Designation: C 926 – 06

Standard Specification for

Application of Portland Cement-Based Plaster1
This standard is issued under the fixed designation C 926; 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 (e) indicates an editorial change since the last revision or reapproval.

1. Scope* C 207 Specification for Hydrated Lime for Masonry Pur-

1.1 This specification covers the requirements for the appli- poses
cation of full thickness portland cement-based plaster for C 219 Terminology Relating to Hydraulic Cement
exterior (stucco) and interior work. C 260 Specification for Air-Entraining Admixtures for Con-
1.2 This specification sets forth tables for proportioning of crete
various plaster mixes and plaster thickness. C 595 Specification for Blended Hydraulic Cements
C 631 Specification for Bonding Compounds for Interior
NOTE 1—General information will be found in Annex A1. Design Plastering
considerations will be found in Annex A2.
C 897 Specification for Aggregate for Job-Mixed Portland
1.3 The values stated in inch-pound units are to be regarded Cement-Based Plasters
as the standard. The SI (metric) values given in parentheses are C 932 Specification for Surface-Applied Bonding Agent for
approximate and are provided for information purposes only. Exterior Plastering
1.4 The text of this specification references notes and C 1063 Specification for Installation of Lathing and Furring
footnotes that provide explanatory material. These notes and to Receive Interior and Exterior Portland Cement-Based
footnotes (excluding those in tables and figures) shall not be Plaster
considered as requirements of the specification. C 1116 Specification for Fiber-Reinforced Concrete and
1.5 Details of construction for a specific assembly to Shotcrete
achieve the required fire resistance shall be obtained from C 1328 Specification for Plastic (Stucco) Cement
reports of fire-resistance tests, engineering evaluations, or E 90 Test Method for Laboratory Measurement of
listings from recognized fire testing laboratories. Airborne-Sound Transmission Loss of Building Partitions
and Elements
2. Referenced Documents E 119 Test Methods for Fire Tests of Building Construction
2.1 ASTM Standards: 2 and Materials
C 11 Terminology Relating to Gypsum and Related Build- E 492 Test Method for Laboratory Measurement of Impact
ing Materials and Systems Sound Transmission Through Floor-Ceiling Assemblies
C 25 Test Methods for Chemical Analysis of Limestone, Using the Tapping Machine
Quicklime, and Hydrated Lime 2.2 ANSI Standard:
C 35 Specification for Inorganic Aggregates for Use in A108.1 Specification for Installation of Ceramic Tile3
Gypsum Plaster
C 91 Specification for Masonry Cement 3. Terminology
C 150 Specification for Portland Cement 3.1 Terms shall be defined as in Terminologies C 11 and
C 206 Specification for Finishing Hydrated Lime C 219, except as modified herein.
3.2 Definitions of Terms Specific to This Standard:
1
3.2.1 accelerator—an admixture that will shorten the set-
This specification is under the jurisdiction of ASTM Committee C11 on
Gypsum and Related Building Materials and Systems and is the direct responsibility
ting time of plaster.
of Subcommittee C11.03 on Specifications for Application of Gypsum and Other 3.2.2 admixture—a material other than water, aggregate, or
Products on Assemblies. basic cementitious material added to the batch before or during
Current edition approved June 1, 2006. Published June 2006. Originally job mixing.
approved in 1981. Last previous edition approved in 2005 as C 926–98a(2005).
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
3
Standards volume information, refer to the standard’s Document Summary page on Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036.

*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.

