Albert Carrillo: Breaking Down the Misconceptions of Lath

To cut or not to cut … The Lath

Control joints used as an architectural feature.

Five wall assemblies with cracks painted for visual location.

It was a year or so ago when my article intimated to a result, to a given test, to a controversial subject, to confirm what us plasterers have known for years. When it comes to control joints in relation to cracking in stucco, let me boldly state: Lath cut or continuous performs the same.

In this month’s article, I wish to delve deep into that dark hole where the perceptions, the myths and just plain old misunderstandings of the control joint in relation to the lath reside.

ASTM C1063 – 17 Section 7.9.1.5 states: “Lath shall not be continuous through control joints, but shall be stopped and tied at each side.” So, there it is, the mandate to cut the lath behind the control joint. This is the golden statement that allows forensic professionals to cease searching for causes of cracking in stucco when the lath is discovered to be continuous behind the control joint. It is presented as the “only” option for stucco assemblies and a violation of code to do otherwise. A few more caveats exist; control joints shall be installed to delineate areas not more than 144 square feet and not more than 100 feet for horizontal areas, and the distance between control joints shall not exceed 18 feet in either direction or a length to width ratio of 2 ½ to 1.

In the Field Applications

In the application of a metal plaster base—what we call lath—ASTM C1063 requires the fastening of said lath to framing members (studs) to be every 7 inches, even at the ends of the lath and prohibits fastening between studs. If the end of the lath terminates between framing members, it must be wire tied every 9 inches. In short, everything must be fastened to a framing member and nothing can be fastened between framing members.

In efforts to allow the option of running continuous lath, many industry experts and former applicators have addressed the ASTM C11 committee offering additional language but to no avail.

In a stucco assembly, the control joints are then required to be fastened to a framing member. Given the width of a control joint this will require two framing members at every location of a control joint so each side flange can be fastened. It is therefore imperative that first the plans and specifications clearly illustrate the location of the control joints and back-to-back stud locations, and second, that the framing and stucco contractors are in concert to ensure proper location. In keeping true to the definition, this would apply to horizontal locations of the control joint too. ASTM however does not clearly state this and in almost all cases, is never considered or enforced by forensic professionals.

So we now have a visual of what the framing should look like in relation to the control joint (let’s just ignore the horizontal control joints, eh?). Next, the sheathing is to be installed onto which the lath and accessories will sit atop, with everything fastened to the framing members. In order to keep with the “shall” mandate, the control joint will be fastened directly on top of the WRB and to the framing members every 7 inches and the lath tied into the control joint every 9 inches. Directly adjacent to every control joint location is the next framing member of the assembly. It may be as far as 16 or 24 inches away, or may be right next to the double framed backing of the control joint. At each adjacent framing member, the lath is fastened every 7 inches.

In viewing the wall in the mind’s eye, we have a robust attachment of the sheathing and lath and accessories to the framing members at a minimum of 16 inches apart. Given the weight of the stucco itself this robust fastening is what would be required to hold everything in place for a long time. History has proven this to be the case.

Movement

The theory of cutting lath is that it allows for the movement of the lath behind the stucco, so cracking does not occur in the stucco. There lies the fallacy. With the robust attachment over an entire wall assembly and proper keying of the stucco into the lath and accessories, how does the stucco move independently of the lath and accessories? It’s all one big assembly, therefore cutting the lath, or not cutting the lath behind a control joint is insignificant. Coming from the field and having done both installations, I can personally attest to the claim. The only movement of the stucco in relation to the control joint occurs during the curing of the stucco, more specifically when it turns from a plastic state to a rigid state. Then, there is some shrinkage within the 144 square feet that occurs but it is very minimal and sometimes not even notable. It is my claim that control joints allow for minimal movement during initial drying only, provide a screed to maintain proper thickness consistency and act as aesthetic grounds for architectural design.

The Proof is in the Testing

In efforts to allow the option of running continuous lath, many industry experts and former applicators have addressed the ASTM C11 committee offering additional language but to no avail. No testing to prove otherwise was their rebuttal and so has waged the battle for the past 20 or so years.

Last year the technical bodies of the four western contractor associations, known as the Wall and Ceiling Conference, got together and conducted such a test. Two separate tests were also conducted in Texas, one in Houston and another in Austin—I’ll touch on those momentarily. Five “L” shaped wall assemblies were constructed (see diagram) with the required 144 square feet maximum area adjacent to smaller sections. On each of the wall assemblies, one side had continuous lath, the other discontinuous at the control joint locations. All walls were constructed with the same materials to final completion of a stucco assembly, identical in every way, except for the configuration of the control joint. One panel had no control joints at all to be referenced as a control panel. Everything was installed per ASTM standards, the stucco wet cured and allowed to sit for over a year.

Initially, there was minimal cracking with only one or two hairline cracks on some of the panels. The cracks were so small they had to be highlighted with markers and paint in order to be found on subsequent visits. And as time passed more cracks appeared. No cracking was noted near the control joints and in fact, was the type typically associated with movement or shrinkage. All the cracks were considered hairline and would not require any addressing if occurring on a construction project. And most notably, in all the panels with control joints, the largest crack was 0.015 or 1/64 of an inch. The panel with no control joints had one crack 0.030 or 1/32 of an inch.

Each panel exhibited the same cracking characteristics on either side. Some exhibiting more than others. For instance, one set of panels showed maybe three to five cracks on each side while another set showed as many as 12 to 15. Again, each set of panels had one side continuous lath and the other discontinuous.

The Last Crack

The results of this extensive test showed what us old timers already knew; the lath at the control joints, cut or not, does-not-a-crack-prevent. And not forgetting about the tests in Texas, these “different protocol tests” showed similar results. One, done the way they do it there, no wet cure, with various forms of floating, the other done in a continuous smaller dimension panel structure, both showed no significant difference between cutting or not cutting the lath.

Cracking in stucco is never deemed acceptable, especially when exhibited at your beautifully designed entryway. And one would never try to sell a cracked job when the owner is unhappy and yes, stucco cracks and yes, there are probably no crack-free stucco assemblies in existence, but for one to state there is only one way to install a control joint (i.e. with cut lath), is a disservice to our industry. You see, the cracks in stucco are more likely to be caused by other forces: wind, drying rates, mix ratios, sand quality, batch consistency, floating processes, thickness variations, experience of the applicators, etc., than by lath which may or may not be cut at the control joint.

Stucco cracks are the building talking to us, we are best to listen. W&C