Stucco can be an extremely strong and durable material, lasting for the life of a home. It's composed of Portland cement the same material that lends strength to concrete driveways and slab foundations. Just like a driveway, though, a stucco wall is prone to cracking, because large areas of stucco can't accommodate expansion and contraction. If you can relieve large spans of stress, you'll prevent large cracks.
Cracking is promoted by insufficient curing, poorly mixed stucco, improper lath type and installation, inconsistent thickness of stucco application, and poor workmanship. But even if stucco is applied properly and all the conditions are right, it will still probably crack.
Cracks provide a pathway for water to reach the drainage plane, and you'll end up with an unattractive exterior and unsatisfied homeowners. But you can anticipate cracks on large slabs, around windows and doors and at any point of transition, such as a corner. To control cracks, you need to create areas that relieve stresses. Here are the tools you can use to do that:
Control joint. A control joint is a gap that's intentionally placed between two stucco areas to direct inevitable cracking in a predictable and aesthetically acceptable way. According to the Portland Cement Association, joint spacing should meet the following criteria:
Casing bead. A casing bead relieves stress, separating stucco from all other materials at vinyl or wood doors and windows, as well as where stucco meets dissimilar material, such as at light fixtures and any penetrations through the wall. Casing bead is also required at soffits.
Corner bead. A corner bead is a metal or flexible plastic strip attached along an inside or outside corner of two stucco walls. A corner bead helps to relieve stress so that stucco won't crack.
Weep screed. A weep screed is a component that allows you to terminate the stucco above grade at the base of a wall. It also provides a way to maintain a consistent thickness of stucco to control cracking. A wall with varying thicknesses of stucco will have different stresses, which promote cracking.
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