Jeff Bowlsby CCS, CCCA
Exterior Wall and Stucco Consultant
Licensed California Architect
Stucco Finish Coat Texture
The stucco finish coat can be used to create a finish texture or pattern which can be an important characteristic of how a stucco installation is perceived, experienced, performs and is evaluated.
Visit the StuccoMetrics Reference Archives webpage for cited references and further information.
In the past, in some but not all regions, the term ‘stucco’ may refer to the finish coat only or to other meanings. Other connotations refer to ‘stucco’ as exterior cement plaster and ‘plaster’ as interior cement plaster. This website construes the term ‘stucco’ more broadly, to include the complete portland cement plaster cladding system, for exterior building applications.
ASTM C1063 Standard Specification Installation for Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster(1)
· (7.1) Workmanship: Erect metal furring and plaster bases to receive specified plaster thickness and achieve finished stucco surfaces true to line within 1/4-in. (6.4-mm) in 10-ft (3.05-m), level, plumb, square or curved as designed.
ASTM C926 Standard Specification for Application for Portland Cement-Based Plaster(1):
· (184.108.40.206) bedding coat: a plaster coat for embedding aggregate or other decorative material as an exposed finish.
· (220.127.116.11) finish coat plaster: Final, exposed plaster layer
· (3.2.15) factory-prepared plaster: Factory-prepared dry-blended proprietary plaster material combinations, requiring only adding water and mixing before application.
· (3.2.17) floating: The process of either using a float tool to densify, level and reasonably true the brown coat plaster, or bringing the aggregate to the surface of finish coat plaster.
· (3.2.27) stucco finish: Factory-prepared dry-blended proprietary plaster material combinations, for finish coat plaster applications.
· (3.2.29) texture: Any plaster surface profile other than a smooth surface
· (4.4.3) Sand, jobsite mixed finish coat plaster: Specification C897
· (6.1.2) Plaster mixes are symbolized as a function of the base over which they are applied: Specified in Table 1
· (6.1.4) Mixture proportions, jobsite mixed finish coat plaster: Specified in Table 3.
· (6.1.5) Factory-prepared finish coat plaster: Reference 3.2.15.
· (6.2.3) Finish coat plaster: Retempering not allowed.
· (7.1.2) Plaster nominal thickness: Measured from outer plaster surface exclusive of texture variations, to either back plane of metal plaster base exclusive of self-furring provisions, or to outer surface of WRB or solid plaster base.
· (7.2.3) Apply finish coat plaster with sufficient material and pressure to assure continuous bond, coverage of base coat and nominal thickness required by Table 4 and 18.104.22.168.
· (22.214.171.124) Three-coat work: Apply finish coat plaster following 7.2.3
· (126.96.36.199) Two-coat work: Apply finish coat plaster following 7.2.3
· (7.4.1) Finish coat plaster, either jobsite mixed or factory-prepared: Apply by machine or by hand, following 7.2.3.
· (7.4.2) Avoid excessive water use while applying and finishing finish coat plaster.
Appendix X1.2 (Nonmandatory Information) Finish Coat Categories (applicable to both natural and colored finishes):
· (X1.2.1-X1.3): Narratives describing a variety of textures.
Why is it in contemporary American architecture where stucco is a predominant exterior wall cladding material, that it is most often used to emphasize a flat, planar, uniform, soul-less surface? It is often placed over irregular substrates, adjacent to other materials that are extruded, machined, cut or fabricated with high precision for linearity and planarity. In that visual context, it is natural to visually compare stucco to those other materials, i.e. with the expectation of as much linear and planar precision as possible. The demand is strong especially in modern architecture, for a machine-like, smooth trowel finish that emulates plaster over masonry, with no cracks and no joints.
Stucco bases including solid masonry walls of brick or concrete masonry units, cast concrete, or scratch/brown portland cement plaster over lath, are in and of themselves durable materials that with minimal maintenance should remain serviceable for the life of any building. However, when exposed, these materials are subject to weathering and deterioration, and sometimes these materials are not attractive aesthetically. A stucco finish coat can be installed over bases which enhance the aesthetics and coloration of the building and improve durability by functioning as the exposed layer of protection against water-related deterioration. Texture is an articulation of the exposed stucco finish surface and only occurs in the stucco finish coat.
This webpage focuses on the textural possibilities available for stucco. Architects realize that textures can be an important aspect of the architectural expression, tactile experience and sculptural qualities of a building. Finish coat texture selection and specification can be an important factor when desiring that the visibility of stucco cracks and base coat surface imperfections be concealed. Cracks can be more noticeable with smooth finish textures, and are less noticeable with more heavily articulated finish textures.
To pursue and achieve that perfectly flat, planar, joint-less, crack-less stucco finish coat aesthetic is an admirable goal but far from reality. The vast majority of substrate construction in the USA is not masonry, a substrate that has few if any joints, but framed construction which requires stucco with movement joints for performance reasons.
Acknowledging the preference for smooth stucco surfaces in certain contexts, it is also important to realize that our buildings can benefit by expressing the unique fluid, plastic qualities of stucco in other settings. Portland cement-based plaster is a fluid, plastic material when it is installed, and it is within its nature to express its fluid, plastic qualities, even more so than when used as a flat plane. Stucco can be used to clad an entire building, or portions of a building for aesthetic or architectural, compositional effect. A range of different textures can be used on the same building, with aesthetic discretion, for various aesthetic effects.
