Stucco: Q&A
Can the type of joint in plaster affect its spacing or location?
Joint spacing in stucco is more of an issue over frame construction. No matter if the joint is called a control joint, a contraction joint, or an expansion joint, it is there to relieve stresses. The rules for the maximum spacing between joints and the maximum size of panel are well established and discussed in a separate FAQ on joint spacing.
Stucco is a like a thin layer of concrete. It typically contains reinforcement when it’s placed over framed construction but may be direct-applied to solid substrates like concrete or concrete masonry. When direct applied to a solid substrate, the jointing rules are simply to follow what is present in the backup. The building itself should contain joints to limit random cracking.
Complex jointing patterns using multiple types of jointing accessories in framed construction can be confusing, because people wonder if one joint is somehow different from the others. They are different, but they have one important similarity: a joint relieves stresses and provides a separation between various sections. Again, because plaster is so thin, it must be sectioned into panels to control stresses due to volume change.
If there is a potential for out of plane movements, then the joint must be able to move in different directions, both in plane and out of plane. If a joint is a contraction or expansion joint, it simply has to move back and forth. “Expansion” joint is somewhat of a misnomer because the plaster does not increase in size beyond the as-placed volume of the individual panels.
For these reasons, it is important to choose the right accessory for the joint. The one-piece accessories usually have a pleat or accordion shape. They can move in plane but do not handle out of plane movements well. For those locations (control joints), such as the interface between different types of substrates, borders, penetrations (doors, windows, etc.), or at the top of a wall where it meets a roof or second story, two-piece assemblies are best. These may be prefabricated or may be field-constructed by placing casing beads or other accessories back to back. Note that at a window or door frame, a casing bead adjacent to the frame serves the same purpose.
Can stucco be applied directly over painted brick?
This is a common question that often arises when people are rehabbing or updating older construction. Plaster is a cost-effective finish, relatively easily installed, that improves the appearance and creates a water-resistant wall surface.
A painted surface will not typically absorb water and, as such, is a substrate to which stucco will not readily bond—at least not uniformly. There are two basic alternatives to covering a painted brick surface with a new coating of portland cement plaster.
Sand blast or water blast to remove the paint in its entirety, then direct apply a two-coat system. It is essential to have a surface that is uniformly absorptive to accept the plaster coating. In addition, it may be beneficial to use a bonding agent or dash bond coat with this approach.
Attach paper backed lath or install appropriate building paper between wall and attached metal lath to provide a moisture barrier and to serve as a bond breaker. Apply traditional three coat stucco to metal lath and accessories. In this approach, the idea is to treat the plaster like a sheathed system, using metal lath to support the plaster on the substrate, while completely isolating the plaster layer from the backup with building paper. This prevents a partial bonding situation, which could set up undesirable stresses in the plaster and lead to cracking.
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