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 C 926 – 06
3.2.3 acid etching—the cleansing and controlled erosion of 3.2.13 factory prepared (“mill-mixed” or “ready mixed”)—
a solid surface, using an acid wash. pertaining to material combinations that have been formulated
3.2.4 air entrainment—the use of an air-entraining admix- and dry-blended by the manufacturer, requiring only the
ture or air-entraining cementitious material in a plaster mix to addition of and mixing with water to produce plaster.
yield a controlled quantity of minute (typically between 10 and 3.2.14 fiber, natural or synthetic—an elongated fiber or
1000 µm in diameter) disconnected air bubbles in the plaster strand admixture added to plaster mix to improve cohesiveness
(see entrapped air). or pumpability, or both.
3.2.5 backplaster—plaster applied to the face of metal lath 3.2.15 floating—act of compacting and leveling brown-coat
opposite a previously applied plaster. plaster to a reasonably true surface plane using a float tool or
3.2.6 bond—the state of adhesion between plaster coats or the act of bringing the aggregate to the surface of finish-coat
between plaster and plaster base. plaster.
3.2.16 key (also mechanical key)—plaster that physically
3.2.7 bonding compound or agent—compounds surface ap-
surrounds, penetrates, or deforms to lock onto the perforations
plied or integrally mixed with plaster to improve the quality of
or irregularities of the plaster base or previous coat of plaster.
bond between plaster and plaster base or between plaster coats.
3.2.17 metal plaster base—expanded metal lath, or welded
3.2.8 cementitious material—a material that, when mixed
or woven wire lath.
with water and with or without aggregate, provides the
3.2.18 plaster—portland cement-based cementitious mix-
plasticity and the cohesive and adhesive properties necessary
ture (see stucco).
for placement and the formation of a rigid mass.
3.2.19 required—pertaining to a mandatory obligation im-
3.2.9 coat—a thickness of plaster applied in a single opera- posed by a force outside of this specification, such as a building
tion. code, project specification, contract, or purchase order.
3.2.9.1 basecoat—all plaster applied before the application 3.2.20 rustication (also “break”)—an interruption or
of the finish coat. change in plane of a plastered surface.
3.2.9.2 bedding coat—a plaster coat that receives aggregate 3.2.21 scoring (also known as “scratching”)—the grooving
or other decorative material impinged into its surface before it of the surface of an unset plaster coat to provide a key for a
sets. subsequent coat.
3.2.9.3 brown coat—in three-coat work, the second coat, 3.2.22 set—the chemical and physical change in plaster as it
applied over the scratch coat. In two-coat work, brown coat goes from a plastic, workable state to a rigid state.
refers to the double-up basecoat. In either use, the brown coat 3.2.23 stucco—portland cement-based plaster used on exte-
is the coat directly beneath the finish coat. rior locations.
3.2.9.4 dash-bond coat—a thick wet mixture of portland 3.2.24 stucco finish—a factory-prepared, dry blend of ma-
cement and water, with or without aggregate, dashed onto the terials for finish coat applications.
surface of a plaster base such as smooth monolithic concrete or 3.2.25 temper, v—to mix or restore unset plaster with water
concrete block surfaces to improve the mechanical key for to a workable consistency.
subsequent plaster coats. 3.2.26 texture—any surface appearance as contrasted to a
3.2.9.5 double-up coat—the brown-coat plaster applied to smooth surface.
the scratch coat plaster before the scratch-coat plaster has set.
3.2.9.6 finish coat—the final layer of plaster applied over 4. Materials
basecoat plaster. 4.1 Materials shall conform to the requirements of the
3.2.9.7 fog coat—a light coat of cement and water, with or referenced specifications and standards and to the requirements
without aggregate or color pigment, applied by machine spray specified herein.
to improve color consistency. 4.2 Cement:
3.2.9.8 scratch coat—the first coat of plaster applied to a 4.2.1 Portland Cement—Specification C 150, Type I, II, and
plaster base. III, as specified. White where specified.
3.2.9.9 skim coat—a thin finish coat applied to an existing 4.2.2 Air-Entraining Portland Cement—Specification
plaster surface or other substrate to improve appearance. C 150, type as specified. White where specified.
3.2.9.10 three-coat work—application of plaster in three 4.2.3 Masonry Cement—Specification C 91, Types N, S,
successive coats with time between coats for setting or drying, and M. White where specified.
or both. 4.2.4 Blended Hydraulic Cement—Specification C 595,
3.2.10 cold joint (“joining” or “jointing”)—the juncture of Type IP, I(PM), IS, and I(SM) as specified.
fresh plaster application adjacent to set plaster, in the same 4.2.5 Air-Entraining Blended Hydraulic Cement—
plane. Specification C 595, Type IP-A, I(PM)-A, IS-A, and I(SM)-A
3.2.11 curing—the act or processes of producing a moisture as specified.
environment favorable to cement hydration, resulting in the 4.2.6 Plastic Cement—Plastic Cement shall meet the re-
setting or hardening of the plaster. quirements of Specification C 1328, Standard Specification for
Plastic (Stucco) Cement.
3.2.12 entrapped air—unintentional air voids in the plaster
generally larger than 1 mm. NOTE 2—Plastic cements are not available nationally.

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 C 926 – 06
4.3 Type “S” Hydrated Lime—A hydrated lime that con- 5.2.2 Smooth or nonabsorbent solid surfaces, such as cast-
tains not more than 8 % unhydrated oxides when tested in in-place or precast concrete, shall be prepared to receive
accordance with Test Methods C 25. See Specifications C 206 portland cement plaster by one of the following methods:
and C 207 for a complete description of a Type “S” hydrated 5.2.2.1 Sandblasting, wire brushing, acid etching, or chip-
lime. ping or a combination thereof,
4.4 Aggregates: 5.2.2.2 Application of a dash-bond coat applied forcefully
4.4.1 Sand for Base Coats—Specification C 897. Aggregate against the surface, left untroweled, undisturbed, and moist
failing to meet gradation limits in Specification C 897 shall be cured for at least 24 h, or
permitted to be used, provided the plaster made with this sand 5.2.2.3 Application of a bonding compound suitable for
has an acceptable demonstrated performance record in similar exterior or interior exposure solid surfaces in accordance with
construction and climate conditions. the manufacturer’s written directions.
4.4.2 Perlite—Specification C 35. 5.2.3 Where bond cannot be obtained over the entire surface
4.4.3 Sand for Job-Mixed Finish Coats—Specification to receive plaster by one or more of the methods in 5.2.2, or
C 897. where total plaster thickness will exceed the total thickness
4.5 Water—Water used in mixing, application, and finishing specified in Table 1 for types of solid bases, furred or
of plaster shall be clean, fresh, suitable for domestic water self-furring metal plaster base shall be installed in accordance
consumption, and free of such amounts of mineral or organic with Specification C 1063.
substances as would affect the set, the plaster, or any metal in
the system. 6. Plaster Proportions and Mixing
4.6 Admixtures—See 3.2.2 and .
4.7 Fibers—Specification C 1116 on alkali-resistant fibers, 6.1 Proportions:
glass fibers, nylon, polypropylene or carbon fibers. 6.1.1 All portland cement plasters shall be mixed and
proportioned in accordance with the following tables and
5. Requirements for Bases to Receive Portland Cement- accompanying requirements, using measuring devices of
Based Plaster known volume with successive batches proportioned alike.
5.1 Metal bases and accessories used to receive plaster shall 6.1.2 Plaster mix used shall be as designated and referenced
be installed in conformance with Specification C 1063, except to Table 2.
as otherwise specified. 6.1.3 Base-coat proportions shall be as shown in Table 3 for
the mix specified from Table 2.
NOTE 3—All metal, or PVC, or CPVC plastic members should be free
of deleterious amounts of rust, oil, or other foreign matter, which could
6.1.3.1 Measurement of Materials—The method of measur-
cause bond failure or unsightly discoloration. ing materials for the plaster shall be such that the specified
proportions are controlled and accurately maintained. The
5.2 Surfaces of solid bases to receive plaster, such as weights per cubic foot of the materials are considered to be as
masonry, stone, cast-in-place or precast concrete shall be follows:
straight and true within 1⁄4 in. in 10 ft (2.1 mm/m) and shall be
Material Weight, lb/ft3(kg/m3)
free of form oil or other elements, which would interfere with
bonding. Form ties or other obstructions shall be removed or Portland cement 94 (1505)
trimmed back even with the surface of the solid base. Blended cement Weight printed on bag
Masonry or plastic cement Weight printed on bag
5.2.1 Solid surfaces shall have the suction (ability to absorb Hydrated Lime 40 (640)
water) or surface roughness, or both, to provide the bond Lime Putty 80 (1280)
required for the plaster. Sand, Damp and Loose (6.1.3.2) 80 (1280) of dry sand

