Stucco Stone FAQ
For more information:
Get a free estimate
Visit our Bay Area showroom
Contact us online
Or Call 888-957-7800
- 1: Can I paint stucco to get the color I want?
- 2: How do you create ornamental shapes on plaster surfaces?
- 3: What are common stucco finish textures? Is there anywhere to view them?
- 4: How long does stucco last on a building?
- 5: In a stucco frame-wall assembly, what purpose does building paper serve?
- 6: What is the correct thickness of stucco?
- 7: Is it necessary to use a bonding agent with stucco?
- 8: Does stucco require curing, and if it does, how is this best accomplished?
- 9: Can stucco (Portland cement plaster) be applied directly over painted brick?
- 10: Plaster is placed in two or three layers, or “coats.” Are there guidelines or restrictions on placing each successive coat?
- 11: What kind of fire rating does plaster provide?
- Stucco Overview
1: Can I paint stucco to get the color I want?
Stucco can be painted. Portland cement-based paints are very compatible with stucco because they are made of the same material. These paints should be scrubbed into the surface and fully cured. Alternatively, you could consider a colored stucco finish. These finish coats are often made with white cement and pigments, providing the widest range of colors. Premixed materials are color matched from batch to batch and are most consistent. Additionally, the fact that you are placing a finish coat with a nominal thickness of 1/8 in. instead of a paint layer usually gives more assurance of complete coverage. It is possible to paint with other types of paint, though these are usually not as long lasting as cement-based paint. Acrylic paints are long lasting and durable but change the permeability of the stucco (make it non-breathable) which in some climates may have adverse effects on the long-term performance of the system.
2: How do you create ornamental shapes on plaster surfaces?
Stucco finishes are popular across North America. They lend themselves to nearly every type of architectural style. Certain styles can be enhanced with built-out shapes, such as cornices, quoins, or decorative tiles. Achieving these details on plaster finishes has evolved over time to today’s simple techniques.
Creating Shapes
Shapes are sometimes referred to as “plant-ons” because that’s how they are attached to stucco surfaces. An expanded polystyrene foam section is bonded to the basecoat with a material made specifically for that purpose. Some people use an EIFS basecoat material as the glue. This is attached to a portland cement plaster base, typically the brown and scratch coats, before final finishing. The shape is then finished like EIFS: covered with a basecoat and mesh, then a finish coat.
What to Consider
The shapes must be securely attached to the wall. The basecoat material acts like a glue to hold the backside, then also embeds the mesh that goes over the top of the shape. As the foam itself has no structural strength, the mesh and basecoat together provide an impact-resistant surface to the shape, protecting it in service.
3: What are common stucco finish textures? Is there anywhere to view them?
The Technical Service Information Bureau (TSIB) is a trade group in southern California serving the needs of the wall and ceiling industry regarding lath, plaster, and drywall. They have an excellent online resource depicting plaster textures.
The 30 textures shown on the site are accompanied by suggested application procedures. This gives material (ingredient) advice, where appropriate, and methods of applying or finishing the plaster to achieve specific appearances. For instance, the sand float finishes are described as light, medium, or heavy, and the grain size of aggregate helps achieve the desired texture. All of the textures can be made with gray or white cement, with or without pigments.View plaster textures.
It is important to be aware of regional differences in naming finishes. Certain parts of the country may call a specific texture by another name than described by the TSIB. That is why this site is so beneficial: it provides visual depictions of each finish to prevent misinterpretations that might occur with verbal descriptions.
4: How long does stucco last on a building?
While the service life of stucco can’t be quantified as a specific number of years, properly applied and maintained portland cement plaster, or stucco, is as durable as any commonly used cladding material. Its hard surface resists abrasion and can take a lot of physical abuse. It stands up to all sorts of climates, from cold to hot and wet to dry. Many older homes built in the early 1900s have had very little maintenance and remain in good shape today.
5: In a stucco frame-wall assembly, what purpose does building paper serve?
