Technical Questions & Answers

Technical Questions & Answers

(Answers provided by the Texas Lathing & Plaster Bureau) - Texas Lathing & Plastering Contractors Association (TLPCA) formerly - South Central Wall, Ceiling & Plaster Association (SCWCPA) Technical Committee.
Answer: Control Joints are the most debated misunderstood, missed and most litigated component of a plaster system.

ASTM C1063 which by reference is a part of the International Building Code (IBC) and the International Residential Code (IRC) states in:

  • Article A!.3 –Control Joints shall be installed to minimize stress out to stucco curing, drying shrinkage and minor movement, along predetermined, usually straight lines and act as a screed to aid in stucco thickness control.
  • Article 7.11.4.1 outlines the placement requirements for control joints.
    • Wall areas not to exceed 144 sf.
    • Ceiling areas not to exceed 100 sf.
    • Distance between control joints shall not exceed 18 ft in either direction.
    • Control joints shall be used to keep the shape of panels length-to-width ratio not to exceed 2-1/2-to-1.

Control Joints are not expansion joints and serve to create panels within a stucco clad wall. Expansion joints are used to accept movement within the structure and between dissimilar materials.

Here is where it gets tricky and complicated:

  • Control Joints do not guarantee against cracking.
  • The curing and drying shrinkage generally occurs within the first 48 – 96 hours during the application of the cement plaster scratch and brown coats.
  • Minor movement has never been defined or clarified.
  • Cracking occurs within a stucco panel when a stress greater than the ability of the cement plaster to resist is placed within or on the cement plaster.

Article 7.10.1.5 states Lath shall not be continuous through control joints. This means that the lath material will be cut behind the control joint and the control joint wire-tied on top of the metal lath. If this is done, a framing member will have to be installed in the wall on each side of the control joint to attach the cut end of the sheet of the metal lath. This is rarely the condition on a job site. The framing contractor and the lathing contractor are not the same subcontractor and are nor generally aware of each other’s needs.

As much as 75% of the country (my estimate) does not cut the lath behind the control joint but ties the control joint to the face of the metal lath which by default is required if there is no framing member installed behind wall at each vertical control joint. Most of the stucco contractors and the contractor associations in the South, Southwest, West, Northwest and Midwest follow the practice of leaving the lath continuous behind the control joint.

This practice of continuous lath behind the control joints is constantly debated and is the basis for many litigation cases involving stucco. It is my opinion that proper curing of the coats of the stucco is far more responsible for reducing cracking than control joint placement or method of installation. In my opinion cutting the lath behind a framed wall with sheathing is unnecessary but beware of the Building Code requirements because you are likely violating one or the other (cutting the lath or placing studs for the lath sheet ends) in a typical project.

Answer: A stucco or One-coat stucco system may be installed onto a metal lath plaster base over an EIFS. This application method is analogous of installing stucco over continuous insulation, which is becoming more commonplace to comply with Energy Code requirements. The metal plaster base must be installed so that it is positively attached to framing members at appropriate intervals per the system manufacturer. Additionally, the fasteners must be of sufficient length and shear strength to support the stucco system.

It is prudent and advisable to consult the EIFS and One-coat stucco manufacturer regarding compatibility of the materials and appropriateness of the assembly. It is my understanding that the manufacturer’s technical department has been assisting with input regarding the stucco over EIFS assembly. Assuming that they are aware of the current asbuilt and intended finished assembly, performance should not be a problematic issue if the materials and assembly is deemed a warrant-able application by the product/system manufacturer.

Answer: Stucco accessories such as corner beads, casing beads, control joints, etc. can be galvanized, zinc, or vinyl and the climate can dictate which is most appropriate. For example, if your project is within the City of Houston, a standard galvanized accessory is suitable. Far Southern Harris county, Galveston county, and surrounding coastal counties do not specifically require the use of a more corrosion-resistant accessory, but it is prudent to specify zinc or vinyl accessories. The accessories have more exposure and are more susceptible to degradation that the metal lath being that the lath should be completely embedded within the stucco.

What I know:

  1. The plaster isn’t rock hard. A screwdriver can be plunged through it.
  2. It’s about 1” thick on metal lathe.
  3. Testing has shown that it doesn’t contain asbestos.