Textural possibilities are limited only by the imagination and variations in the finish coat application methods, tool selection and materials used. Textures from the smoothest surfaces (including metallic emulations), to fine-, medium-, and coarse- grained textures, to embossing, scoring, and finally to three-dimensional sculptural effects that are molded integral with the wall but project from the wall. Textures are influenced by the selections of materials used such as aggregate type, size and density. Small dimension aggregates typically result in smooth finishes, coarse aggregates allow heavy articulation. Aggregates can be mixed into the finish stucco matrix or embedded by casting onto a bedding coat. Decorative inorganic exposed aggregates can be implemented for a variety of effects – glass fragments and beads, glazed tile, mica flakes, stone chips, sea shells, crushed pebbles and small gravel – what other materials can be used?
Stucco finish coat materials consist primarily of an aggregate which provides durability and color, a binder which adheres to the aggregate and stucco base, and often an integral pigment or pigmented surface coating. To be durable, aggregates must not be adversely affected by water (non-ferrous) or by alkalis from the stucco base or binder. Aggregates may include silica sand, marble, granite and similar inert materials of various gradations. Aggregate gradations range from the finest granularity (almost a powder) for smooth finishes to small dimension gravels for the coarsest, most articulated, tactile textures. Stucco finish coat binders can be portland cement-based, lime-based or polymer-based and durability is primarily determined by the binder material characteristics. Portland cement-based finish coat binders are durable and common, whether they include integral pigments or are painted. Lime-based finish coat binders were the traditional finish coat material until portland cement became more abundant and popular, but remain available. Polymer-based (typically acrylic) finish coat binders, as a carry-over from the EIFS industry, are becoming prevalent in contemporary construction. Each of these different finish coat materials has different aesthetic and performance characteristics. Not all textures can be created using each different binder material.
Textures are also influenced by the installation techniques and tools used by craftsman. Stucco finish coats may be simply trowelled or sprayed on and textured during installation. Smooth surface trowels, steel and plastic, result in smooth finish textures, sponge and wood floats create rougher textures. What other tooling can be used to create desirable, unique pattern or texture variations? With imagination, implementing a variety of interesting finish textures in addition to the standard sand float and spray-on knockdowns that are so common, can be explored and implemented. Make the stucco finish texture an important aesthetic feature of the spatial experience. Use your imagination. Be creative.
Plastering and Texturing Tools
From Plasterer’s Manual P21, Portland Cement Association, 1948(2)
Photo used with permission of the Portland Cement Association
One of several illustrated texturing techniques
From Plasterer’s Manual P21, Portland Cement Association, 1948(2)
Photo used with permission of the Portland Cement Association
Anticipate that manually-applied finish texture coats will not be perfectly uniform as when produced with machine assistance. Variances in manually-applied finish coats are germane to its character and artistic expression. A higher level of artistic craftsmanship is required for the most creative textures.
Artistic expressions of stucco finish textures by imaginative and experienced craftsman can include emulated naturescapes with rocks, boulders, naturalistic stone outcroppings, tree trunks, roots, which can integrate water features, creeks, streams, waterfalls and landscaping materials. Stucco can be an excellent emulation media for stone and specialty naturalistic environmental settings and where integrated with a building design can become “one with nature”.
More sophisticated decorative patterns and effects similar to Venetian plasters such as Sgrafitto may be possible using portland cement plasters. It is time to experiment.
Stucco finish coat textures are an important tool for aesthetic expression and are virtually limitless in variation.
Minimum Standard of Care: Architect specifies the required stucco texture or specifies which entity selects and approves the stucco texture, which could be the owner, the architect, the installer or another entity.
Stucco Best Practices:
· Architect specifies finish coat material and texture requirements by including a physical sample or a photo (use the images from this website or other resource) and inserting them onto the construction drawings or specifications.
· Architect specifies that mock-ups of the required finish coat material and texture be submitted for review and approval on a free-standing mock-up on site, before installation. An accepted mock-up becomes the standard to judge installed work.
· The stucco craftsmen must understand the specified finish coat material(s) and texture(s) and the requirements necessary to achieve the specified results during bidding and submits the texture for review and approval before installation.
· Select medium to rough textures to minimize the appearance of cracks. Cracks and base coat surface imperfections appear more noticeable in smooth trowel finishes, than in heavier textures.
Stucco textures can include a full array of not only artistic workmanship based texturing techniques, but also embedded and exposed aggregates, mica, glass, embossed patterns – the possibilities are almost endless. Experimentation is suggested, to explore the possibilities for architectural expression.
Stucco Best Practice: Where more than one stucco craftsman applies the texture on a project, practice and coordinate the installation to minimize and avoid visible variations in the texture.
(1) ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
(2) Portland Cement Association, 5420 Old Orchard Road, Skokie, IL 60077-1083
Consultation with licensed and experienced stucco professionals is recommended for stucco-related endeavors. No liability is accepted for any reason or circumstance, specifically including personal or professional negligence, consequential damages or third party claims, based on any legal theory, from the use, misuse or reliance upon information presented or in any way connected with StuccoMetrics.com.