Stucco is known to be a weather resistant building finish, but it is part of a system. In order for the wall to resist water penetration effectively, the system must be properly designed and detailed, then built according to plans.
The main purpose of building paper is to keep water from contacting the substrate and structural support members—very commonly sheathing like plywood or OSB (oriented strand board) and wood or metal studs—so that these materials stay dry. Metal can rust and wood can rot. Also, wood is prone to expand and contract with changes in moisture, so it’s essential to keep sheathing dry to provide the plaster with a sound substrate. Minimizing the changes in moisture minimizes the stresses that might be placed on plaster from behind. In addition to structural considerations, excess moisture within a wall creates a potential for mold or mildew inside buildings.
Building paper prevents moisture-related problems in stucco walls. Several industry documents, such as PCA’s Portland Cement Plaster/Stucco Manual(EB049), ACI’s Guide to Portland Cement-Based Plaster, and building codes across the country, recommend 2 layers of paper. During construction, paper can be damaged. Two layers of paper provide greater assurance that water won’t get to the sheathing or support members. Paper should be lapped like siding, meaning that upper layers are placed over lower layers. This facilitates drainage toward the outside. Where the edges of paper-backed lath meet, connections should be lath-to-lath and paper-to-paper.
Building paper should comply with the current requirements of UU-B-790a, Federal Specifications for Building Paper, Vegetable Fiber (Kraft, Waterproofed, Water Repellent, and Fire Resistant). This specification differentiates weather resistive Kraft papers by types, grades, and styles. Grade D is a water-vapor permeable paper. Grade D paper with a water resistance of 60 minutes (or more) works well for stucco applications, and is often preferred to Grade D paper having the minimum 10-minute resistance required by UU-B-790a.
Some specifiers are turning to house wraps for stucco underlayment. While these materials may be more rugged than paper—and therefore less prone to damage during installation—a single layer is still not adequate according to many industry professionals. At best, a hybrid system, with the house wrap closest to the sheathing and covered with the paper, seems to be an acceptable alternative.
6: What is the correct thickness of stucco?
Stucco thickness depends on the backup system and on whether or not lath is present. In ASTM C 926, the Standard Specification for Application of Portland Cement-Based Plaster, thicknesses are provided for scratch, brown, and finish coats.
Over frame construction, lath must be used. Over solid substrates—which include concrete masonry, cast-in-place concrete, and precast concrete—lath is sometimes used. When lath is present, three-coat plaster is recommended. Note that frame construction—metal or wood studs—may or may not have sheathing present, but that plaster thickness is independent of sheathing. With lath, total plaster thickness is 7/8 in.
Three-coat work can also be specified for solid plaster bases without metal lath. The correct thickness is then 5/8 in.
Two-coat applications are only for use over solid plaster bases without metal lath. For unit masonry, that thickness is ½ in. For cast-in-place or precast concrete, the thickness for two-coat work is 3/8 in. These are direct-applied systems, meaning that there is no metal lath involved.
It is important to note that the committee in charge of ASTM C 926, the reference document for this application, has decided to keep the term “nominal thickness” in the title of the table for two- and three-coat work. This term takes into account that walls are built to certain tolerances and may not be exactly plumb or plane. The reference to a nominal thickness allows for small variations from an exact dimension. The intent of the specified thickness is to provide a reasonable system that, over many years, has proven itself to be weather resistant and durable. Local building officials should be consulted for further information about variations from the specified thickness.
7: Is it necessary to use a bonding agent with stucco?
Products that increase the adhesion of plaster to substrate or plaster to plaster are called bonding agents, and are either surface applied to a substrate or integrally mixed into the plaster.
A distinction should be made between framed construction and solid backing (such as masonry or concrete). Framed construction requires the installation of moisture-resistant paper behind the lath. You do not want or need to have plaster bond to the paper, so bonding agents are not used with framed construction, only solid surface substrates.