TABLE 1 Nominal Plaster ThicknessA for Three- and Two-Coat Work, in. (mm)
Vertical Horizontal
BASE 1st Coat 2nd Coat 3rd CoatB Total 1st Coat 2nd Coat 3rd CoatB Total
Interior/Exterior
Three-coat work:C
Metal plaster base ⁄ (9.5)
38 ⁄ (9.5)
38 ⁄ (3)
18 ⁄ (22)
78 14 ⁄ (6) ⁄ (6)
14 ⁄ (3)
18 ⁄ (16)
58

Solid plaster base:

Unit masonry 14 ⁄ (6) ⁄ (6)
14 ⁄ (3)
18 ⁄ (16)
58 Use two-coat work
Cast-in-place or precast 14 ⁄ (6) ⁄ (6)
14 ⁄ (3)
18 ⁄ (16)
58 ⁄ (9.5), max
38

concrete
Metal plaster base over solid ⁄ (12.5)
12 ⁄ (6)
14 ⁄ (3)
18 ⁄ (22)
78 ⁄ (12.5)
12 ⁄ (6)
14 ⁄ (3)
18 ⁄ (22)
78

base
Two-coat work:
Solid plaster base:
Unit masonry ⁄ (9.5)
38 ⁄ (3)
18 ⁄ (12.5)
12 ⁄ (9.5)
38

Cast-in-place or pre-cast 1⁄4 (6) ⁄ (3)

18 ⁄ (9.5)
38 ⁄ (9.5)
38

concrete
A
Exclusive of texture.
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.