Two questions:

  1. Are there any tests I can do to determine what kind of plaster it is?
  2. What time period were perlite and vermiculite gypsum plasters available?

Answer: When you say soffit, I am assuming this is an exterior installation which would lean toward cement plaster. If this is an interior installation, it is likely gypsum plaster.

However, when you say a screwdriver can “plunge” through it, it does not sound like cement plaster. Gypsum plaster with a lightweight aggregate could have been used for fireproofing of the truss.

The easiest way to tell is immerse the product in water, if it swells and/or deteriorates it is gypsum plaster. If it remains hard it is cement plaster.

Perlite and Vermiculite plasters are still available.


Answer: Yes, you can attach the lath direct to the metal studs and be in compliance with the code. However, it is not recommended. With thermal heating of the stucco exterior cladding and the expansion of the metal stud framing, many times there is a vertical crack in the stucco that mirrors the metal stud layout. Additionally, I would not want my stucco to be able to reach the mineral wool and probably cause it to lose some of its thermal resistance value.

It would be my recommendation to use another layer of sheathing to separate the metal lath from the studs and insulation. It is my opinion that using paper between the metal lath and the studs would not be sufficient.

You have the Z furring properly shown in the vertical direction. The metal lath has to be placed at right angle to the framing member to which it is attached, To turn the Z furring horizontal would have the lath running vertical which make it very difficult for the application of the stucco material.

I would also run a Water Vapor Transmission Analysis on the complete wall once the final design is reached to insure that there is no dew point which may occur interior of the weather barrier that could result in moisture damage.

The use of the mineral wool to achieve Continuous Insulation is unique in an exterior plaster wall. My question to you is “with the probability of humidity and condensation occurring in the wall cavity, how much is the R-value of the mineral wool reduced and will the wool begin to sag in the cavity”? I do not see any provision for clips or other means to maintain the mineral wool continuous in the cavity.

Answer:

Brick is an acceptable substrate for the application of stucco (Portland cement plaster). Stucco can be applied with or without a metal plaster base (lath) depending on the condition of the brick and the ability of the stucco to bond to the brick. A solid surface should have roughness and the ability to absorb water so a bond between the brick and stucco can be achieved. Typically, a metal base (lath) would be used if the brick is painted or another condition is present that would prevent a bond over the entire brick surface.

When applying stucco over brick on vertical surfaces (provided that bond can be achieved), it can be applied in two or three coats. See the following:

Three coat work over unit masonry (brick):

  • 1st coat: ¼”
  • 2nd coat: ¼”
  • 3rd coat: 1/8”

Two coat work over unit masonry (brick):

  • 1st coat: 3/8”
  • 2nd coat: 1/8”

The last coat in both of the applications above is 1/8” thick and is so that the finished texture can be achieved. It is common in a 3-coat system for the 1st and 2nd coats to be installed in fairly quick succession. The 2nd coat of a 3-coat application may be applied as soon as the 1st coat is rigid enough to hold the next coat without damage or cracking to the 1st basecoat.

Also, if the chimney is clad with brick veneer in lieu of a solid masonry fireplace/chimney assembly, ensure that any weeps or other provisions for air flow and/or drainage are maintained through the stucco assembly where they occur in the brick. We appreciate you contacting the SCWCPA! Let us know if we can be of further assistance.

Answer:Under the Dallas Building Code (2012 IBC with amendments), we are working on a church school project with cement plaster and brick/stone veneer.

The building is primarily steel-framed with metal stud infill, though there are some areas where CMU is used to create fire walls. To meet the requirements of the IECC, we felt compelled to go with 2” continuous insulation. The lower 10 feet of the building’s elevations are brick/stone veneer with cement plaster above that point. Thus, over the metal studs and ext. gyp. sheathing, a fluid-applied air barrier has been applied. On top of that goes 2 inches of XPS rigid insulation and either 3 coat cement plaster or a 2” air space and a brick or stone veneer. Where the wall substrate is CMU, the stucco is applied to metal lath installed directly on the CMU or it is applied to ext. gyp. sheathing and insulation separated from the CMU by metal furring channels.

