Technical Bulletin Corporate: P.O. Box 397 Fortson Georgia FAX Technical: FAX

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1 Technical Bulletin Corporate: P.O. Box 397 Fortson Georgia FAX Technical: FAX MW# Topic: Stucco Cracking Stucco is a field mixed and applied product. Sourcing can vary from a Master Wall One Coat Stucco (OCS), Master Wall Cemplaster Stucco or traditional stucco, which is a mixture of cement, sand and lime and a Master Wall Superior Finish. While all stuccos are manufactured with quality materials, even the best mixed and applied stucco can crack from time to time. This bulletin explores some of the various causes and strategies for minimizing cracking in stucco. Like any construction project, there are several factors involved including the owner s expectations, basic stucco design, project scheduling & construction, shrinkage cracks and structural cracking. All have a bearing on the quality of the finished project. Owner s Expectations Some owner s expect the best. In stucco that would mean a very smooth looking project with very minimal cracking, if any. Specifiers and applicators can be prepared to deliver with an improved stucco, which would include Stucco co Ad-Liquid Liquid, a second reinforced base coat using F&M, MBB or F&M Plus and Standard Mesh, Primecoat, and a Superior Elastomeric Finish. Lesser stucco systems can still perform extremely well, but the owner or architect needs to fully describe the expectations and the degree of quality required. Disclaimer This Technical Bulletin is published for general informational purposes only and is not intended to imply that these are the only materials, procedures, or methods, which are available or suitable. The products and manufacturer trade names and product use are believed reliable and not guaranteed. Materials, procedures, or methods may vary according to the particular circumstances, local building code requirements, design conditions, or statutory and regulatory requirements. While the information in this Technical Bulletin is believed to be accurate and reliable, it is presented without guarantee or responsibility on the part of Master Wall Inc Master Wall Inc. Page 1 of 6 6/05

2 Basic Stucco Design There are two basic standards for stucco, ASTM C926, Standard Specification for Application of Portland Cement-Based Plaster and ASTM C1063, Standard Specification for Installation of Lathing and Furring to Receive Interior and Exterior Portland Cement- Based Plaster. These publications are available from the American Society for Testing and Materials ( Parge coats may be applied directly to clean, unpainted concrete and masonry according to this standard. More commonly, stucco is applied over a sheathing, which requires the use of control joints These joints help to control the cracking in stucco. In general, the following guidelines apply for stucco over sheathing: A. Structural 1. Design for maximum allowable system deflection, normal to the plane of the wall, of L/ Design wall assembly for wind load in conformance with code requirements. B. Moisture Control Prevent the accumulation of water into or behind the Portland cement-based plaster (stucco), either by condensation or leakage into the wall construction, in the design and detailing of the wall assembly. a. Provide corrosion resistant flashing to direct water to the exterior where it is likely to penetrate components in the wall assembly, including, above window and door heads, beneath window and door sills, at roof/wall intersections, decks, abutments of lower walls with higher walls, above projecting features, and at the base of the wall. b. Wall System Design design wall to eliminate vapor condensation within the wall assembly. c. Weather Resistant Barrier Provide a barrier over framed construction consisting of a minimum of one (International Residential Code) to two-layers (International Building Code) asphalt felt meeting ASTM D-226, Type 1, Grade D or building code-approved alternate. Verify requirements with local building code authority. 1. Protect sills of rough openings with water resistant barrier or peel and stick type membranes recognized by local codes. Where casing bead is used back-to-back at expansion joints back joints with barrier membrane. Refer to Master Wall details. 2. Slope sills and other areas a minimum of 1:2 to shed water. 2

