Question:
There is considerable discussion about the proper
installation of control joints in stucco, specifically:
Is it necessary to cut entirely through the
stucco membrane, or will installing a groove in
the surface suffice? —via e-mail
Answer:
There are a few questions I receive with almost
predictable regularity, and as often as not I
attempt to cite a credible source for my answer
so the reader knows that I’m not making up the
answer. However, I have heard this particular question debated
at length during several technical committee meetings, so I’m
not certain there is consensus on this one. Fortunately, I recently
acquired another source for those pesky lathing and stucco
questions: The Metal Lath Handbook from Gary Maylon.
Maylon has written several articles on lath and stucco for this
magazine, and he is chairman of the ASTM committee on lathing,
so I consider him one of my gurus.
In his book, Maylon explains: “It is absolutely imperative that
the lath be discontinuous behind the expansion/control joint
and that the lath be wire tied on either side of the cut (ASTM
C1063 section 7.10.1.4). Uncut lath will hinder the proper
functioning of the expansion joint. Where expansion joints
cross, the vertical joint should be continuous at the junction
with the horizontal joint split. These junctions should be laid
in a bed of caulk and caulked with a flexible caulk after the stucco
has cured and finished shrinking. Where even moderate
winds or moisture are expected, it is advisable to apply an 8- to
10-inch strip of construction paper (grade “D”), 15# felt or peel
and stick flashing material under all expansion joints to serve as
secondary flashing.”
Another common lathing question concerns the proper installation
and preventing cracks in suspended stucco ceilings. So
while I’m touting this newly discovered resource, I’ll offer Maylon’s
explanation on the topic, including an ingenious method
for avoiding cracks due to excessive uplift: “Most suspended
ceilings are attached to framing members such as bar joist or
truss members, or they are attached directly to the ceiling or the
floor above by means of hanger wires and a cold rolled channel
grid work. ASTM C1063 requires that the main runners be
held off any penetration or walls a minimum of 1 inch. Cross
furring channel members and all accessories such as casing beads
and expansion joints must maintain a minimum clearance of
3/8-inch from all abutting surfaces. All of this simply means
that the ceiling membrane must have enough clear space
around the perimeter so that it will freely float.
“This rule is critical to the properly functioning stucco ceiling.
A ceiling that is restricted on the perimeter, but experiences
movement up or down in the center beyond the allowable
deflection of L/360 or about 1/3 inch per 10-feet of length, will
exhibit structural cracking. This type of damage usually requires
extensive repair work to fur. Most of the time we see this type
of ceiling restriction performed for reasons of security concerns
or to prevent uplift due to wind pressures, such as ceilings for
exterior porticos. Uplift prevention can easily be accomplished
by the use of two cold rolled channels placed toe to toe forming
a box around the hanger wires (a length of rigid electrical
conduit may also be used for this purpose). The length of these
channels should be approximately 1 1/2 inches shorter than the
hanger wires in order to accommodate any movement of the
ceiling. These can be place at each corner of more often if
desired for security reasons. Remember that a lo-foot square
portland cement ceiling with 7/8-inch stucco weighs approximately
1,200 pounds, and is not easily lifted.”
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,
or send your e-mail question to jones@awci.org.
