A common misconception in the lath and plaster industry
is that if a window, particularly a plastic or aluminum
window has a return lip on the outside edge, the edge of
the window can serve as the casing bead for stucco applications.
Nothing can be farther from the truth. In my most recent
trips to jobs in Florida and Minnesota, I viewed jobs that
exhibited this problem. On some of the Florida jobs the
contractors tried this stucco application on plastic (PVC)
windows (note: there was an absence of casing beads, no
window flashing, and the poor application of building paper
on the job in question). Some of these contractors tried
to remedy water intrusion problems by applying multiple
layers of caulk on several occasions to try to stem the
tide of water intrusion. Some of the structures that the
author visited were less than a year old and well on their
way to water damage and mildew problems.
Why did this catastrophe occur and how could it have been
avoided? The stucco was applied directly into the casing
shaped edge of the vinyl window instead of a casing bead
as required by the International Building Code, and ASTM
C1063, the metal lath installation specification. Anyone
familiar with the action of stucco during the curing or
hydration process knows that the volume of water in the
stucco mix hydrates from the total stucco application resulting
in a certain amount of shrinkage. This shrinkage should
be expected and dealt with by use of the proper accessories.
Combine this shrinkage with the fact that portland cement
stucco and metal lath have a very different coefficient
of expansion than that of vinyl or aluminum windows and
one has to know that a crack will occur at this juncture.
These cracks will change size with the expansion and contraction
of the window as it heats and cools. Caulking does little
good because; although it will adhere well to the window
it does not adhere very well to the stucco edge.
To perform this application properly the window should
have been flashed prior to the installation of the window
unit. Most window manufacturers recommend flashing before
and after window installation and warn that the installation
flange or fin should not be considered to be part of the
The window opening is then covered with a peel and stick
type flashing starting at the sill and working to the top.
If the wall is covered first with water resistant paper
or one of the geotextile products they can be used to flash
or wrap the window opening. Some experts recommend the use
of a fabricated steel or vinyl sill pan to be preinstalled
at the bottom of the window opening prior to the insertion
of the window unit. The window is then installed.
Following this installation a flashing that sticks only
on the top edge is applied to the bottom edge of the window
and a drip edge, usually "Z" shaped is placed
over the window. Next the side flashing is installed followed
by the top flashing. A peel and stick flashing is recommended
for the sides and top. The reader should go to www.fortifiber.com
for additional detailed information on flashing applications
and products available.
Once the window is flashed properly the casing bead can
be installed next to the window. It is recommended that
the window have an "L" shaped flange rather than
one that has a return leg on top that would form a "C"
In this way the casing can be installed approximately 3/8
inch from the upright leg of the window providing the perfect
sized channel for the application of a backer rod and caulk.
Why is backer rod important? The answer is simple; the round
shape of the backer rod provides the perfect surface to
receive caulk. As the caulking installers fill the channel
and use their finger to smooth the caulk, they impart a
slightly concave top surface. It is the feeling of some
experts in the caulking industry that an hour glass shaped
caulking application is considered ideal because it provides
maximum surface for the two bonding edges and a thinner
section in the middle of the span to facilitate elongation
of the caulk. It is important to choose a caulking material
that will adhere well and maintain its flexibility and durability
for the longest possible period. The author usually recommends
a silicone caulk but the reader should research this area
further before making that determination.
Once the casing bead is in place the lather can then lap
the metal lath over the expand flange of the casing bead
to assure that the two products are joined together to form
a solid bond when the stucco is applied.
On these jobs the contractor actually used a casing bead
but only at the bottom of the framed wall where he was required
to use a foundation weep screed.
Had this contractor followed the above procedure during
he initial construction he wouldn't now be faced with devastating
leakage problems at every opening in the stucco on this
"The author offers no warrantees,
implied, stated, or expressed regarding the information
found in this article including techniques, construction
methods, drawings or materials identified in this article.
To the best of his knowledge the information within is correct
and up to date as of its publication date. The author is
not responsible for typographical errors. This article is
protected by all copyright laws and shall not be photocopied,
stored in any electronic format or distributed without express
written consent of the author. All rights reserved by Metal
Lath Consulting Co, LLC."
A-1 Note repairs around and below all openings caused by
failure to use casing beads.
A-2 Note that contractor used casing bead for repair but
failed to leave 3/8" gap to allow room for backer rod
A-3 Rot and mold resulting from water leaks from leaking
windows located above.
A-4 Contractor filled original window edge with caulk to
try and stem the flow of water however this caulking didn't
A-5 Contractor flashed the outside of the window flange
on the second go round. It will help to some degree.