Images courtesy Simpson, Gumpertz and Heger Scratching the Surface with Stucco Control Joints by Jeff Bowlsby, CCS, CCCA ortland cement plaster, also known as stucco, is a popular and common exterior wall cladding finish. One concern of this otherwise excellent material is its inherent cracking—not only causing visual blemish, but also leading to possible water intrusion that could result in concealed damage.1 Stucco cracks are caused by numerous factors, but most are the expression of internal stress relief resulting from more movement than the finish can accommodate. The internal stresses creating cracks may derive from: • stucco mortar shrinkage occurring shortly after installation; • ambient in-service temperature and re-hydration cycling; • freeze−thaw cycling; and • similar small-scale building movements. Where stucco control joints are installed, these forces may cause control joint movements. These control joints are one of the stucco industry’s solutions for minimizing and controlling cracking, but they can be controversial. Control joint products are not created equal—they come as sheet metals (including galvanized steel and solid zinc alloy) or extruded polyvinyl chloride (PVC), with each type offering different structural performances. P Two standard one-piece stucco control joint profiles are available—the #15 (M-fold or double-V style) and the XJ-15 (locking flange or double-J style), depicted in Figure 1. Mounting flanges are either expanded sheet metal or a solid flange in either sheet metal or PVC, with spaced round perforations that key with the stucco mortar. Current dilemmas surrounding stucco control joints include: • How does each different stucco control joint product perform (and which is best?) • Should the lath be continuous or discontinuous at the control joint? • If the lath is continuous at the control joint, what effect does its product type have on control joint performance? • If the lath is discontinuous at the control joint, should it be fastened to supports? • Should the control joint flanges be fastened directly to the supports or wire-tied to the lath? Stucco control joint performance depends on the combined influences of many variables, ranging from product characteristics and lath type to mortar properties, installation configurations, and fastening/support conditions.2 However, no published control joint performance data or testing 48 The Construction Specifier April 2009 CS_APRIL_09.indd 48 3/16/09 4:45:22 PM