Carefully installed flashing paper and caulk are the keys
to preventing leaks
There are three stucco wall systems in use today: traditional
three-coat stucco, so-called one-coat or synthetic stucco
systems, and EIFS (Exterior Insulating Finish Systems). While
water damage in EIFS walls has made the news in recent years
because of class-action lawsuits, the fact is water can and
does penetrate the finish in all three systems. This is
especially true in houses with little or no roof overhang,
where excessive amounts of water can run down the wall.
The solution to building a water-tight stucco wall is to
view it as a system in which all parts must be correctly
installed, particularly at penetrations, joints, openings, and
other areas susceptible to leaks. Too often, however, the
stucco finish gets all the attention, while the underlying
protective membranes and flashings -- which are the main
defense against water intrusion -- are overlooked.
In my 24 years as a stucco contractor, I have gone beyond
the minimum requirements of the code to devise flashing details
that have withstood the test of time. In this article, I will
explain the materials and techniques I use to prevent leaks in
stucco walls. In every case, you can tell if you have installed
the flashing and building papers correctly by imagining the
path water will take if you were to spray the wall with a hose
before applying the plaster. If the water can flow freely over
the paper from layer to layer without ponding or finding its
way inside, then the wall is properly flashed.
I work primarily with one-coat and traditional stucco, both of
which rely on water-resistant membranes in conjunction with
flashing at all wall openings and penetrations to direct any
water that makes it through the stucco down towards the bottom
of the wall. EIFS systems, which are applied directly over
foam, originally relied on a water-tight skin to keep all water
out. But because water from leaks at penetrations can become
trapped behind the foam, EIFS manufacturers are developing
systems that also use a water-resistant membrane behind the
The UBC currently requires Grade D (10-minute) paper for the
membrane. While this is adequate, during periods of heavy rain,
the water will eventually saturate the paper and find its way
to the wood framing through any holes and tears. If left
unchecked, this moisture can damage the structure, but at the
very least will cause excessive expansion and contraction of
the wood frames. The resulting cracks in the stucco base and
finish coats are not only unsightly, they could allow even
greater water intrusion, leading to more cracks.
The code also calls for horizontal laps of 2 inches and
vertical laps of 6 inches. I think these minimum overlaps
should be doubled. I have seen water travel sideways and back
up behind the overlaps, especially if the paper has wrinkled,
which it tends to do when it gets wet.
Heavier paper. To break the cycle of leaks
leading to cracks leading to more leaks, I recommend upgrading
the paper. I have done extensive testing of various papers, and
at a minimum I prefer to use a 28-pound, 30-minute paper, which
is made by a number of companies (see "
Sources of Supply"). The 30-minute
paper is easier to fold and bend than heavier (60-minute)
paper, and unlike lighter weight paper, it is more resistant to
splitting when exposed to moisture, sunlight, and extreme
I have also tested Dupont's Tyvek Stuccowrap, although I
haven't yet used it for as long as the other papers, and I find
it has a number of advantages when installed behind stucco.
Tyvek bends more easily than paper and it's less likely to tear
during installation. Tyvek flashing paper is not available, but
Dupont does make a special tape, which I have found to be easy
to work with at seams and joints. Tyvek is also dimensionally
stable, both when wet and when subjected to wide temperature
swings, so the holes around nails and staples won't gradually
grow larger the way they do in conventional building papers. In
addition, Tyvek won't wrinkle like building paper when it gets
wet, an important advantage because wrinkles are a likely place
for holes to develop. Stuccowrap also appears to be beneficial
to the stucco curing process. By not drawing the water out of
the scratch coat, it allows the stucco to hydrate more
effectively, developing greater strength and density.
1. Carefully layered flashing paper and
strategically placed caulk make for a leakproof window.
When nailing the window in place, make sure all nails
are driven flush; bent and protruding nails should be
pulled so they won't snag and possibly tear subsequent
layers of paper.
Around openings and penetrations, many stucco contractors use
sisal kraft paper, which sandwiches a layer of asphalt paper
between brown kraft paper, but we have experienced many
problems with it. Sisal kraft paper is easily cut by sharp
objects, such as the corner of window frames; when wet, it
develops wrinkles that make it difficult to get it to lie flat,
leading to rips if the winds pick up.
One alternative is Moistop, which is made by sandwiching
kraft paper between two layers of polyethylene. It bends easily
around flashing and holds up well over time. Moistop does not
wrinkle when wet like sisal kraft, but it does rip in the
I now prefer to use Future Flash, a 20-mil rubberized
asphalt membrane sandwiched between 4-mil polyethylene on the
face and polyester on the back. One reason is its strength:
During fall and winter we can get winds that blow 50 miles an
hour or more, and Future Flash won't tear apart like sisal
kraft and Moistop. Paper products won't stretch either, whereas
Future Flash has 128% elongation.
I also like Future Flash because it's not self-sticking, so
any water that does make its way behind the membrane has a
chance to find a way out. If all the horizontal laps are
sealed, they may create a dam that will catch water. When
water-testing stucco walls, I have watched water that leaked
through nail penetrations run down the backside of the paper
and find its way out again between the overlaps.
At horizontal surfaces, I prefer to use Vycor (formerly
called bituthene), a rubberized-asphalt and poly membrane
originally designed for use as roof flashing. It's thick enough
to be durable, and adheres well to itself and to most other
materials. Vycor also seals itself around any fasteners that
are driven through it, and it's self-sticking in temperatures
above 60 degrees F. When it's very hot, though, Vycor becomes
so sticky it can be difficult to handle, especially when
working alone. Vycor will also bleed if left exposed, so it
must be used only where it will be covered by other
2. For doors installed over wood subfloors, the
author uses a soldered metal pan at the sill, making
sure the metal deck-to-wall flashing overlaps the side
flanges and that the flashing paper around the opening
is properly layered. All doors should be securely
shimmed and nailed through the jambs to prevent cracks
in the stucco from door movement.
We also use a lot of caulk, both to repair pinholes and tears
in the paper and to seal flashing at penetrations and openings.
Caulks vary greatly in quality, and so do the conditions under
which they are applied. A caulk that works well in hot, dry
weather may not perform as well in cool, damp conditions;
similarly, a caulk may have to bond to surfaces that are moist
or dirty, shiny or dull.