Plastering isn't the oldest profession, but it may be close to
it. Coatings have been applied to the outside of structures for
almost as long as people have been building shelter.
Originally, plaster consisted of mud alone. As technology
advanced, lime-based compounds were used. Modern-day Portland
cement was invented in England in the early 1800s. It gradually
gained popularity in the United States and was widely available
by the 1890s.
Traditional three-coat stucco is nothing more than a
Portland cement-based exterior plaster, which is a mixture of
cement, sand, and water. Cement-based stucco has a long history
of proven performance in a variety of climates here in the
U.S., from the hot, dry Southwestern deserts to cold Northern
winters and the humid Southeast.
Any problems with stucco are more related to a lack of
uniformity and skill in stucco application than to the products
themselves. In the past 15 years, in response to this,
manufacturers have introduced many new stucco products,
including fiber-reinforced "one-coat" stuccos with acrylic
polymers. These products have the perceived advantage of faster
installation time and fewer shrinkage cracks as the coating
cures. Despite the apparently "enhanced" performance of these
products, I believe that the original recipe for stucco still
best withstands the test of time.
Properly installed three-coat Portland cement stucco
provides the building with a dense, monolithic, protective
shell. It requires very little maintenance and can easily last
over 100 years. Three-coat stucco is easy to repair or patch
and resists weather, impact, fire, insects, and rot. It is also
vapor-permeable, as well as watertight in its entire depth.
These are clear advantages over new synthetic exterior-finish
systems.
Preparing the Substrate
Stucco can be applied to just about any wall system, including
wood frame, metal frame, masonry, ICF block, Rastra (a
particular type of ICF with a textured surface made of 85%
recycled foam plastics and Portland cement), pumicecrete,
poured concrete, straw bales, or adobe. Regardless of the wall
system you use, quality stucco starts with a suitably rigid
structure. Good stucco will be strong enough to resist cracking
as the building undergoes normal movement, but stucco is not
structural. Stresses from ground movement, foundation
settlement, or most commonly, framing or structural movement
will crack stucco. Earthquakes may be beyond your control, but
foundation settlement and framing movement are not. Most of the
structural movement I see is the result of changes in the
moisture content of framing and sheathing materials. Plywood
can move enough to crack stucco if it is improperly spaced or
nailed. For every 4% change in moisture content, lumber will
change 1% in size. These dimensional changes primarily occur
perpendicular to the grain and can result in significant
structural shifting.
The finished thickness of stucco is only 7/8-inch. Irregular
or uneven framing at plate lines, along shear panels, or at
bowed studs can leave the stucco too thin at these places,
resulting in cracks. Keep the framing straight enough so that
the stucco won't have any excessively thin areas. If I happen
to see bowed out framing before starting a job, I'll recommend
furring out the rest of the wall so that the stucco can achieve
an even 7/8-inch thickness in that area.
Flashing and Weather
Barrier
The fact that stucco is highly vapor-permeable doesn't mean
that you can forget about the effects of water and water vapor
on framing and sheathing. It's important to protect the framing
and sheathing from any rainwater that does get behind the
stucco (see Figure 1).
|
Figure
1. A properly installed weather-resistive
barrier is essential to protect the structure from
moisture intrusion. Here, 60-minute Type D kraft paper
is being installed. |
Make sure that the backup system of flashing, weep screeds,
and a weather-resistive barrier is carefully installed. I have
covered this subject at length in another article (see
Stucco Flashing
Details, 10/98), so I won't repeat it here. But the best
way to think about the backup system is to imagine that you are
all done with the prep work but the stucco hasn't been applied
yet. If you were to spray water on the building with a hose, it
should all run off without any water getting to the framing or
sheathing (Figure 2).
|
| Figure
2. Lath can either be stapled -- the fastest
method -- or nailed. Special furring nails (inset) are
designed to hold the lath off the wall so that it's
properly embedded in the stucco. |
Lath
Some stucco-compatible surfaces -- for example, porous
masonry, concrete, and Rastra -- will accept stucco without the
use of metal lath. The surface should be clean and sufficiently
damp and rough to ensure a proper bond. I'll often verify that
I'm getting a good bond by doing a test patch. If I'm not
satisfied, I can always attach lath or use a bonding agent. A
Portland cement dash coat of one-and-a-half parts sand to one
part Portland cement will also promote adhesion.
Wood sheathing, ICFs, and adobe all require metal lath.
Metal lath comes in many forms, including wire fabric, commonly
called "stucco netting" (which looks a lot like chicken wire
but isn't); expanded, or diamond mesh; and expanded metal mesh
with flat or 3/8-inch ribs. Lath must be corrosion resistant
for exterior use, and must be overlapped and fastened properly
(see Figure 2, above). While pneumatic staples have become
common on many sites, furring nails are a better option. The
advantage of furring nails is that they put the wire in the
center of the scratch coat, thus providing better
reinforcement.