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Q: What is the proper spacing for contraction/expansion
joints in portland cement plaster/stucco applications?
A: The proper use of contraction joints in stucco
systems will depend on a number of variables, including: the type
of construction materials to which the stucco will be applied; the
orientation of the construction—vertical (walls) or horizontal
(ceilings); and whether the surface is curved or angular.
Stucco may be direct-applied to concrete or masonry substrates;
however, if these materials are used together, as in the case of
a concrete framework of beams and columns with masonry block infill,
a joint may be required at the transition of one material to another.
Stucco that is direct-applied to concrete or masonry requires contraction
joints only where there is a change in material or where there are
joints in the concrete or masonry structure.
Metal lath may be used over concrete or masonry construction and
should be used in sheathed frame and open frame construction. When
stucco is applied to any construction using metal lath, joint spacing
recommendations should be implemented. The recommendations found
in the Portland Cement Plaster/Stucco Manual, EB049, are based on
ASTM C1063, Standard Specification for the Installation of Lathing
and Furring to Receive Interior and Exterior Portland-Cement Based
Plaster. Applications that use metal lath require three layers of
plaster: scratch, brown, and finish coats.
The joint spacing should meet the following criteria:
- no length should be greater than 18 ft. in either direction
- no panel should exceed 144 sq. ft. for vertical applications
- no panel should exceed 100 sq. ft. for horizontal, curved,
or angular sections
- no length-to-width ratio should exceed 2 ½ to 1 in
any given panel.
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