I wouldn't be in favor of cutting the lath. The main reason for control joints is to allow thermal movement in the stucco and continuous lath would negate this movement.

Theoretically, wire tying the lath on both sides is preferable from a thermal movement standpoint. Properly done the control joint shouldn't need backing. Sometimes the control joint is nailed into a stud on one side, then wire tied on the other. Nailing both sides leads to cracking or a situation where the stucco pulls away from the control joint.

Lath curling is another issue altogether, but a good one to be discussed. I see it on some installations as well and it's usually the result of many small installation errors resulting in problems later.

Flat walls - make sure they're straight and make corrections. If the wall is shaped like a banana your lath job is doomed to look the same.

Self-furring lath is sometimes nailed randomly up the stud line rather than at the dimple indentations. This causes the lath to buckle in and out rather than laying plumb a small distance out from the wall.

The corrective measure for wavy lath is staples. Many, many staples. Let's just call them holes. The lath is now tight to the wall, the stucco above is essentially unreinforced with the lath in the wrong place and the holes complete the destruction.

The process we use in our training is to strike the level/plumb lines where the control joints are going, tack them in place and set ends in sealant, then run your lath and wire tie every 6". Remove the tacks.

THANK YOU MARK.
I THINK THE LATH SHOULD BE CUT AND ATTACH TO FRAMING PREVIOUSLY INSTALLED TO HOLD SHEATHING AND THE CONTROL JOINT ITSELF.
BEST REGARDS.

Mark, this is a little more than a comment, but, hey, you asked!

CODE ANALYSIS AND INTERPRETATIONS REGARDING ONE COAT STUCCO SYSTEMS

There is an issue with the IRC codes that needs to be addressed by architects, designers, builders, manufacturers, installers, and ultimately the Code Authority.

IRC codes have adopted specific ASTM standards that were originally written to address conventional, full thickness 3-coat plaster. Although many parts of the standard do pertain to both thin-coat or one-coat stucco and 3-coat conventional plaster systems, there is one area that demands a separate standard or modified language to the existing standards in order to enable installers to prepare both quantifiable and consistent bid amounts and expectations in the residential production housing market.

The section in question is the language regarding “control joints”. The C 1063 ASTM standard states that in the absence of any direction or location of control joints on the plans, the installer shall use conventional 3-coat plaster details.

The issue with this blanket statement is that in many markets, the production housing industry no longer builds to a 3-coat standard, especially in the South and Western areas of the continental U.S.
Most wood frame housing in these areas is not engineered to specifically accommodate true expansion joints.

Before proceeding, a discussion of terms is in order here.

“Expansion joint” is the term for the separation of stucco or plaster sections of work, usually delineated by area size, but also location in relation to doors, windows and other openings in the wall or ceiling. This term is usually reserved for true separations in the structure beneath the lath bed to allow for anticipated movement.
The same term is used in concrete work, to describe a purposeful separation of monolithic slabs. This separation is total, through the structure beneath and allows for expansion and contraction in the structure, expansion and contraction of the soil, and in the sections of concrete itself. On flatwork, the void in this separation is usually filled with a compressive material such as asphalt saturated strips of cellulose in ½ inch thicknesses. Standard width is approximately 3 inches and is placed before the pour, along with the forms for the concrete.

This separation in plaster walls & ceilings is accomplished in a similar fashion, and a wide variety of metal and plastic or vinyl accessories are commercially available. In plaster and stucco work the lather installs each side of this joint separately, and fastens each to its’ respective lath base. Each side is of these two panels are supported by independent framing and sheathing components engineered to allow for anticipated and unanticipated movement forces within and without the total structure envelope.

“Control joint” on the other hand, is a term used to describe an intentionally applied weakness to the cement membrane, or surface. Since concrete will usually separate at the weakest point, these “joints” create a place for the curing forces within the cement to release the heat buildup and energy created during the cure process. They are not structural in nature and do not require the total separation of the substrate.
In concrete flatwork these joints are tooled into the wet concrete after the pour and before the concrete sets. This creates a weakness in the slabs that allow a crack to occur along these lines and relieve the internal stresses present after the pour.



In Plaster and Stucco, these control joints are usually created with a one-piece, folded sheet metal with expanded wings to allow the sides of the stucco membranes to key into and lock onto the sides of the cement membrane. Separation of the lath and underlying substrate is not necessary, and indeed may be harmful to the integrity of the lath base.

