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Q: Is stucco suited for use in a coastal environment?
A: Portland cement plaster is suited
to virtually every type of climate, whether wet or dry and hot or
cold. It is popular in coastal environments because it is unaffected
by moisture, but when metal lath is part of the system, there are
potential concerns due to the elevated chloride levels in the environment.
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| Applying stucco over metal lath. |
In coastal areas, air is literally salty: it carries chlorides.
This is an aggressive ion that corrodes metal. Hardened plaster
is not affected by chlorides. If the plaster is direct-applied to
concrete or masonry backup, there is little or no metal reinforcement
in the plaster. If a 3-coat system is installed, whether the backup
is frame construction or to achieve mechanical bond over a solid
substrate, metal lath is attached to structural supports to hold
the plaster to the wall. The lath is required to be galvanized,
and embedding it in a highly alkaline environment like portland
cement plaster provides added protection from corrosion.
The lath should be fully embedded by the first base coat, called
the scratch coat. This assures that the lath will have at least
½ in. (12.5 mm) of cover—the second base coat, a 3/8
in. (9.5 mm) thick layer, plus the finish coat at 1/8 in. (3 mm).
The combination of galvanized coating and plaster cover protects
the metal. This makes it critical to minimize cracking of the plaster,
which would otherwise provide a direct path for chlorides to reach
the steel. Methods to minimize cracking include proper consolidation
during installation, appropriate contraction joint spacing and proper
curing of the freshly placed stucco material.
There are other protective measures that can be taken in extreme
exposures. These include stainless steel lath, which is more resistant
to corrosion due to chlorides, and polyvinyl chloride (PVC) lath,
which is not susceptible to corrosion, but does not have the same
strength or thermal coefficient of expansion as steel, and may not
bond well to plaster.
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