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 C 926 – 06
TABLE 2 Plaster Bases—Permissible Mixes NOTE 5—On horizontal ceiling supports or roof soffits protected by a
drip edge, gypsum board products shall be permitted to be used as backing
NOTE—See Table 3 for plaster mix symbols.
for metal base to receive portland cement plaster.
Mixes for Plaster Coats
Property of Base
First (Scratch) Second (Brown)
7.1.4 Separation shall be provided where plaster abuts
dissimilar construction materials or openings. (See A2.1.4.)
Low absorption, such as C C, CL, M, or CM
dense, smooth clay tile, 7.1.5 Each plaster coat shall be applied to an entire wall or
CM or MS CM, MS, or M
brick, or concrete
P P
ceiling panel without interruption to avoid cold joints and
abrupt changes in the uniform appearance of succeeding coats.
High Absorption, such as CL CL
concrete masonry, absorp- Wet plaster shall abut set plaster at naturally occurring inter-
M M
tive brick, or tile
CM or MS CM, MS, or M
ruptions in the plane of the plaster, such as corner angles,
P P rustications, openings, and control joints where this is possible.
Metal plaster base C C, CL, M, CM, or MS Joinings, where necessary, shall be cut square and straight and
CL CL not less than 6 in. (152 mm) away from a joining in the
CM or MS CM, MS, or M preceding coat.
M M
CP CP or P 7.1.6 Metal plaster base shall be covered with three-coat
P P work with or without solid backing. The combined total
nominal thickness shall be as shown in Table 1. A dash-bond
coat shall not replace one of the specified number of coats.
7.1.7 Two-coat work shall be used only over solid bases
6.1.3.2 For purposes of this specification, a weight of 80 lb meeting the requirements of 5.2. The combined total nominal
(1280 kg) of oven-dried sand shall be used. This is, in most thickness shall be as shown in Table 1. A dash-bond coat shall
cases, equivalent to one cubic foot of loose, damp sand. not replace one of the specified number of coats.
6.1.4 Finish-coat proportions for job-mixed finish coats 7.1.8 Backplaster where required, shall be applied only after
shall be as specified in Table 4. the coat on the opposite side has set sufficiently to resist
6.1.5 Factory-Prepared Finish Coats—See 3.2.14. breaking or cracking the plaster keys.
6.1.6 Dash-bond coat proportions shall be 1 volume part 7.1.9 Each coat shall be permitted to set before the next coat
portland cement and not more than 2 volume parts of aggregate is applied. (See X1.4.2.)
mixed to a consistency that will permit application as specified 7.1.10 Plaster coats that have become dry shall be evenly
in 7.1.5. dampened with water prior to applying subsequent coats to
6.1.7 Admixtures shall be proportioned, mixed, and applied obtain uniform suction. There shall be no visible water on the
in accordance with the printed directions of the manufacturer. surface when plaster is applied.
(See A2.5.) 7.2 Plaster Application on Metal Plaster Bases:
6.2 Mixing:
7.2.1 The first (scratch) coat shall be applied with sufficient
6.2.1 All plaster shall be prepared in a mechanical mixer,
material and pressure to form full keys through, and to embed
using sufficient water to produce a workable consistency and
the metal base, and with sufficient thickness of material over
uniform color. (See X1.1.)
the metal to allow for scoring the surface.
6.2.2 Base-coat plasters that have stiffened because of
7.2.1.1 As soon as the first (scratch) coat becomes firm, the
evaporation of water shall be permitted to be tempered one
entire surface shall be scored in one direction only. The vertical
time only to restore the required consistency. Plaster not used
surfaces shall be scored horizontally.
within 11⁄2 h from start of initial mixing shall be discarded.
7.2.1.2 The first (scratch) coat shall become sufficiently
NOTE 4—Severe hot, dry climate conditions accelerate the stiffening of rigid to support the application of the second (brown) coat
plaster and require reduction of this limit. The use of cold waters will slow without damage to the monolithic continuity of the first
the stiffening process.
(scratch) coat or its key.
6.2.3 Finish-coat plaster shall not be tempered. 7.2.2 The second (brown) coat shall be applied with suffi-
cient material and pressure to ensure tight contact with the first
7. Application (scratch) coat and to bring the combined thickness of the base
7.1 General: coat to the nominal thickness shown in Table 1.
7.1.1 Portland cement plaster shall be applied by hand or 7.2.2.1 The surface of the second (brown) coat shall be
machine to the nominal thickness specified in Table 1. brought to a true, even plane with a rod or straightedge, filling
7.1.2 Plaster nominal thickness shall be measured from the surface defects in plane with plaster. Dry rodding the surface of
back plane of the metal plaster base, exclusive of ribs or the brown coat shall be permitted.
dimples, or from the face of the solid backing with or without 7.2.2.2 The surface shall be floated uniformly to promote
metal plaster base, to the outer surface exclusive of texture densification of the coat and to provide a surface receptive to
variations. bonding of the finish coat.
7.1.3 Portland cement-based plaster shall be applied on 7.2.3 The third (finish) coat shall be applied with sufficient
furred metal plaster base when the surface of solid backing material and pressure to ensure tight contact with, and com-
consists of gypsum board, gypsum plaster, wood, or rigid foam plete coverage of the base coat and to the nominal thickness
board-type products. shown in Table 1 and 7.3.1.1.

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 C 926 – 06
TABLE 3 Base-Coat Proportions,A Parts by VolumeB
Cementitious Materials Volume of Aggregate per Sum of
Plaster Mix Separate
Portland Cement Masonry Cement Volumes of Cementitious Materials
Symbols Plastic
or Blended Lime
Cement
Cement N M or S 1st Coat 2ndC Coat
C 1 ... ... ... 0– ⁄34 2⁄ 12 –4 3–5
CL 1 ... ... ... ⁄ –11⁄2
34 2 1⁄ 2 –4 3–5
M ... ... 1 ... ... 2 1⁄ 2 –4 3–5
CM 1 ... 1 ... ... 2 1⁄ 2 –4 3–5
MS ... ... ... 1 ... 2 1⁄ 2 –4 3–5
P ... 1 ... ... ... 2 1⁄ 2 –4 3–5
CP 1 1 ... ... ... 2 1⁄ 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
support a higher aggregate content without loss of workability. The workability of the plaster mix will govern 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.

TABLE 4 Job-Mixed Finish Coat Proportion Parts by Volume

Cementitious Materials Volume of Aggregate
per Sum of Separate
Plaster Mix SymbolsA Masonry CementA Volumes of
Portland Cement or
Plastic Cement Lime Cementitious
Blended Cement
N M or S MaterialsB
F 1 ... ... ... ⁄ –11⁄2
34 1 1 ⁄2 –3
FL 1 ... ... ... 11⁄2 –2 11⁄2 –3
FM ... ... 1 ... ... 1 1 ⁄2 –3
FCM 1 ... 1 ... ... 1 1 ⁄2 –3
FMS ... ... ... 1 ... 1 1 ⁄2 –3
FP ... 1 ... ... ... 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.