All of a sudden, we are being given this argument by the sub:

“Rigid insulation expands (thermal) with a force 6-times greater than EPS foam. When it is installed behind stucco (cement plaster) and expands, the added force can exceed the ability of the stucco to bear it and results in excessive stucco cracks. This issue is multiplied exponentially when it is installed over CMU. The CMU has little to no deflection, so all of the expansion force is directed outward into the backside of the stucco assembly resulting in devastating stucco cracks. As dire as that sounds, there is a very simple and affordable solution.

If we change all of the stucco at these locations to 3" Drainage EIFS we can provide the same thickness of wall as the 2" rigid plus the 7/8" stucco as well as maintain an R-value of over 11. This would include all locations on the building other than the Cross area, which is already being completed... and does not have the rigid insulation behind it. We can provide this system with intermediate mesh for added durability/reinforcement. Also, the finish coat material is the exact same as has been used on the area for the Cross, so the finished appearance will be the same so there should be no concerns with it matching. As noted above, this would be Drainage EIFS which means it will also have the liquid weather barrier over the sheathing behind all of EIFS which is an upgrade and will provide a 10-year system warranty from the EIFS manufacture vs the standard 1-year warranty stucco provides.”

On the surface, this sounds good, but after searching the internet, I was unable to find support for the expanding XPS theory. (The attachment seems to say that all insulation expands and it shouldn’t be used as a weather barrier.) There seems to be some bad blood between EPS and XPS manufacturers, but I cannot find any independent sources that discuss the problem. And TSIB (second attachment) doesn’t really discuss it either. When I pointed out that we had followed TSIB recommendations, I received this e-mail from a friendly competitor (Parex) of the sub (STO):

“We are experiencing problems with the use of Extruded Polystyrene (XPS by Dow Corning etal..) behind plaster, especially when installed over a solid substrate such as CMU or Cast Concrete. What is happening is that the forces with which XPS expand and contract are strong enough to cause significant cracking in plaster. Over a solid substrate like CMU or Cast Concrete this force cannot go inward due to the rigidity of the substrate, so the forces are applied laterally and perpendicular to the substrate. As a result, we are CONSISTENTLY seeing stucco crack when installed over a solid substrate and XPS. I noticed in your email you referenced TSIB, I too use them as a good resource, but I believe the one thing that is different with regard to TSIB being a West Coast firm, is that the stucco industry west of Texas uses expanded polystyrene (EPS) versus extruded polystyrene (XPS) behind plaster. EPS, when it expands and contracts, does so with very little force and as such does not add to any cracking that stucco might experience.

So what to do? XPS is being specified quite frequently behind stucco now since it has a higher R-Value than EPS (r-5, vs r-3.8). And in cases where the owner or architect wont budge I tell them to use lots of expansion joints (#40 Expansion Joints vs #15 Control Joint) since we now know that XPS is problematic. If they are willing to look at EIFS, this is the best solution since EIFS is now water managed, can be made highly impact resistant and has all the necessary fire testing. If stucco has to be used, I recommend that the architect look at using Expanded Polystyrene (EPS) in lieu of XPS to help control the stress that XPS can impart on stucco laminas. ”

The Owner is leery of EIFS type systems but I’m less so because I know that improvements have been made. Though I recently found out that on multi-family projects in our office, cement plaster is almost exclusively used, but not over continuous insulation – some manipulation of the COMcheck calculation.

Answer: ASTM C926.3.2.10.5 defines the double-up coat the brown-coat plaster applied to the scratch coat plaster before the scratch-coat plaster has set.

C926.7.2.1.2 The first (scratch) coat shall become sufficiently rigid to support the application of the second (brown) coat without damage to the monolithic continuity of the first (scratch) coat or its key.

I further refer you to Titan Americas tech bulletin on this subject authored by Mike Starks and The Portland Cement Association’s Stucco manual. There are many publications endorsing this method as the scratch and brown dry as one. It is not advisable on open framing or horizontal surfaces.

Answer: There is no established standard requiring periodic painting of stucco. Stucco can be painted for many reasons: one might desire to change the color, the color has faded over time, or one may need to paint to fill and bridge hairline cracks in stucco panels. It is most important to know what type of textured finish is applied so that you can choose the appropriate coating. In our region, it is common to have one of two types of stucco finish: an integrally textured and tinted proprietary finish (with or without elastomeric properties) or a painted (typically elastomeric paint) traditional Portland cement plaster textured finish.

