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In
much of the world, masonry walls are the most common substrate for
stucco. ASTM refers to this as a “solid base.” Europe uses red clay
units (see picture). Similar to our CMU, it is a great substrate for
cement plaster. They both are absorptive and ideal substrates. Why does
the U.S. not build like the rest of the world? The reason masonry walls
are not as common in America is basically a two-fold answer.
While
it is possible to add paper, felt or housewrap over the CMU before
plastering, it is generally not advised. A sheet good becomes a bond
breaker. Losing the intimate contact between the CMU and cement plaster
is not a good thing. The belief that you are adding water resistance is a
fallacy. Attaching the paper or felt to a masonry substrate cannot be
done practically with nails, screws or staples. Using a power-actuated
fastener is no better. This application will force fasteners into the
masonry with high pressure. This will likely spall the masonry and tear
the paper/felt. How can this be more water-resistant? In addition,
fasteners are recommended to be placed into the grout joints. How can
you see the masonry joints through a black felt? If a designer really
wants extra water protection, there are fluid-applied WRBs that can be
used. Make sure they are meant to have cement plaster bonded to them. Do
not use bituminous-type compounds.