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Q: Does the type of joint in plaster affect its spacing or location?
A: 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.
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Joint accessories:
one-piece and two-piece. |
Jointing rules, placement and curing, and other design and construction
aspects of plaster are discussed in PCA’s Plaster/Stucco
Manual (EB049).
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