7.3 Plaster Application on Solid Plaster Bases: 7.4.1 Job-mixed or factory-prepared finish coats shall be
7.3.1 High-suction bases shall be evenly dampened with applied, by machine or by hand, as specified in 7.2.3.
clean water prior to the application of plaster. Do not dampen 7.4.2 The use of excessive water during the application and
low-suction solid bases, such as dense concrete or smooth finishing of finish-coat plaster shall be avoided.
brick. 7.5 Fog-Coat Application—Job-mixed or factory-prepared
7.3.1.1 Where masonry or concrete surfaces vary in plane, fog coats shall be applied in accordance with the directions of
plaster thickness required to produce level surfaces shall not be the manufacturer.
required to be uniform.
8. Curing and Time Between Coats
7.3.2 Three-Coat Application on Solid Bases:
7.3.2.1 The first (scratch) coat shall be applied with suffi- 8.1 Provide sufficient moisture in the plaster mix or by
cient material and pressure to ensure tight contact and complete moist or fog curing to permit continuous hydration of the
coverage of the solid base, to the nominal thickness shown in cementitious materials. The most effective procedure for curing
Table 1. As soon as the first (scratch) coat becomes firm, the and time between coats will depend on climatic and job
entire surface shall be scored in one direction only. The vertical conditions. (See X1.4.2.)
surfaces shall be scored horizontally. 8.2 Sufficient time between coats shall be allowed to permit
7.3.2.2 The second (brown) coat shall be applied using the each coat to cure or develop enough rigidity to resist cracking
same procedures specified in 7.2.2 and 7.2.2.1, bringing the or other physical damage when the next coat is applied. (See
surface to a true, even plane with a rod or straightedge, filling X1.4.2.)
any defects in plane with plaster and darbying. The surface 9. Product Marking
shall be floated uniformly to provide a surface receptive to the 9.1 Packaged materials shall be clearly marked or labeled to
application of the third (finish) coat. indicate product, brand name, the manufacturer, and the weight
7.3.2.3 The third (finish) coat shall be applied as specified in of the material contained therein. Similar information shall be
7.2.3. provided in the shipping advices accompanying the shipment
7.3.3 Two-Coat Application on Solid Plaster Bases: of bulk materials.
7.3.3.1 The first (scratch) coat shall be applied as specified
in 7.3.2.1. 10. Delivery of Materials
7.3.3.2 The second (finish) coat shall be applied as specified 10.1 Packaged materials shall be delivered in factory-
in 7.2.3. sealed, unopened, and unbroken packages, containers, or
7.4 Finish-Coat Application: bundles.

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10.2 Bulk materials shall be delivered in clean transport 12.3 Plaster Application—When artificial heat is required,
vessels, free of contaminates. heaters shall be located to prevent a concentration of heat on
uncured plaster. Heaters shall be vented to the outside to
11. Protection of Materials prevent toxic fumes and other products of combustion from
11.1 Weather-sensitive materials shall be kept in a dry adhering to or penetrating plaster bases and plaster. Adequate
condition until ready for use. (See A2.4.) ventilation shall be maintained in all areas, particularly in
11.2 Bulk materials shall be stored to prevent subsequent interior areas with little or no natural air movement.
contamination and segregation. 12.3.1 Interior environment shall be maintained at a tem-
perature above 40 °F not less than 48 h prior to and during
12. Environmental Conditions application of portland cement-based plaster. Interior tempera-
12.1 Portland cement-based plaster shall not be applied to ture shall be maintained above 40 °F until normal occupancy.
frozen base or to a base containing frost. Plaster mixes shall not 12.3.2 For exteriors, plaster shall be applied when the
contain frozen ingredients. Plaster coats shall be protected ambient temperature is higher than 40 °F (4.4 °C), unless the
from freezing for a period of not less than 24 h after set has work area is enclosed and heat is provided as described in 12.3.
occurred.
12.2 Portland cement plaster shall be protected from uneven 13. Keywords
and excessive evaporation during dry weather and from strong 13.1 brown coat; cementitious; exterior plaster; fog coat—
blasts of dry air. bond; portland cement; scratch coat; stucco

ANNEXES

(Mandatory Information)

A1. GENERAL INFORMATION

A1.1 The work shall include all labor, materials, services, examined before plastering is applied thereto. The proper
equipment, and scaffolding required to complete the plastering authorities shall be notified and unsatisfactory conditions shall
of the project in accordance with the drawings and specifica- be corrected prior to the application of plaster. Unsatisfactory
tions, except heat, electric power, and potable water. conditions shall be corrected by the party responsible for such
conditions.
A1.2 Where a specific degree of fire resistance is required
for plastered assemblies and constructions, details of construc- A1.6.1 Metal plaster bases, backing, attachment, and acces-
tion shall be in accordance with official reports of fire tests sories to receive plaster shall be examined to determine if the
conducted by recognized testing laboratories, in accordance applicable requirements of Specification C 1063 have been met
with Test Methods E 119. unless otherwise required by the contract specifications.
A1.6.2 The construction specifier shall describe, in the
A1.3 Where a specific degree of sound control is required proper section of the contract specifications, the physical
for plastered assemblies and constructions, details of construc- characteristics of solid surface bases to receive plaster. The
tion shall be in accordance with official reports of tests plane tolerance shall be not more than 1⁄4 in. in 10 ft (3.1
conducted by recognized testing laboratories, in accordance mm/m). The mortar joints shall be flush and not struck.
with applicable sound tests of Test Methods E 90 or E 492. Dissimilar materials such as ties, reinforcing steel, and so forth,
shall be cut back 1⁄8 in. (3 mm) below the surface and treated
A1.4 Scaffolding shall be constructed and maintained in with a corrosion-resistant coating. Masonry shall be solid at
strict conformity with applicable laws and ordinances. corners and where masonry changes thickness in a continuous
A1.5 Work schedules shall provide for completion of work construction. Form release compounds shall be compatible
affecting supports, framework or lath of a suspended ceiling with plaster or be completely removed from surfaces to receive
(such as loading) before plastering work is accomplished. plaster. The plastering contractor shall use this portion of the
construction specifications for acceptance or rejection of such
A1.6 Surfaces and accessories to receive plaster shall be surfaces.

