ensure tight contact and complete coverage
of the solid base, to the nominal thickness shown in Table 4. As soon as the first (scratch) coat becomes firm, the entire surface
shall be scored in one direction only. The vertical surfaces shall be scored horizontally.
7.5.2.2 The second (brown) coat shall be applied using the same procedures specified in 7.4.2 and 7.4.2.1, bringing the surface
to a true, even plane with a rod or straightedge, filling any defects in plane with plaster and darbying. The surface shall be floated
uniformly to provide a surface receptive to the application of the third (finish) coat.
7.5.2.3 The third (finish) coat shall be applied as specified in 7.4.3.
7.5.3 Two-coat Application on Solid Plaster Bases:
7.5.3.1 The first coat shall be applied to the nominal thickness shown in Table 4 using sufficient material and pressure to ensure
tight contact and complete coverage of the solid base. The applied material shall be brought to a true, even plane with a rod or
straightedge. Dry rodding the finish surface of the applied material shall be permitted.
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7.5.3.2 The second (finish) coat shall be applied as specified in 7.4.3.
7.6 Finish-coat Application:
7.6.1 Job-mixed or factory-prepared finish coats shall be applied, by machine or by hand, as specified in 7.4.3.
7.6.2 The use of excessive water during the application and finishing of finish-coat plaster shall be avoided.
7.7 Fog-coat Application—Job-mixed or factory-prepared fog coats shall be applied in accordance with the directions of the
manufacturer.
7.8 Curing and Time Between Coats:
7.8.1 Provide sufficient moisture in the plaster mix or by moist or fog curing to permit continuous hydration of the cementitious
materials. The most effective procedure for curing and time between coats will depend on climatic and job conditions. (See
X1.5.2.)
7.8.2 Sufficient time between coats shall be allowed to permit each coat to cure or develop enough rigidity to resist cracking or
other physical damage when the next coat is applied. (See X1.5.2.)
7.9 Environmental Conditions:
7.9.1 Portland cement-based plaster shall not be applied to frozen base or to a base containing frost. Plaster mixes shall not contain
frozen ingredients. Plaster coats shall be protected from freezing for a period of not less than 24 h after set has occurred.
7.9.2 Portland cement plaster shall be protected from uneven and excessive evaporation during dry weather and from strong blasts
of dry air.
7.9.3 Plaster Application—When artificial heat is required, heaters shall be located to prevent a concentration of heat on uncured
plaster. Heaters shall be vented to the outside to prevent toxic fumes and other products of combustion from adhering to or
penetrating plaster bases and plaster. Adequate ventilation shall be maintained in all areas, particularly in interior areas with little
or no natural air movement.
7.9.3.1 Interior environment shall be maintained at a temperature above 40 °F not less than 48 h prior to and during application
of portland cement-based plaster. Interior temperature shall be maintained above 40 °F until normal occupancy.
7.9.3.2 For exteriors, plaster shall be applied when the ambient temperature is higher than 40 °F (4.4 °C), unless the work area
is enclosed and heat is provided as described in 7.9.3.
8. Keywords
8.1 bond; brown coat; cementitious; exterior plaster; fog coat; portland cement; scratch coat; stucco
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ANNEXES
(Mandatory Information)
A1. GENERAL INFORMATION
A1.1 Where a specific degree of fire resistance is required for plastered assemblies and constructions, details of construction shall
be in accordance with official reports of fire tests conducted by recognized testing laboratories, in accordance with Test Methods
E119.
A1.2 Where a specific degree of sound control is required for plastered assemblies and constructions, details of construction shall
be in accordance with official reports of tests conducted by recognized testing laboratories, in accordance with applicable sound
tests of Test Methods E90 or E492.
A1.3 Scaffolding shall be constructed and maintained in strict conformity with applicable laws and ordinances.
A1.4 Work schedules shall provide for completion of work affecting supports, framework, or lath of a suspended ceiling (such as
loading) before plastering work is accomplished.
A1.5 Surfaces and lathing accessories to receive plaster shall be examined before plastering is applied thereto. The proper
authorities shall be notified and unsatisfactory conditions shall be corrected prior to the application of plaster. The plastering
contractor shall use this portion of the construction specifications for acceptance or rejection of such surfaces.
A1.5.1 Lath, solid plaster bases, the water-resistive barrier, attachment, and accessories to rece
...