The Metal Lath Newsletter
Volume 03, Issue 04, December, 2003

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Foundation Weep Screed -- the Most Under-Used and Misunderstood of all the Metal Lath Accessories

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?

  1. 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.
  2. 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.
  3. 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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Photos

 
The photo depicts the junction of a one-piece expansion/ control joint with the foundation weep screed.
  This photo shows the junction of a corner bead with the foundation weep screed.
     
 
The Metal Lath Handbook

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The answers to commonly asked metal lath installation questions derived from job experiences and client questions.

 
 


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