If contamination is present on the substrate surface, good bond is inhibited. This is generally not a concern with new masonry walls, but can be an issue with new cast-in-place concrete as it may have residual form release agent on its surface. Older concrete or masonry walls may have bond-inhibiting characteristics, in the form of paint, sealer, some other coating, or dirt on the surface. As such, bonding agents are more likely to be considered for repair and renovation work over either concrete or concrete masonry.
It is generally good practice to prepare the solid substrate so a bonding agent is not necessary. The prepared surface should be clean (all surface materials removed), sound (hard surface), and mechanically roughened. Methods for achieving these criteria include sand blasting and high-pressure water blasting. When this type of preparation does not result in a clean, sound, and roughened substrate, bonding agents offer another solution.
Bonding agents have different chemical formulations, so they have different performance characteristics. Bonding agents do not guarantee performance. You will need to research the material to find out which is best suited to your particular conditions. But where prepared surfaces seem questionable, and lathing is not an option, a bonding agent may be beneficial.
Surface-applied bonding agents should conform to the requirements of ASTM C 932 (www.astm.org). Integral bonding agents should be used only after review of the manufacturer’s documentation of testing and past performance.
Dash-Bond Coat Rather than using bonding agents, another option for low-absorption surfaces is to apply a dash-bond coat. This cement-rich slurry is dashed against the base surface by hand with a brush, trowel or paddle, or by machine. Most of the surface is covered with the plaster. The high cement content provides a tenacious bond. This material is left unfinished so that a rough base is created for the scratch coat.
back to top
8: Does stucco require curing, and if it does, how is this best accomplished?
For cement-based materials, curing is defined as maintaining an appropriate temperature and moisture content for a specific period of time during the early life of the material. All portland cement-based materials, such as stucco, require curing. Since the addition of water to portland cement sets off a chemical reaction called hydration, it’s important to provide excess water to the cement particles so that they develop a good bond with their surrounding environment: aggregate and other cement particles. This is how plaster (and concrete, mortar, and grout) hardens. Plaster sections are quite thin, ranging from about 3/8 in. to 7/8 in. total, and individual coats may be only 1/8 in. thick. Thin layers such as this must be protected from conditions that interfere with cement hydration: things that dry them out or heat or cool them excessively.
Wet and Dry
Sun and wind, alone or in combination, drive moisture out of fresh plaster. To be applied to a wall, plaster must be fluid enough to be troweled, screeded, and floated, but not too wet that it sags or won’t stick. Base coats, of which there may be one or two (sometimes scratch and brown are combined), can be wetted once they have developed adequate strength so that they are not washed away by the water. Since the coats are thin, they can’t hold as much moisture as is ideal for curing—especially if they are competing with sun or wind, which both cause evaporation.
Plaster can be wetted periodically throughout the day to supply additional curing moisture; usually one or two times per day should suffice. In extreme conditions, sun and wind breaks can be used to provide extra protection from the elements. The first two days are the most critical period. The entire first week is important, however, so it is a common recommendation that the base coat stucco be misted or fogged periodically for the first three to seven days after placement. A sheet of polyethylene can be placed over the moistened surface to hold the water in. If the relative humidity of the air is greater than 70%, moist curing may be accomplished without additional wetting of the surface.
A caution about moist curing is that colored finishes can be affected by water application. Finish coat stucco is not moist cured since this may promote mottling and discoloration. Curing of colored finishes is typically done by wetting the base coat to provide curing moisture from behind the finish and ensuring that the surface is shielded from drying.
Hot and Cold
Proper curing also requires that plaster be in a medium temperature range. Usual recommendations range from 40F on the low side to 90F on the high side (about 4C to about 32C). Too cold and there is a risk that water in fresh plaster would freeze. As this is an expansive process, cracking could occur. Cement hydration can be interrupted, too. Too hot and there is a risk of drying—which, like freezing, can also suspend cement hydration—or of accelerating the hydration process to a point where strength development in the longer term is negatively impacted.
