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TECHNIQUES and COMMENTS is a Stucco Newsletter and a division of Roberts Consulting Group
Providing information about 3-coat stucco, One-coat Stucco and Exterior Insulation Finish Systems (EIFS).
278 Stucco Newsletters published over a 40 year period.
By Robert R. Tellez
There are two fundamental types of stresses that cause stucco cracks—internal and external.
Internal stresses are due to the natural curing and drying process of stucco application, which usually occurs within 1 to 2 days. This is why the California Building Code calls for 48 hours of moisture curing for the hydration of stucco. This 48-hour period is when most stucco “internal stress” occurs, causing shrinkage cracks. This shrinkage, when controlled, may create hairline cracks less than 1/16 inch. All internal stress cracks will appear within 14 days. Once the 14-day period is reached, external stress cracks will begin to develop.
External stresses are due to the transfer of outside forces into the stucco assembly. Common examples are at the corners of windows and doors. These stresses can cause movement within the stucco, which can result in wider cracks than internally generated cracks. The following are common, but not exclusive, sources of external stresses that are associated with stucco cracking.
• Movement of framing and sheathing: Lumber and wood-based sheathing are subjected to dimensional variations associated with the loss or gain of moisture. Wood framing is subjected to dimensional variations due to thermal expansion and contraction. This has been proven by Purdue University and the Forest Products Laboratory studies according to USG.
• Improperly applied wood-based sheathing: Wood-based sheathing not properly gapped can swell and put pressure on the stucco. The minimum requirement is 1/8 inch, but we have witnessed that this margin is not sufficient and should be a minimum of 1/4 inch. In our experience, when the OSB is removed, the OSB or plywood is tight at all of the seams. Moreover, the 4x8 sheathing is one of the primary causes of external stress onto the stucco panel. It will expand in all five directions as the wall temperature increases.
• Soil Movement: Seismic events, settlement, and expansion of soil under the building can cause the foundation to move.
• Loads on Building: Live loads (i.e., traffic, furniture, and occupants) and dead loads (i.e., roof tile and equipment).
• Wind & Solar Energy: Wind loads can place pressure on stucco wall panels, due to the kinetic energy force. Solar energy is the primary cause for external stress on the building and can create stucco cracks.
• Choosing your Finish Texture: To ensure full satisfaction with the stucco installation, reasonable expectations should be established with respect to cracking prior to selection of the stucco type of finish, including acrylics. The building owner should expect hairline cracks and diagonal cracks emanating from the corners of windows and doors. By following best industry standards and practices, the potential for stucco cracking can be reduced but not eliminated. Santa Barbara and other steel trowel smooth finishes are more likely to exhibit micro to hairline cracks in the color coat. These steel trowel finishes will also show micro hydration mapping cracks, as opposed to heaver texture finishes that will help conceal cracks. When choosing any type of stucco finish, the designer, owner, and general contractor should be aware that any steel trowel finish will crack. These texture choices are usually out of the plastering contractor’s control.
• Different Textures: Heavier textures can make normal cracking less conspicuous.
• Properly installed weather-resistant barriers, integrated with flashings, are critical to minimize moisture penetration.
• Framing: The builder should ensure that framing is straight and true and will not subject the stucco assembly to undue stress or excessive thickness variation due to bowed rough framing members. The stucco thickness at weep screed, J-metal, and other plastering termination points are 7/8 inch. As the stucco is installed towards the center of the wall, the bowed rough framing members will cause the stucco to be thicker. This is because the rough framing wall is assembled on the ground and all bowed studs will be assembled with the crown facing up towards the interior of the building.
• Plastering Materials: Use good quality ASTM aggregate, stucco, lath, weather-resistant barrier and acrylic finishes per ASTM standards.
• APA: Install wood-based sheathing according to APA industry guidelines and meet or exceed the minimum requirements. The American Plywood Association (APA) recommends leaving a 1/8-inch space at panel edge and end joints, unless otherwise recommended by the panel manufacturer. Proper nailing at edge and end joints is critical. Over-driven, undersized or improper nailing at edge, field and end joins can greatly influence stucco cracking.
• Rough Framing: Measure the face of studs and sheathing during the framing stage of a construction project prior to installing lathing. It will be more beneficial to find a building without windows installed first. Nail a 7/8 inch shim first to the lowest and highest point of the wall and attach the string line to the face of the shim. The string line now represents the finish plane of the stucco wall. Then measure the face of the stud or structural sheathing materials to the string line at different locations. At this point, you will have the overall picture of how the rough framing can directly affect the stucco thickness and thus create a weakened plane of thicker and thinner stucco behind the scratch coat. Then do the same thing horizontally from corner to corner of the rough framing and find out the truth of the unevenness of the rough framing. This technique will help you think the next time you are standing in front of a stucco crack.
• Roofing Materials: Load roofs prior to application of scratch and brown coats.
• Drywall Materials: Install drywall prior to application of the first scratch coat, including all field nailing or screw fastening applications. Any field nailing or installing of field screws on the exterior walls will cause each framing member to vibrate and thus crack the stucco wall.
• Soil Testing: Builders should test soils prior to the installation of the stucco and ensure proper compaction/fill to minimize foundation movement and have the testing reports available.
• Petrographic Examination Reports: Assessing “causation” of plaster cracking is, at times, very complex, difficult and extremely limited when restricted to only observing a Petrographic Examination Report. Contrary to belief, when evaluating the nature of Portland cement plaster cracking, one must consider other factors besides presumed negligence or poor materials, mixing or workmanship by the plastering subcontractor. Plaster cracks form when a stronger force exceeds the restraint capacity of Portland cement.
It is obligatory to prepare a complete Petrographic report listing all of the external stress factors that were or were not associated to any stucco cracks. Any professional opinion and Petrographic Examination reports shall address how external stress cracks developed and need to be listed in each of their reports per this Stucco Bulletin, or they are considered to be incomplete. The reports shall address the commonly developed stress crack’s origin such as framing, wind loads, and any other external stress factors that have damaged the stucco system. Some normally expected movements in the supporting structure will generate external stress cracks in the stucco system, although there is no way to predict when or where these cracks may occur. The inability of Portland cement plaster to flex or resist perpendicular forces will soon cause it to crack. This is why the most experienced architects will specify relief joints, elastomeric coating or an acrylic finish with a base and mesh system to minimize the cracking before the bidding process.
Penetrations: Penetrations that cut • through the plaster system (window and doors, electrical panel boxes, vents, pipes, water spigots, etc.), create “high stress” points. Typically, these locations are prone to, and more frequently, manifest cracks.
Vinyl Windows: Vinyl windows are large • producers of plaster cracking because they expand and contract much more than other types of window materials. Vinyl windows are constantly moving as temperatures fluctuate. There are other contributing factors of composite/simulated wood trim components that are also related to movement. The coefficient of expansion/contraction of vinyl windows is about six times that of stucco. For example, an 8-foot wide window can expand 7/19 of an inch. During hot weather, a vinyl window can expand with enough force to crack stucco. This is a key reason why most vinyl windows are white. White generally reflects and absorbs less heat than darker colors. Contort deformation of the window frames can easily be seen when temperatures rise during the day. Cracks in the plaster typically emerge and radiate from the outer perimeters of the confined vinyl window frames and structural framing members.
ROBERTS CONSULTING GROUP