Concrete Home Building Council

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.

Q: In a stucco frame-wall assembly, what purpose does building paper serve?
A: 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.

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.

Frequently Asked Questions

Q: How long does stucco last on a building?
A: 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.

Q: How thick does stucco have to be applied on walls?
A: Stucco thickness depends on the backup system. Solid surfaces like concrete or concrete masonry make excellent substrates. Depending on the application two or three coats may be applied with thicknesses ranging from 3/8 in. over cast-in-place concrete in a two-coat system to 5/8 in. over masonry.

Stud wall construction is also very common. These systems use either metal or wood studs with or without sheathing, a moisture resistant paper, and metal lath, and in all cases the total of the three plaster coats is a nominal 7/8 in.

Q: Can I paint stucco to get the color I want?
A: 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.

Q: What is a fog coat?
A: A fog coat is a light application of a cement-based slurry, the same proportions of cement, lime (if any), and water as used in the original application minus the sand, used to even out a surface’s appearance. It is typically sprayed or rolled onto the surface, similar to painting with a cement-based paint. Fog coating improves the look of stucco without changing its ability to transmit moisture vapor.

Q: Where can I buy stucco?
A: You don’t really “buy” stucco so much as you buy the materials to mix stucco onsite or hire a contractor to do the work. You can purchase materials to make stucco throughout the country at material supply houses and home improvement centers.

There are a variety of acceptable mixture proportions for stucco, and the proportions of each successive coat vary. The individual materials may include portland, masonry, or plastic cement, lime or other plasticizers, sand, and water. The following documents contain tables of mixture proportioning recommendations:

• Portland Cement Plaster/Stucco Manual (sold through BuilderBooks.com)
• Repair of Portland Cement/Stucco
• Guide to Portland Cement Plastering, ACI 524
• Standard Specification for Application of Portland Cement-Based Plaster, ASTM C 926

It is not impossible to handle plastering repairs as a do-it-yourself project, but it is a fairly large undertaking for the average person. You can look to your local Yellow Pages or the Blue Book of Construction, available on line at http://www.thebluebook.com or at your local library to identify contractors in your area.

Q: What is the proper spacing for contraction/expansion joints in portland cement plaster/stucco applications?
A: The proper use of contraction joints in stucco systems will depend on a number of variables, including: the type of construction materials to which the stucco will be applied; the orientation of the construction—vertical (walls) or horizontal (ceilings); and whether the surface is curved or angular.
Stucco may be direct-applied to concrete or masonry substrates; however, if these materials are used together, as in the case of a concrete framework of beams and columns with masonry block infill, a joint may be required at the transition of one material to another. Stucco that is direct-applied to concrete or masonry requires contraction joints only where there is a change in material or where there are joints in the concrete or masonry structure.

Metal lath may be used over concrete or masonry construction and should be used in sheathed frame and open frame construction. When stucco is applied to any construction using metal lath, joint spacing recommendations should be implemented. The recommendations found in the Portland Cement Plaster/Stucco Manual, EB049, are based on ASTM C1063, Standard Specification for the Installation of Lathing and Furring to Receive Interior and Exterior Portland-Cement Based Plaster. Applications that use metal lath require three layers of plaster: scratch, brown, and finish coats.

The joint spacing should meet the following criteria:
• no length should be greater than 18 ft. in either direction
• no panel should exceed 144 sq. ft. for vertical applications
• no panel should exceed 100 sq. ft. for horizontal, curved, or angular sections
• no length-to-width ratio should exceed 2 ½ to 1 in any given panel.

Concrete Home Building Council - 10/20/2005 (Plain Text Version)

Stucco: A Weather Resistant Building Finish