3 C. Grade Condition 1. Keep stucco a minimum of 6 to 8 above grade in framed construction (varies by Code). D. Aesthetic Projections 1. Aesthetic projections may be formed out of stucco or EPS Foam. Slope all projections a minimum of 1:2 to shed water. Follow local code guidelines for the installation of foam plastic insulation (maximum 4 thick). E. Expansion Joints 1. Provide expansion joints where directed by the design professional. Common locations include the following: A. Where building movement is anticipated. B. At dissimilar substrates. C. At floor lines in certain wood framed constructions. D. Where the stucco meets dissimilar materials. 2. Expansion joint design depends upon the anticipated movement. Master Wall suggests the following minimum sizes, subject to design acceptance and detailing: Windows/Doors 3/8, Building Expansion/Dissimilar Substrates & Materials ½, Floor Line (shrinkage) ¾. F. Control Joints 1. Provide control joints where directed by the design professional. Common locations include the following: A. To limit cracking in the system at a maximum area of 144 ft 2 (13.4 m 2 ). B. Length to width ratio should not be more than 2.5:1. C. At dissimilar substrates. 2. Increase joint requirements where thicker stucco or special structural conditions exist. 3. Typically control joints are tied to the metal lath, which is cut to make two discontinuous slabs. 4. When stucco is bonded to a solid substrate such as concrete or masonry the control joint requirements may be revised. Control joints may be aligned with any control joints in the plaster base. G. Sealing or caulking of V-grooves, exposed ends, and edges of plaster panels exterior work to prevent entry of water shall be provided by sealant trades (07920). H. The designer shall indicate location of joints, accessories and accessory type on architectural drawings. 3

4 Project Scheduling & Construction Scheduling and construction has a bearing on the amount of stucco cracking. Preferably, the stucco should be installed near the end of the project for best results. Builders should look to correct the following conditions prior to the application of stucco: Build on a solid foundation and verify that site has proper compaction prior to stucco application. Make sure carpenters install outside framing members crown out for a consistent, even wall. Make sure the straightest framing members are used where cabinets are placed Verify that wood panels are properly spaced to allow for structural, thermal or seasonal movement (generally 1/8 to 1/4 gap). Refer to the APA publication, Installation of Stucco Exterior Finish Over Structural Wood Panel Wall Sheathing at Make sure a proper gap in the sheathing is provided at the floor lines for crossgrain shrinkage, if required. Please reference Technical Bulletin MW# for additional considerations. At the very least, load the home or building with drywall prior to stucco application. Preferably have the drywall installed as nailing or screwing can affect the stucco. Load home or building with roofing materials prior to lath application. Have cabinets on outside walls installed prior to stucco application. Shrinkage Cracking Shrinkage Cracking Shrinkage cracks, commonly called hairline, eggshell or check cracks are usually caused when the stucco cures or dries quickly. Rapid drying under hot or windy days also contributes to the problem. 4

5 These small cracks are generally less than 1/16 wide and can be somewhat normal for stucco. The small cracks are usually covered with a Master Wall Superior Finish or Superior Elastomeric Finish, which is designed specifically for stucco hairline cracks. Obviously, hairline cracks that occur after the finish is applied will show up in the finish. Hairline cracks are rarely a major source of water entry for properly designed stucco walls. Beyond the design basics, there are several strategies to reduce the amount of hairline cracks in a wall. They can include the following: Fog the wall during hot or windy conditions. See Technical Bulletin MW# for specific recommendations. Let the stucco cure a minimum of seven days or until the stucco is clean, dry and hard. Add Master Wall Stucco Ad-Liquid to the stucco mix. This acrylic modifier forms a film that helps reduce cracking and strengthens the stucco mix. It also eliminates the need to fog the wall if the levels are high enough. Add fibers to the traditional stucco. Cemplaster and Master Wall One Coat Stucco both have fibers for better crack resistance. Add a leveling base coat and mesh to the stucco. Using either F&M, MBB or F&M Plus and Standard Mesh will not only help level the surface, but it will cover and reinforce any hairline cracks. Use Superior Elastomeric Finish over the hairline cracks. This product is designed to bridge minor hairline cracks in the stucco. 5

6 Structural Cracking Stress transferrred to the stucco often results in structural cracking. These cracks are a direct result of some external force placed on the stucco. Determining the exact cause of the cracking may require the involvement of a stucco expert or engineer and may include the following conditions: Ground or foundation movement Earthquakes Structural or frame movement Point Loads Green Lumber Wood panel expansion due to improper gapping Structural, mechanical or other movement Wind loads that exceed design Excessive deflection In summary, even the best applied stucco can crack. A thorough review of the owner s expectations tempered with good design, application and construction will minimize cracking issues. For any specific concerns please contact Master Wall Technical Services at