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 C 926 – 06

A2. DESIGN CONSIDERATIONS

A2.1 Exterior plaster (stucco) is applied to outside surfaces A2.3 Relief from Stresses:
of all types of structures to provide a durable, fire-resistant
A2.3.1 For information on control joints and perimeter
covering. Interior plaster is applied to inside surfaces that will
relief see the Installation Section of Specification C 1063.
be subjected to various exposures, such as abrasion, vibration,
or to continuous or frequent moisture and wetting, or to A2.3.1.1 Control joints shall be cleaned and clear of plaster
freezing or thawing. within the control area after plaster application and before final
plaster set.
A2.1.1 Sufficient slope on faces of plastered surfaces shall
be provided to prevent water, snow, or ice from accumulating A2.3.1.2 Prefabricated control joint members shall be in-
or standing. Air-entrained portland cement plaster provides stalled prior to the application of plaster; therefore, the decision
improved resistance to freeze/thaw deterioration. Resistance to to use them, the type selected, their location, and method of
rain penetration is improved where plaster has been adequately installation shall be determined and specified in project speci-
densified during application and properly cured. Plaster shall fication sections other than the section on plastering.
not, however, be considered to be “waterproof.” A2.3.1.3 A groove or cut in plaster only shall not be
A2.1.2 The construction specifier shall describe, in the considered a control or expansion joint.
appropriate section of the contract specifications, the require- A2.3.2 Where plaster and metal plaster base continues
ments for furnishing and application of flashing. Flashing shall across the face of a concrete column, or other structural
be specified at openings, perimeters, and terminations to member, water-resistive building paper or felt shall be placed
prevent water from getting behind plaster. Flashing shall be between the metal plaster base and the structural member
corrosion-resistant material. Aluminum flashing shall not be (paper or plastic-backed metal plaster base shall be permitted).
used. Flashing supplemented with sealant shall be permitted. Where the width of the structural member exceeds the ap-
A2.1.3 Sealing or caulking of V-grooves, exposed ends, and proved span capability of the metal plaster base, self-furring
edges of plaster panels or exterior work to prevent entry of metal plaster base shall be used and sparingly scatter nailed to
water shall be provided. bring paper and metal base to general plane.
A2.1.4 To reduce spalling where interior plaster abuts A2.3.3 Where dissimilar base materials abut and are to
openings, such as wood or metal door or window frames, or receive a continuous coat of plaster: (1) a two-piece expansion
fascia boards, the edge of three-coat plaster shall be tooled joint, casing beads back-to-back, or premanufactured control-
through the second and finish coats to produce a continuous expansion joint member shall be installed; or (2) the juncture
small V-joint of uniform depth and width. On two-coat work, shall be covered with a 6-in. (152-mm) wide strip of galva-
the V-joint shall be tooled through the finish coat only. nized, self-furring metal plaster base extending 3 in. (76 mm)
A2.1.5 Provide in the appropriate project specification sec- on either side of the juncture; or (3) where one of the bases is
tion that solid bases to receive plaster shall not be treated with metal plaster base, self-furring metal plaster base shall be
bond breakers, parting compounds, form oil, or other material extended 4 in. (102 mm) onto the abutting base.
that will prevent or inhibit the bond of the plaster to the base.
A2.1.6 Maximum allowable deflection for vertical or hori- A2.4 Weather-Sensitive Materials—Water-sensitive mate-
zontal framing for plaster, not including cladding, shall be rials shall be stored off the ground or floor and under cover,
L/360. avoiding contact with damp floor or wall surfaces.
A2.2 Provisions for Drainage Behind Exterior Plaster: Temperature-sensitive materials shall be protected from freez-
ing. Bulk materials shall be stored in the area of intended use
A2.2.1 In multistory construction where lath and portland and caution shall be exercised to prevent contamination and
cement plaster exterior walls are continuous past a floor slab, segregation of bulk materials prior to use.
tracks or plates and studs shall be offset to provide a space not
less than 3⁄8 in. (9.5 mm) between the inner face of the exterior A2.5 Admixtures—Admixtures shall be proportioned and
plaster and the edge of the floor slab. mixed in accordance with the published directions of the
A2.2.2 At the bottom of exterior walls where the wall is admixture manufacturer.
supported by a floor or foundation, a drip screed and through-
wall flashing or weep holes or other effective means to drain A2.5.1 The quantity of admixtures required to impart the
away any water that may get behind the plaster shall be desired performance is generally very small in relation to the
provided. quantities of the other mix ingredients. Batch-to-batch quanti-
A2.2.3 Where vertical and horizontal exterior plaster sur- ties shall be measured accurately.
faces meet, both surfaces shall be terminated with casing beads A2.5.2 Air-entraining agents cause air to be incorporated in
with the vertical surface extending at least 1⁄4 in. (6 mm) below the plaster in the form of minute bubbles, usually to improve
the intersecting horizontal plastered surface, thus providing a frost or freeze-thaw resistance, or workability of the plaster
drip edge. The casing bead for the horizontal surface shall be during application. Air-entraining agents for portland cement-
terminated not less than 1⁄4 in. from the back of the vertical based plaster shall meet the requirements of Specification
surface to provide drainage. C 260.