Use of an acrylic finish does not preclude painting. If it is determined that the stucco has a proprietary acrylic finish, it is advisable to use coatings from the same manufacturer; however, manufacturer information is not always known. Therefore, high quality exterior acrylic latex paint is suitable for re-coating acrylic finishes that are not elastomeric finishes (meaning that they are stretchy). An elastomeric paint would be necessary if the acrylic finish has elastomeric properties. Whichever the case, the paint chosen should have a permeability rating that will allow water vapor transmission.

Skybeck Construction (GC) 4300 Cromwell Drive, Kyle, TX / Carrington Oaks multi-family

  • Zip Wall OSB
  • (2) Ply 60 Minute
  • 2.5 SF Metal Lath
  • 3/4" Western 1-Kote
  • Senergy Sahara with Primer

Lath system was completed on Leasing Office. My team did not continuously wrap metal lath around corners purposely, cut at 90 degree and installed K-lath corner aid. GC's consultant required us to remove corner aid, wrap corners with metal lath continuously and then install corner aid. Am I correct, based on ASTM 7.11.2.1, that if a corner bead is installed I am not required to wrap lath continuously based on the system described above?

Answer: ASTM C-1063 is the reference document which, by reference, is a pert of the International Building Code as the association/bureau source for its position in this matter. When a corner bead and/or corner aid is used at external corners there is no requirement for the metal lath to be wrapped continuously around the corner. Please refer to ASTM C-1063-14a, Article 7.11.2.1 which reads as follows: 7.11.2.1 External Corner Reinforcement - External corner reinforcement shall be installed to reinforce all external corners where corner bead is not used. Where no external corner reinforcement or corner bead is used, lath shall be furred out and carried around corners not less than one support on frame construction. The requirement to wrap the metal lath around the corner is only required where corner bead is not used. The K-Lath Corner Aid qualifies as a corner bead in this condition.

Answer:The 2" requirement can be a catch 22 item, In ASTM C-1063, article 7.11.5 states that the bottom edge of the Weeps Screed shall be placed not less than 1" below the joint formed between the foundation and the framing along with the 4" and 2" clearance requirements. Sometime complying with both requirements is not possible.

In my opinion, this is the controlling factor for the termination of the stucco. The stucco installer is not in charge of where finished 'raw earth' is to be or where the paving is to be placed. Those items are under the purview of the architect (designer) and/or builder, not the stucco contractor.

If the stucco is terminated as outlined above, this does not appear to be a stucco issue. To raise the termination of the stucco if at the foundation/framing line would create more problems than you have now.

I agree with the Sierra Inspector as to the need for positive slope away from the foundation. Any fix that is recommended should be permanent and long-term, not a band-aid fix that required periodic maintenance of repair. Removal of the stucco wall and adding additional concrete beams/foundation and/or flashing may be the only solution.

Answer: There is no simple black and white answer to this question. First, you need to know what type and what thickness of Continuous Insulation you will be using. Next you have to determine the moment-arm placed on the fastener to determine the diameter of the screw fastener to be used. (RE: Foam Sheathing Coalition research study "Guide to Attaching Exterior Wall Coverings through Foam Sheathing to Wood or Steel Wall Framing" Released September 20, 2010)

Answer Part B: The thickness of the C1 required is based upon the zone placement and the date of the code being designed for. You will have to refer to the appropriate IECC document to get that information. Would "guesstimate" somewhere between 1" and 1-1/2" would meet the requirements. The only typical detail that I am aware of to date have been published by the TSIP out of California. There is just not enough history and successful projects to review to publish standard details.As for the placement of the insulation, it should be outboard of the sheathing and weather barrier.

The Texas Lathing and Plastering Contractors Association (TLPCA) is an organization for the promotion of quality methods and practices in these trades. Since 1952 the Association members have established the workmanship standards for the industry in Texas.

P. O. Box 152282 Arlington, Texas 76015

info@tlpca.org or yvonne@tlpca.org

(817) 461-0676