The forces that these type of joints are designed to handle are far less than the Expansion type of joint described above. The Control type of joint creates a break within the stucco membrane to allow stresses during cure to be released through compression and expansion of the stucco membrane itself, not the structure beneath. Some structural movement is accommodated by this type of joint, but far less than is possible in a true expansion/separation joint.

ASTM C 1063 was written specifically to address the installation of the true separation, or expansion joint used most commonly in full thickness, three coat plaster systems as stated in the first section of the standard:
“This specification covers the minimum requirements for lathing and furring for the application of exterior and interior Portland cement-based plaster as in specification C 926.”

ASTM C 926 then states, again in the first section:
“This specification covers the requirements for the application of full thickness Portland cement-based plaster for exterior and interior work.”

The standard within most ICBO reports for one coat stucco products states that “Control joints must be installed as specified by the architect, designer, builder or exterior coating manufacturer in that order.
In the absence of details, conventional three coat plastering details must be used.”
This section clearly lays out the hierarchy for deciding the location of the control joints. Note that the installer is not mentioned except in the last sentence, then only by vague reference to knowledge used in another type of plaster system.

The performance and structural design requirements of thin coat stucco are not interchangeable with three coat plaster and the material specifications differ substantially.

Applying the spacing and location information given in the standard is prudent advice, and should be retained; however, the sentence that references separation of the lath behind the “control” joints should be deleted when applied to any thin coat stucco system built without the required engineering to support true expansion joints.

Application of this standard to residential wood framing is possible only when the structure is engineered to address excessive structural movement, and the stress points are located by the architect or engineer. Thus a set of plans can be produced that all bidders can see the amount and locations of the required expansion and control joints. The result is a quantitative basis upon which consistent estimates can be produced.

Without the underlying engineering present to guide them, with true separation of the structure, and separate framing panels with the attendant components such as double studs and separated sills and plates, the installer is unable to consistently determine locations and quantity of control joints accurately. Thus the standard must revert back to the inherent architectural responsibilities of design and location of control joints. One could make the argument that only the “control” type of joints and detailing be employed in thin coat residential stucco systems, at least when the design lacks true separation joints.
Application of the standard as written will result in out of compliance liabilities for the majority of subcontractors, and all of the work performed since the code was adopted.
With these conditions in mind, we propose the following language be inserted into the standard for thin coat stucco systems:

Architectural design of the project shall include locating any control joints on the project drawings at time of bid that the architect, designer, builder or exterior coating manufacturer determines to be necessary for the optimal performance of the stucco system specified. Plans without control joints shown are to be interpreted to mean that none are necessary according to the architect, designer or builder. All flush mount window and door openings will have butterfly type metal reinforcing installed at each corner. If a foam pop-out is installed at the corners, no butterfly patch will be necessary.

Cracks in stucco are a natural occurrence when a brittle exterior stucco system is placed on a wood frame structure subject to dry warp and movement. Excessive cracking can be caused by a multitude of factors. Many of the conditions that contribute to stucco cracking are out of the applicators control.

Stress points in the structure should have control joints located by a certified structural engineer or Architect.

Wood based sheer panels should be gapped at all edges 3/16” minimum.

Wood used at exterior walls should be less than 19% total moisture content. Most of the shrinkage of wood occurs as the moisture content drops from 28-19%, during the drying process. Even properly dried wood will absorb and release moisture introduced during construction. Dry wood that has been re-wetted has a higher tendency to warp during dry-out due to uneven evaporation rates. Houses that receive rain during the period before lath should be allowed to dry completely before proceeding.

Wood members that exhibit warp or bowing should be replaced before lath installation. Tolerance for the finished stucco plane is ¼” in 10’. The substrate tolerance should be at least that required for Stucco.

Subcontractor will perform crack repair of minor cracks in excess of 1/16” when such cracks are clearly the fault of the applied stucco mix, curing process, or other such cause within the control of Subcontractor, or its forces. Cracks clearly resulting from one or more of the above listed factors that are out of Subcontractor’s direct control will be repaired after the issuance of a repair order, with the cost for such repair born by others, and returned to Subcontractor in the form of a Purchase Order from the General Contractor.

We need a position paper from the various manufacturers of thin coat stucco on this issue asap.

Please feel free to call with any questions you may have,

Sincerely,

______________________________
John Carroll
jcarroll@desertvistainc.com

I have plastered hundreds of projects with continuous lath behind the control joints and had no problems