The Foundation Weep Screed is an
Important Part of the Stucco Drainage System
In the early 1990’s the requirement for the application
of foundation weep screeds at the base of all framed walls
at the juncture of the framing base and the foundation was
placed in ASTM C1063 and already existed in the Uniform
Building Code. In fact although the use of the foundation
weep screed was commonly required in the Uniform Building
Code for a long time, and it first appeared in ASTM C1063
in the 1994 edition of volume 04.01.
The large 3 1/2” inch back or nailing flange is intended
to provide not only a firm method and secure flange for
positive attachment to the framing and plate or track but
also serves as a flashing element behind the building paper
and the metal lath.
Unlike the traditional casing bead or the one-coat punched
casing bead used extensively by the one-coat stucco industry,
the foundation weep screed does not have a 900 angle at
the bottom of the stucco membrane but rather is angular
in shape. The angular ground, which slopes down at approximately
a 300 angle and returns to the wall at approximately the
same angle, provides a surface that is conducive to drainage
rather than forming a dam like the casing bead’s square
angles tend to produce.
You might ask several questions at this point: 1. What
keeps the stucco cement from sliding off of the angular
bead? 2. Where does the water drain from, the holes in the
ground or the surface plane of the angle? 3. If the angle
is covered with stucco, how can it drain moisture?
- The stucco is embedded into the lath and the bonding/drainage
holes in the surface of the angular bead. If the lath
is installed properly with the diamonds sloped down and
away from the lather, then the stucco will naturally bite
into the lath. While the holes do provide a minute amount
of drainage they are primarily there as bonding holes
in which the stucco can embed.
- Since the stucco actually rest on the surface of the
angular bead of the foundation weep screed I am often
asked why or how it is able to provide drainage. The answer
is simple. Stucco is composed of cement, aggregate (sand)
and water (sometimes with a little lime or acrylic modifier).
Since the water evaporates or hydrates out of the stucco
during the curing process the total mass or volume of
the stucco shrinks. While we are only talking about 1/32
inch or so it is quite sufficient to create a drainage
outlet for moisture that might find its way to the surface
of the weather barrier and flow to the bottom of the cementitious
membrane.
- Note: it is important not to block this opening. Some
contractors finish the wall with trowlable or roll on
acrylic or Elastomeric finishes. It is important that
these finish materials do not fill the shrinkage crack
formed at the juncture of the stucco and the edge of the
foundation weep screed. While the holes do provide some
drainage after stucco shrinkage that is not their main
function. They are primarily bonding holes. There are
not enough of them and no efficient way to clean them
out to perform satisfactorily as weep holes. We must rely
on the stucco shrinkage along the entire angular surface
of the screed.
It is critical that the foundation weep screed, the first
accessory installed, be placed over a ten to twelve inch
strip of paper flashing and that it be solidly nailed in
place with the bottom of the flange in contact with the
foundation. It must be located a minimum of 2 inches above
paved surfaces and 4 inches above finished grade. Do not
let the landscaper add fill or top soil and change these
minimum space requirements. It is also a good idea to make
sure that the sprinkler system if present does not flow
onto the foundation weep screed, as this may cause premature
deterioration.
As mentioned earlier the foundation weep screed is the
first accessory installed and should be placed over grade
D building paper or a strip of building paper flashing.
The second layer of building paper or the paper backed lath
is then installed over the back flange of the screed to
the top edge of the angular screed. In this way any moisture
that makes its way through the cementitious membrane to
the building paper will by way of gravity be drawn to the
bottom of the wall and flow over the angular screed and
exit via the drip screed. In this way we have several redundancies
that will help to prevent moisture from penetrating the
cementitious membrane and the weather barrier. We must keep
moisture off of the sheathing and out of the wall cavity.
It is important to state at this point that no matter how
carefully the weather barrier, lath and accessories are
installed on the wall assembly, it is only as good as the
entire system including the sheathing, the windows and the
flashing around these items and all openings and the caulking
used to seal all openings.
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