Housing Giants - July 24, 2008 - (Page 19)

To prevent this issue, the scratch and brown coats have to be mixed according to the manufacturer’s instructions. According to ASTM standards, the brown coat should have slightly more sand than the scratch coat. “Sand is typically added to the stucco mix by the shovel full,” Baker says. “While it may sound trivial, it’s important that everyone on-site uses the same size and shape shovel in order to get a consistent mix from batch to batch.” Enough sand has to be added for shrinkage resistance while not affecting the stucco’s workability. morE rEasons stucco cracks n n n Lack of control and expansion joints Poor sheathing installation Incorrect application of metal lath common Error no. 2: inconsistEnt coat tHicknEss It’s important to apply the scratch and brown coats at consistent thicknesses over the surface of the wall. When a coat is inconsistent, for instance, ¼-inch thick in one area and ¾-inch thick in another, its strength varies. Thin areas aren’t strong enough to resist cracking. Achieving a uniform thickness can be particularly difficult with open-frame construction; if sheathing is installed only at corners or above and below windows, it creates an uneven surface for applying the stucco. As a result, the stucco is likely to be thinner over studs and thicker across stud bays. Stucco is particularly susceptible to cracking around windows and trim boards, where it’s applied thicker to create architectural character. The scratch coat should be applied at a uniform thickness of 3⁄8 inch to ½ inch and be embedded well in the metal lath. Baker tells Housing Giants, “It’s important to the strength of the stucco to fur the metal lath out from the wall, allowing it to be embedded in the scratch coat.” The brown coat should be applied at a uniform thickness of ¼ inch to 3⁄8 inch, except at architectural details, where it can be thicker. The brown coat should be thinner than the scratch coat, so the scratch coat is the strongest layer. common Error no. 3: insufficiEnt Hydration After a coat of stucco is applied, it should be hydrated to allow the stucco to cure to full strength. Once the coat has chemically reacted with a sufficient amount of water and is allowed to dry, it reduces in volume and shrinks. The shrinkage builds up www.Housinggiants.com 7.24.08.Housing giants 19 http://toolbelt.buildiq.com/topics/exterior-finishes/stucco-3-coat/cracking.aspx http://www.cement.org/stucco/faq_curing.asp http://www.housinggiants.com

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Housing Giants - July 24, 2008

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