Laths and Stone
Lath Integrity: You (May Not) Get What you Expect
Photos courtesy of Alabama Metal Industries Corp., Birmingham, Ala.
Expanded metal lath has been a staple of the plastering industry for more than 100 years. From its beginnings in the 1900s, the basic design of diamond mesh lath hasn’t changed a great deal. What has changed, however, is a growing segment of lath production that does not meet the building code. The problems, damages and associated responsibility of the installation of these inferior laths are allocated to the designer and/or the installer of record. The inherent risk of purchasing, selling and installing a non-compliant lath deserves discussion to avoid performance of the stucco cladding.
Characteristics of code-compliant expanded metal laths are defined in ASTM C-847-10 Physical Properties of Expanded Metal Lath. In this ASTM standard, the lath dimensions (length, width and expanded thickness), weights (expressed in pounds per square yard) and allowable tolerances are defined for the industry. A variety of ways exist to satisfy these criteria, such as selection of base thickness of galvanized steel; the configurations of the diamond cutters; and the amount of “stretch” the machine applies to the base galvanized coil. U.S. lath producers configure manufacturing protocols differently, but share a common objective to produce lath sheets that satisfy the required weight and dimensions within the allowable tolerances.
The machinery used to produce expanded metal lath are huge mechanical machines – typically more than 100 feet in length. The machines can be configured to apply Grade D asphalt building paper from jumbo rolls during lath production and packaging. The automated machinery may only require one or two men to run the lath and can produce up to 1,000 sheets per shift.
The head of the machine houses the Galvanized Coil Spooler, where the seven- to eight-inch-wide, G-60 pre-galvanized coils, weighing more than 10,000 pounds, unwind. This yields up to 3,000 sheets. The process is continuous; with unwound coil is fed into the cutting rollers that apply 20,000 pounds of pressure to cut through the solid sheet. Then, stretcher-arms attach to the coil to literally stretch the compacted diamonds out about 27 inches, as the sheet continuously runs. The machine has a sheet length cut-off that is set to the required length, typically (not less than) 97 inches, as required by ASTM C-847-10. After a period, the “cutters” must be re-tooled, to make clean cuts into the solid coil. Worn cutters yield undesirable effects of unexpanded diamonds, or diamonds that are sheared or fractured in their configuration.
The process described above produces flat-expanded lath sheets. For those flat sheets to become “self-furred,” they continue through an additional process, where either one-quarter-inch dimples or v-grooves are embossed into the face of the lath sheet. These dimples or v-grooves provide one-quarter inch of furring, measured from the back of the metal dimple or v-groove, to the face of the lath strands. This furring process also is rolled in line with the equipment and wears over time, requiring the lath manufacturer to monitor the wear. Over time, this wear can result in furring falling below the one-quarter inch that is required in ASTM C-1063, Table 3, footnote C.
Other “alternative” laths, like pure zinc alloy (a softer metal versus galvanized steel) and non-metallic scrims, are being advertised as acceptable laths for stucco and stone as well. While alternative products may carry a third-party evaluation report, they are not necessarily recognized by the local building code. Moreover, these laths may not be recognized by the ASTM C-1063 as acceptable materials for installation of Portland cement-based plaster systems. Without this documentation, the selection and installation of these laths present designers and installers with an assumed risk, in terms of liability for long-term performance.
Unfortunately, contractors across North America may be installing sheets that fall short of current ASTM standards in terms of weight, length, width and/or metal galvanization. This sub-standard quality is being produced and marketed as “utility” or “nominal” lath, without intention to meet current ASTM C-847-10 standard. Examples of “utility” or “nominal” lath include G-40 galvanizing, (versus required G-60), lath sheet lengths of 96 inches (versus the minimum of 97 inches) and lath sheet weights that fall below the advertised 2.5 pounds to a 2.1 pounds per square yard. The production of utility lath offers manufacturers raw material savings that, in turn, is offered as a reduced price that attracts consumers. To further complicate the lath dealer’s purchase process, some manufacturers offer a “code compliant lath,” but only when specifically requested and sold at a premium price.
Yet, contractors who install stucco or stone systems according to ASTM standards depend on the dealer to provide lath products meeting industry standards. This product expectation typically is not questioned, but should be. There are various ways lath manufacturers can assure the lath dealer and installer of producing and supplying quality products. One method is requiring the lath manufacturer to submit to third-party testing reports, demonstrating compliance to ASTM C-847-10, defined under ICC’s Acceptance Criteria 191. This third-party testing can be performed by an IES accredited laboratory, or another third-party testing laboratory. Typically, these tags are attached to the pallet during manufacturing.
One manufacturer has developed a packaging method to take the third-party identification all the way down to individual bundles that are supplied to the jobsite; each bundle has the lath weight. Whatever the method chosen, the intent is to provide full disclosure to the dealer and contractor purchasing lath sheets produced to proper dimensions and weights as defined in the ASTM C-847-10 standard.
The Expanded Metal Lath Association (EMLA) Division of NAAMM -National Association of Architectural Metal Manufacturers is a trade association formed back in the 1970s. EMLA’s goal is to promote the use of expanded metal lath in building construction by providing technical data to support the architect or engineer’s design needs to for lath and framing design.
In 2009, EMLA members performed an exhaustive edit to the original EML/SFA-920, first published in 1991. This renamed document “EMLA-920-09,” sub-titled “Specifications for Lathing & Plastering of Steel Framing Systems,” was updated with new terminology, installation details, etc. That revised document is available as a free download at www.naamm.org/technical. More recently, EMLA saw the need to publish quantitative values on transverse loading, using expanded metal lath on three-coat stucco. So, EMLA developed an RFQ for running transverse load testing on six framed wall scenarios, following the criteria outlined in ICC’s -AC191 (Acceptance Criteria for Evaluating Expanded Metal Lath). EMLA hired an IES accredited lab in Los Angeles, and asked the Technical Services Information Bureau (TSIB, a division of the Western Wall and Ceiling Conference) to assist by having the local union build the four- X 4-foot test frames of the six separate wall systems. The AC 191 requires six tests of each wall system, requiring 36 panels to be tested.
These systems include:
- Both 2.5 pound 3.4 self‐furring, Expanded Metal Lath in a three‐coat stucco over exterior gypsum sheathing attached to 3 5/8 inches X 33mil X 33ksi min. (20 ga) metal studs spaced at 16 inches on center with #8 modified truss screws.
- Both 2.5 pound and 3.4 self‐furring, Expanded Metal Lath in a 3‐coat stucco over exterior gypsum sheathing attached to six inches X 54mil X 50ksi min. (16 ga) metal studs spaced at 16 inches on center with #8 modified truss screws.
- Both 2.5 pound and 3.4 self‐furring, Expanded Metal Lath in a three‐coat stucco over exterior gypsum sheathing attached to two- X six-inch wood studs (No. 2, SPF or So. Pine) spaced at 16 inches on center with #8 modified truss screws.
Test results are being evaluated now, and will be published for the design industry to access.
In conclusion, expanded metal lath, like many other building materials, is under extreme pricing competition in the marketplace. This competition has resulted in the production, promotion and marketing of laths that do not comply with industry standards or the building code. With these laths offered at discounts to the code-compliant laths, dealers and contractors often choose these products, because they assist with offering a bid with a lower overall cost.
To be certain you are receiving a code-compliant expanded metal lath, have your dealer send a certification from the manufacturer that the metal lath being supplied to your project has been made to the ASTM C-847-10 standard. This also can be noted on markings and or tags on each bundle shipped to your project. This provides additional assurance of installing code-compliant lath.