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 C 926 – 06

APPENDIX

(Nonmandatory Information)

X1. GENERAL INFORMATION

X1.1 Additions—Bonding compounds or agents may be of the base coat from one area or location to another, the type
pre-applied to a surface to receive plaster. In this usage it is not of finish selected, the migration of color pigments with
considered an admixture. Bonding compounds that are inte- moisture, and with job site climatic and environmental condi-
grally mixed with plaster prior to its application are considered tions. These factors are rarely under the control of the
admixtures. Where exterior exposure and cyclic wetting are applicator.
anticipated, the re-emulsification capability of the bonding
material must be considered. Bonding agents are only as good X1.2 Finish Coat Categories (applicable to both natural
as the material surface to which they are applied; therefore, and colored finishes):
form release materials must be removed from concrete or be X1.2.1 Texture, as a description of surface appearance, is
compatible with the bonding material used. Bonding agents in identified generally with the method and tools used to achieve
plaster mixes may increase the cohesive properties of the the finish. Texture can be varied by the size and shape of the
plaster. Bonding agents should meet the requirements of aggregate used, the equipment or tools employed, the consis-
Specifications C 631 or C 932. tency of the finish coat mix, the condition of the base to which
X1.1.1 By the use of a suitable admixture or additive, it is it is applied, and by subsequent decorative or protective
possible to improve plaster’s resistance to moisture movement. treatment.
However, the use of the terms damproofing or water proofing X1.2.2 There are many factors that affect the ultimate
is misleading, and their use shall be discouraged. appearance of textured and integrally colored plaster. A suit-
X1.1.2 Natural or synthetic fibers, 1⁄2 to 2 in. (13 to 51 mm) ably sized sample panel should be submitted for approval by
in length and free of contaminates may be specified to improve the architect and the owner. Once approved, the sample should
resistance to cracking or to impart improved pumpability be maintained on site for reference and comparison.
characteristics. The quantities per batch shall be in accordance X1.2.3 With the almost limitless variations possible for
with the published directions of the fiber manufacturer. No finish appearance or texture, the same term may not have the
more than 2 lb (0.90 kg) of fiber should be used per cubic foot same meaning to the specifier, the contractor, and the actual
of cementitious material. Asbestos fibers should not be used. applicator. The specifier is cautioned to use an approved range
Alkaline-resistant glass fibers are recommended where glass of sample panels. To provide some guidance, the following
fiber is used. categories are generally understood and recognized to imply a
X1.1.3 Plasticizers containing hydrated lime putty, air- particular method of application technique or resulting finished
entraining agents, or approved fatteners to increase the work- appearance:
ability of a portland cement plaster may be used. Plaster X1.2.4 Smooth Trowel—Hand- or machine-applied plaster
consistency and workability are affected by plasticizers that are floated as smooth as possible and then steel-troweled. Steel
beneficial in proper quantities from an economic standpoint, troweling should be delayed as long as possible and used only
but in excess can be detrimental to the long-term performance to eliminate uneven points and to force aggregate particles into
of the plaster in place. the plaster surface. Excessive troweling should be avoided.
X1.1.4 Color material for integral mixing with plaster X1.2.5 Float—A plaster devoid of coarse aggregate applied
should not significantly alter the setting, strength development, in a thin coat completely covering the base coat, followed by a
or durability characteristics of the plaster. Natural or mineral second coat that is floated to a true plane surface yielding a
pigments that are produced by physical processing of materials relatively smooth to fine-textured finish, depending on the size
mined directly from the earth appear to offer the best long-term of aggregate and technique used. It is also known as sand
performance with respect to resistance to fading. Plaster color finish.
is determined by the natural color of the cementitious materi- X1.2.6 Trowel-Textured (such as Spanish Fan, Trowel
als, aggregate, and any color pigment, and their proportions to Sweep, English Cottage)—A freshly applied plaster coat is
each other. The use of white cement with the desired mineral given various textures, designs, or stippled effects by hand
oxide pigment color material may result in truer color. troweling. The effects achieved may be individualized and may
X1.1.4.1 The uniformity of color cannot be guaranteed by be difficult to duplicate by different applicators.
the materials manufacturer of the component materials or by X1.2.7 Rough-Textured (such as Rough Cast, Wet Dash,
the applicating contractor. Color uniformity is affected by the Scottish Harl)—Coarse aggregate is mixed intimately with the
uniformity of proportioning, thoroughness of mixing, cleanli- plaster and is then propelled against the base coat by trowel or
ness of equipment, application technique, and curing condi- by hand tool. The aggregate is largely unexposed and deep
tions and procedure, which are generally under the control of textured.
the applicator. Color uniformity is affected to an even greater X1.2.8 Exposed Aggregate (also known at Marblecrete)—
degree by variations in thickness and differences in the suction Varying sizes of natural or manufactured stone, gravel, shell, or