Curing compounds are effective for concrete but are not used regularly on plaster. These materials might interfere with subsequent coats of plaster and might lead to discoloration of the stucco finish.
9: Can stucco (portland cement plaster) be applied directly over painted brick?
This is a common question that often arises when people are rehabbing or updating older construction. Plaster is a cost-effective finish, relatively easily installed, that improves the appearance and creates a water-resistant wall surface.
A painted surface will not typically absorb water and, as such, is a substrate to which stucco will not readily bond—at least not uniformly. There are two basic alternatives to covering a painted brick surface with a new coating of portland cement plaster.
1. Sand blast or water blast to remove the paint in its entirety, then direct-apply a two coat system. It is essential to have a surface that is uniformly absorptive to accept the plaster coating. In addition, it may be beneficial to use a bonding agentor dash-bond coat with this approach.
2. Attach paper-backed lath or install appropriate building paper between wall and attached metal lath to provide a moisture barrier and to serve as a bond breaker. Apply traditional three-coat stucco to metal lath and accessories. In this approach, the idea is to treat the plaster like a sheathed system, using metal lath to support the plaster on the substrate, while completely isolating the plaster layer from the backup with building paper. This prevents a partial bonding situation, which could set up undesirable stresses in the plaster and lead to cracking.
10: Plaster is placed in two or three layers, or “coats.” Are there guidelines or restrictions on placing each successive coat?
Industry documents contain guidelines for placing each coat of stucco in two-coat or three-coat systems. The recommendations are there to ensure 1) proper curing and 2) strength development of the previously installed coats.
To cure cement-based materials means to allow them the opportunity to hydrate, and this requires both adequate moisture and proper temperature. The International Building Code (IBC) provides rules for the minimum time delay between each coat of plaster (see table). Exterior plaster has more stringent requirements because it is typically subject to more severe exposure than interior plaster.
Time Delay Between Each Coat of Exterior Cement Plaster Based on IBC Requirements:
| Coat | Moist curing (minimum period) | Minimum interval between coats |
| First | 48 hours* | 48 hours* |
| Second | 48 hours | 7 days |
| Finish |
— |
— |
|
*An exception to these time limits occurs with a method called double-up or double-back. See below. |
||
There are two related, but distinct, reasons for delaying application of a subsequent layer of plaster on top of a previously installed layer. One reason is that there has to be adequate moisture retained in each layer. Usually, this is accomplished by misting the plaster once or twice during the day. It can, however, be adequately accomplished by preventing evaporation of moisture from the plaster—and placing the next layer of plaster on top of the other does that. But it’s only allowed if the underlying layer has gained adequate strength. That is the second reason for a time delay between coats: making sure the plaster isn't’t damaged by application of another layer.
This exception to the rule—referred to as a double-up or double-back procedure—allows for more efficient labor practices. The wording indicates that a second coat of plaster (brown coat) can be applied to the first base coat (scratch coat) as soon as the first coat has attained sufficient rigidity to receive the application without damage. This often occurs on the same day (see the footnote to the table above). Curing of the second coat then proceeds per the table.
Following guidelines for curing and delay between coats usually result in the most consistent surface finish—uniform texture and color.
11: What kind of fire rating does plaster provide?
Portland cement-based plaster, commonly called stucco, has long been and continues to be a popular choice for finishes on buildings. It allows for a wide expression of aesthetics, is a cost effective finish, is durable in all types of climates (especially wet ones), and offers fire resistance. Fire resistance is typically classified by a fire rating, but what kind of fire rating does plaster provide?
Things that influence the fire rating of a plaster system include the type of material used for the support member, the size of the support member, the presence/absence and type of exterior sheathing, the aggregate in the plaster mix, the presence/absence of insulation, presence/absence of interior wall finishing materials (gypsum wallboard, etc.) and the thickness of the section. The type of member—wall, partition, ceiling, or other, and member classification (load bearing-LB or non-load bearing-NLB)—also influences the rating.
