After reading (lurking!) the postings for some time by the many inspectors that are involved in this forum, allow me to introduce ourselves. We are a structural engineering firm that investigates “distressed” structures. The last few years we’ve concentrated on stucco cracking and what factors of the structural design (particularly residences) are part of the cause of stucco distress. These are problems such as differential settlement, lumber shrinkage, out-of-plane wind loads (i.e. wall fluttering), in-plane wall movement from wind (serviceability issues) and building to the minimum code standards which usually only consider earthquake resistant construction or the wind codes, which basically are to insure that you house doesn’t blow down. Reading your posts continue to be informative and provide us with additional real world insights.

Neil Moore, SE, SECB

Check our web page: Neil Moore & Associates | Distressed Structures Investigations and Solutions

Ok, well, gee, thanks. Tho, I'm not an Inspector, only an Industry professional, with a small "p".
A question for you then.
Much of the "stucco" seen in the SW U.S. is of the thin-coat (3/8-1/2") variety. Products such as Western 1-Kote, and other variations of the same theme. There is an ongoing battle here over whether this is actual "stucco" per se., but I don't want to pick any more nits here...

This system is usually stapled onto wood framed houses using a 1" thick T & G foam board over a grade D vapor barrier, and 20 ga. woven wire lath. The base coat is then applied to the nominal thickness mentioned above and finished with a 1/8" thick finish coat, that may or may not get painted. Sometimes integral colored finish products are used.

The question revolves around the presence of the foamboard, and the value it lends to the overall strength of the system. Does it provide a degree of "flexability" to mitigate the minor flaws of a wood substrate, or is that a myth?
We see a lot of hairline cracking that is becoming the standard of the industry, as most are less than the width of a dime, and thus fall into the "acceptable" range of most code enforcement agencies. Nevertheless, it is unsightly, and can be quite extensive depending on many, many factors, some of which you listed. I have been involved in some forensic testing on both 1-coat and 3-coat systems, and the issue of oversanding is always a concern, as is the hydration method, as well as the physical properties of the aggregate used. (sand). I would be interested in any comments you may have about the leading causes you have come across in your practice.
Thanks, in advance.
j.

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Welcome Neil,

Join in.

We welcome Your input.
.

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Hillbilly Bears.jpg (95.2 KB, 7 views)

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NM:

From your site:

"NMA assumes stucco is applied in accordance with the requirements of building codes, and the proven methods of many stucco bureaus and professional stucco associations."

In my 35 years of experience in the field I find that you are assuming too much. The majority of installations I see are not installed according to the mandates of the PCA, ACI, et al.

John:

These are basically one-coat or EFIS systems and their construction or application is suppose to conform to the particular ICBO report of the product or system that they are using. From a structural standpoint we ignore whatever value of the 1” foam board might provide. One thing it does do is place the tension face of the 1-coat material further away from the center-line of the stud and this would stretch this surface more that traditional stucco when considering the outward bowing of the stud from wind suction pressures on the leeward side of the house.

A.D. Miller:

You’re correct as to workmanship of the stucco contractor installing the stucco correctly, but we are not stucco experts per se! And usually during our investigations the stucco experts will find a lot of problems but sometimes there’s good contractors out there that really do try to do things right. If the stucco has been repaired or replaced correctly and inspected as well and it’s keeps cracking, then you have to look someplace else. Stucco installation is to be per Section 2512 of the IBC which also refers on to ASTM C926. But the industry standards are also ASTM C1063. We start looking at things like the wind design loads and whether the studs are big enough to resist puffing out and showing a horizontal crack at the mid-height of the wall or blowing in and showing a horizontal crack at the floor level. We’ll also be looking at the sill crushing as well as the shrinkage of the plates. You know that there can be up to 9 sill and top plates in a 2-story house with a cripple wall with each of these 2x’ shrinking a little bit in their drying out which can add up to a significant number in terms of stucco distress. We also look at sill plate crushing when you’ve got a truss spanning across the house with a tile roof and maybe a ½ span of a 2nd floor load plus the weight of the stucco. Actually there’s a lot of little movements here and there – say like a little settlement – either differential or poor compaction and the wall racks a little bit or there’s not enough shear walls and they move a little bit in a occasional big gust all causing some sort of diagonal cracks.
Then there’s the sequence of the construction of the building – putting the stucco on before the roofing or lots of banging on the building after the stucco is placed. One of the important things that we find is the studs are too small or the engineer used the wrong wind loads and ignored that fact that the building is on the top of a hill or near a lake with a long fetch. I wager that in your inspections you’ll find drainage issues which cause the foundation to move or the slab to crack – which can also affect the cracking of the stucco. Sometimes we find that the contractor has forgotten a shear element and the building is moving around in low winds. Once we found a multi-million dollar home where the structural design only had two shear walls per level and the house was moving around in low winds and continually cracking at the same place.

NM: Interesting stuff. Do you suppose you would have the time over the next few posts to go into a bit more detail on each of the factors you have mentioned?

I'd be interested to know what your perspective is on the ASTM 1063 requirement for lath to be cut behind the control joint. It seems that no sub does that except on true expansion joints. This is especially true for the thin-coat systems.

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JC:

From the attached .pdf:

ASTM C1063 states, “Control joints shall be formed by using a single prefabricated member or fabricated by installing casing beads back to back with a flexible barrier membrane behind the casing beads. The separation spacing shall be not less than 1/8 inch or as required by anticipated thermal exposure range (per author and to accommodate stucco shrinkage).” It goes on to state that it has to conform to the previous section that states the following: “Lath shall not be continuous through control joints but shall be stopped and tied at each side.”

The first rules of control joint (one-piece) installation after proper placement is to cut the lath behind the joint and wire tie it in place. Wire tying is not conducive to high-production installations but it is necessary and required by ASTM C1063, and every code that references C1063. It is important to cut the lath and not the paper backing when this procedure is performed. The easiest way to accomplish this is to pop a chalk line in the proper locations and use electric shears to cut the lath.

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ASTM 1063 Stucco Lath Article.pdf (131.5 KB, 5 views)