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 C 926 – 06
ceramic aggregates are embedded by hand or machine propul- X1.4.1 Hand mixing should not be permitted, except as
sion into a freshly applied finish “bedding” coat. The size of the approved by the contract specifier.
aggregate determines the thickness of the “bedding” coat. It is X1.4.1.1 After all ingredients are in the mixer, mix the
generally thicker than a conventional finish coat. plaster for 3 to 5 min.
X1.2.9 Spray-Textured—A machine-applied plaster coat di- X1.4.1.2 The amount of water used in the plaster mix
rected over a previously applied thin smooth coat of the same should be determined by the plasterer. Factors such as the
mix. The texture achieved depends on the consistency of the suction of the base, or of the previous coat, water content of the
sprayed mixture, moisture content of the base to which it is aggregate, drying conditions, and finishing operations should
applied, the angle and distance of the nozzle to the surface, and be considered in determining water usage. Use of excessive
the pressure of the machine. water may result in dropouts, fall or slide off, excessive
X1.2.10 Brush-Finish—A method of surfacing or resurfac- shrinkage, high porosity, and lower strength.
ing new or existing plaster. The plaster is applied with a brush X1.4.2 Time Between Coats and Curing for Portland
to a thickness of not less than 1⁄16 in. (1.6 mm). For an existing Cement-Based Plaster:
plaster surface the bond capability must be determined by test X1.4.2.1 The timing between coats will vary with climatic
application or a bonding compound must be applied prior to the conditions and types of plaster base. Temperature and relative
brush application. humidity extend or reduce the time between consecutive
X1.2.11 Miscellaneous Types—This finish coat category is operations. Cold or wet weather lengthens and hot or dry
somewhat similar to trowel-textured finishes, except that the weather shortens the time period. Moderate changes in tem-
freshly applied plaster is textured with a variety of instruments perature and relative humidity can be overcome by providing
other than the trowel, such as swept with a broom or brush, additional heating materials during cold weather and by
corrugated by raking or combing, punched with pointed or reducing the absorption of the base by pre-wetting during hot
blunt instrument, scored by aid of a straightedge into designs of or dry weather.
simulated brick, block, stone, and so forth. A variation of X1.4.2.2 In order to provide more intimate contact and bond
texturing a finish coat involves waiting until it has partially set between coats and to reduce rapid water loss, the second coat
and then flattening by light troweling of the unevenly applied should be applied as soon as the first coat is sufficiently rigid to
plaster or by simulating architectural terracotta. resist cracking, the pressures of the second coat application,
X1.2.12 Scraffıtto—A method of applying two or more and the leveling process.
successive coats of different colored plaster and then removing X1.4.2.3 The amount of water and the timing for curing
parts of the overlaid coats to reveal the underlaying coats, portland cement plaster will vary with the climatic conditions,
usually following a design or pattern. This is not generally the type of base, and use or nonuse of water-retentive admix-
considered a finish coat operation because of the number of tures.
thickness of coats. X1.4.2.4 Some moisture must be retained in or added back
to freshly applied portland cement-based plaster. If the relative
X1.3 When specified as alternate for final coat, trowel- or humidity is relatively high (above 75 %), the frequency for
plaster machine-applied textured acrylic finishes containing rewetting a surface may be reduced. If it is hot, dry, and windy,
aggregate may be substituted for portland cement finish coats, the frequency of rewetting must be increased.
provided brown coat is properly prepared and finish is applied X1.4.2.5 Consider the physical characteristics of the struc-
according to the manufacturer’s directions. ture as well as the previously mentioned conditions when
X1.3.1 Staining of Plaster—Staining and discoloration of selecting the method of curing. The method can be one or a
plaster, caused by free water draining from one plane of plaster combination of the following:
to another or from a dissimilar material onto a plaster surface, (1) Moist curing is accomplished by applying a fine fog
can be minimized by providing sufficient depth and angle for spray of water as frequently as required, generally twice daily
drip caps and the use of water-resistive surface coatings. in the morning and evening. Care must be exercised to avoid
X1.3.2 Staining of plaster due to entrapment of moisture erosion damage to portland cement-based plaster surfaces.
behind the plaster, can be avoided or minimized by providing Except for severe drying conditions, the wetting of finish coat
an air space for ventilation between the back of the plaster and should be avoided, that is, wet the base coat prior to application
adjacent material. This type of staining may occur where of the finish coat.
insulation with or without vapor barrier, or other material (2) Plastic film, when taped or weighted down around the
containing asphaltic or coal tar derivatives, fireproofing salts, perimeter of the plastered area, can provide a vapor barrier to
and so forth, can migrate with moisture movement to the retain the moisture between the membrane and plaster. Care
finished plaster surface. must be exercised in placing the film: if too soon, the film may
X1.3.3 Integrally colored plaster can be discolored or al- damage surface texture; if too late, the moisture may have
tered in shade if subjected to moisture, either from uncured already escaped.
base coats or external sources, such as rain, too soon after (3) Canvas, cloth, or sheet material barriers can be erected
applications. to deflect sunlight and wind, both of which will reduce the rate
of evaporation. If the humidity is very low, this option alone
X1.4 Installation Instructions: may not provide adequate protection.

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 C 926 – 06
SUMMARY OF CHANGES

Committee C11 has identified the location of selected changes to this specification since the last issue,
C 926 – 98a(2005), that may impact the use of this specification. (Approved June 1, 2006)

(1) Removed reference to “plastering section” in A2.3.1.3.

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