In 1991, the Foundation of the Wall and Ceiling Industry published a reference guide on portland cement-based plaster/stucco systems used for fire protection, the Single Source Document on Fire-Rated Portland Cement-Based Plaster Assemblies. Designers, specifiers, building code officials, contractors, and the general public are the intended audience. The information contained therein is “not intended as design or installation criteria,” but can help people determine how to assess their assemblies using the referenced publications, fire test reports, industry standards, and codes.
For example, a typical residential application might be a 3-coat system of plaster over 2x4 wood studs using metal lath attached to the studs, either with or without a layer of sheathing, like plywood. On the interior side would be a layer of gypsum board. The detail for a system made with these components is assigned a 1-hour fire rating based on 1988 Uniform Building Code information.
Stucco, or portland cement plaster, is a versatile facing material that can be applied to flat or curved surfaces either inside or outside any building or structure. Stucco has great appeal as a surface finish because of its utility, low first cost, and minimum need for maintenance.
In its hardened state, plaster is a desirable facing material: hard, strong, fire-resistant, and color-retentive. Because plaster is also breathable, orable to transmit moisture vapor, water that gets behind it does not become trapped. That makes it resistant to rot and fungus. Plaster has proved to be a durable wall cover in all climates, whether wet, dry, hot, or cold.
The final appearance of the finish coat can be varied by changing the size and shape of the aggregate, using colored cement, adding pigments, changing the consistency of the finish mix, the method or equipment used for plastering, and the plasterer's skill in manipulating the finish coat.
Stucco
Traditional Portland cement plaster (stucco) is a time-tested exterior finish. It consists of Portland cement-based materials and sand, mixed with water to form a workable plaster. Portland cement, the same material that is the basis for the hardened properties of concrete used to build super-highways, bridges, and skyscrapers provides strength, durability, and toughness in Portland cement plaster.
Portland cement plaster is applied either by hand or machine to exterior and interior wall surfaces in two or three coats. It may be applied directly to a solid base such as masonry or concrete walls, or it can be applied to a metal lath attached to frame construction, solid masonry, or concrete construction. Applied directly to concrete masonry, Portland cement plaster provides a tough 1/2-inch thick facing that is integrally bonded with the masonry substrate. When applied to metal lath, three coats of plaster form a 7/8-inch total thickness. A vapor-permeable, water-resistant building paper separates the plaster and lath from water sensitive sheathing or framing. Portland cement plaster has high impact resistance, sheds water, but breaths, allowing water vapor to escape. It’s a proven system that works in all climates.
Traditional Stucco Is Strong And Beautiful
Traditional Portland cement stucco provides a tough exterior that resists attack from woodpeckers to weed whackers. That inherent toughness is beautifully complemented by the variety of colors and textures available. Appearance is an important consideration in selecting a wall cladding. You can select a color and texture consistent with the message and image you wish to convey.
Color is determined by selecting cement and aggregate color, and quite often modified by adding mineral oxide pigments to the plaster mix. In many areas, factory-prepared finish-coat products are available. Pre-pigmented packaged cements also can be used to achieve the desired stucco color or pre-weighed mineral oxide pigments can be added to the finish coat stucco during mixing.
Texture is achieved by selecting aggregate size, controlling finish mix consistency, and using special treatment techniques during and after application of the finish coat plaster. Texture gives substance and character to the plaster surface. It can be used to provide highlights, depth, continuity, segmentation, and even achieve the look of a completely different construction material such as wood timbers, brick, or stone masonry construction. To confirm the suitability of a desired color and texture, be sure to ask your plasterer to provide a sample panel for evaluation prior to starting work.
Portland cement plaster has a well documented history of proven performance in extreme climates from the desert southwest of Arizona, to the severe winters of Minnesota, to the hot-humid Florida summers. It performs in all climates.
Portland cement plaster has a well documented history of proven performance in extreme climates from the desert southwest of Arizona, to the severe winters of Minnesota, to the hot-humid Florida summers. It performs in all climates.
