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Q. We have moisture problems with a stucco exterior.
What are the possible causes? —E-mail

A. AWCI’s Technical Manual No. 15, “Evaluation
of Three-Coat Portland-Cement Plaster (Stucco),”
identifies four major causes of water penetrating
the envelope of a stucco clad building:

Loss of “cohesive value.” If the several layers of plaster begin to
come apart, or lose cohesive value, there is a much greater prob- A ability that cracks that allow water intrusion will occur.

Gravity. Gravity causes rain water to run down the stucco surface,
and if there are any openings in the surface, the water will
find it and enter the building envelope. In many cases, the moisture
will be diverted out of the building envelope by the weather-
resistive barrier.

Kinetic pressure caused by kinetic energy. The manual explains
(far better than I could): “Kinetic energy is the energy caused by
a moving body due to its own weight and motion. Two raindrops
of equal mass will create a kinetic pressure different from each
other if one of the two raindrops is driven faster than the other
by wind or some other external force.” In other words, winddriven
rain is more likely to penetrate cracks or other openings
the surface than a windless, free-falling rain.

The pressure differential from inside to outside. When air pressures
are lower on the inside of a structure than on the outside,
water can literally be sucked into the building. This condition.
may result from exhaust air being removed faster than replacement
air can enter. Not only does this condition lead to moisture
intrusion, it can also contribute to excessive dust accumulation.

To address these conditions, the installation of weather-resistive
barriers is recommended on all weather-exposed surfaces to protect
against intruding moisture. The model building codes require
that exterior openings be properly flashed and integrated into a
properly installed weather-resistive barrier. Such a system provides
a means of diverting intruding moisture to the exterior of the

Q. How often should control joints in 1-inch
cement plaster on a concrete-masonry-unit wall
occur! —E-mail

A. The control joint, when properly placed, provides
a pre-weakened place for the crack to naturally
occur. Ideally, a control joint is placed in
such a stucco surface using the appropriate trim element fastened
to the CMU; however, a groove or cut in the surface of the stucco
serves the same purpose. An expansion joint is a joint that
allows movement between either large building elements of the
same material, like long runs of masonry or cement walls, or
where two different types of building material, like wood framing
and masonry, meet. The expansion joint accommodates the
independent movement of each of the elements without causing
damage to the other element that doesn’t expand and contract
at the same rate.

The Northwest Wall and Ceiling Bureau’s Portland Cement
Plaster Stucco Resource Guide recommends such a control joint
every 200 square feet. However, CMU walls frequently have
expansion joints occurring roughly every 20 linear feet (although
the formula for determining the exact placement of an expansion
joint involves size of the joint, temperature changes, mass of the
wall and enough other variables to keep an engineer occupied
until lunch). So if the stucco is applied to a CMU surface with
expansion joints every 20 feet, the stucco should start and stop
at those joints and should not need additional control joints
unless the wall is over 10 feet tall.

About the Author

Lee G. Jones is AWCI’s director of technical services. Send your
questions to him in care of AWCI’s Construction Dimensions.

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