That durability is complemented by its versatility. Portland cement plaster provides an ideal finish or cladding for any building construction system including concrete, concrete masonry, brick masonry, wood frame, or steel frame. Portland cement stucco can be applied to any flat or curved surface either inside or outside.
Don't Mistake Traditional Stucco For "EIFS"
Portland cement plaster (sometimes called traditional stucco) should not be confused with the exterior insulation and finish systems (EIFS) or synthetic stucco systems that have become popular but more recently have been the subject of controversy as a result of performance problems, including water leakage and low impact resistance.
Synthetic stucco consists of a polymer based laminate that is wet-applied, usually in two coats, to rigid insulation board that is fastened to the wall with adhesive, mechanical fasteners, or both. Polymer based (PB) systems, sometimes known as thin coat, soft coat, or flexible finishes, are the most common. The base coat for PB systems is usually only 1/16 in. thick and finish coat thickness is typically no thicker than the maximum sand particle size in the finish coat.
While the PB skin repels water very effectively, problems arise when moisture gets behind the synthetic stucco and is trapped inside the wall. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.
Polymer modified mineral based (PM) exterior insulation and finish systems are sometimes known as thick coat, hard coat, or rigid finishes. The mineral base is Portland cement, while the polymer is usually an acrylic. PM systems have greater impact resistance than the PB systems, but are typically less than 1/3 the thickness of traditional three-coat stucco.
Traditional Stucco is Ideal For Concrete Wall Systems
The durability and versatility of traditional Portland cement stucco provide a great finish for other concrete building systems including concrete panels, insulating concrete forms (ICFs), and autoclaved aerated concrete. Depending on the condition of the concrete surface, Portland cement plaster may be directly bonded to the concrete or applied to metal lath attached to the wall.
When directly bonded to the concrete a dash bond coat or surface applied bonding agent is typically needed to enhance the bond between the plaster and the concrete. When concrete surfaces are extremely smooth or contaminated with excessive form oil metal lath is used as a plaster base. Metal lath can be mechanically attached to concrete, ICF, or to autoclaved aerated concrete walls to provide a base for durable, traditional three-coat Portland cement plaster exterior.
Concrete masonry provides an excellent base for direct application of Portland cement plaster in 2 or 3 coats, consisting of base coat(s) and a finish coat. The use of open-textured concrete masonry units promotes mechanical keying between the plaster and base masonry wall. Because plaster and concrete masonry undergo similar volume changes, they are inherently compatible.
Traditional Stucco Works Great On Frame Construction
Portland cement plaster has been used over frame construction in the United States since before the turn of the century, long enough to develop proven procedures for installation. When Portland cement plaster is selected as the exterior surface for frame construction, metal lath is attached to framing members. Vapor permeable, water-resistant paper is applied over sheathing before attachment of the lath. The paper protects the sheathing and interior of the wall from outside moisture intrusion without trapping moisture vapor in the wall.
Furring dimples in self-furring lath, furring nails, or other furring attachments hold the lath about 1/4 inch away from the sheathing, frame, or solid base wall. This permits the Portland cement to penetrate lath openings and surround the metal strands, embedding them. As the scratch coat plaster hardens, the lath and plaster become rigidly interlocked with the lath, essentially performing as reinforcement. Encasement of the metal also protects the metal lath against corrosion.
Lath may be expanded-metal lath, woven-wire lath, or welded-wire lath. A variety of accessories are also available for use in different plaster installations. Movement joint accessories isolate plaster panels from building movement and accommodate thermal- and moisture-induced volume changes in the finished plaster. Flashing redirects moisture to building exteriors. Foundation weep screeds serve as a plaster stop and directs moisture to wall exterior. The availability of different types of metal lath and accessories increases the versatility of Portland cement plaster and contributes to its serviceability in many applications.
For more information:
Visit our Stucco & Stone Testimonials page
Get an estimate
Contact us online
Or Call 888-957-7800
Email this page
Printer-friendly version