Expanded metal lath has been a staple of the plastering industry for more than 50 years. From its beginnings in the 1800s, the basic design of diamond mesh lath hasn’t changed much. What has changed is the number of manufacturers that say they produce a “quality” product that doesn’t actually meet the building code. Unfortunately, market deception has placed responsibility for the product on the installer who must grapple with a legal term caveat emptor, Latin for “let the buyer beware.” The basic premise at work now is that the buyer purchases at his or her own risk, and therefore should examine and test a product themselves for obvious defects and imperfections.
Quality products 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. There are different ways to achieve these standards, such as adjusting the basic thickness of the galvanized steel coil, modifying the configurations of the diamond cutters and changing the amount of “stretch” the manufacturers apply to the base galvanized coil. U.S. lath manufacturers all configure their set-up a bit differently, recognizing the goal is to achieve target weights and dimensions within the allowable tolerances.
The machinery used to produce expanded metal lath are huge; typically more than 100 feet in length (Photo 1) and can be configured to apply Grade D asphalt paper from jumbo rolls (in-line) during lath production and packaging (Photo 2). This automated machinery may only require one or two men to run the lath and can produce more than 1,000 sheets of lath on an eight-hour shift. The head of the machine houses the galvanized coil spooler, where the 7 to 8 inch wide, G-60 pre-galvanized coils weighing more than 10,000 pounds unwind, yielding many thousands of finished sheets. The process is continuous; unwound coil fed into cutting rollers which apply thousands of pounds of pressure to cut through or perforate the solid sheet (Photo 3). Next, stretcher-arms invade the coil to literally “stretch” the compacted diamonds out approximately 27 inches in one continuous run. The machine has a sheet length cut-off that can be set to various lengths, typically (not less than) 97 inches, as required by ASTM C-847-10, to sell as a standard sheet. After a period of time, the cutters must be re-tooled in order to make clean cuts into the solid coil. Worn cutters result in the formation of unexpanded diamonds, or diamonds that are sheared or fractured in configuration.
The expanded lath sheet is made flat and must continue through an additional furring process, where either dimples or v-grooves are embossed into the face of the lath sheet. This process is known as “self-furring” and is required to provide 1/4 inch of furring, measured from the back of the dimple or groove to the face of the lath strands. This furring process is also rolled in-line with the equipment and wears over time, requiring the lath manufacturer to monitor the wear. This wear can manifest in furring falling below the 1/4 inch required in ASTM C-1063, Table 3, footnote C. Lath manufacturers have to replace this furring tooling to maintain the 1/4 inch.
Unknowingly, contractors across North America may be installing sheets that fall below the ASTM standards in weight, length, width and/or metal galvanization. These products are marketed as “utility” or “nominal” lath, and are not manufactured to the ASTM C-847-10 standard. To further complicate the lath dealer’s purchase process, some manufacturers offer a “code compliant lath,” but only when asked for and at a premium price. So, if the sheet has G-40 galvanizing (like non-load bearing studs), or 96-inch long lath advertised as 2.5 pounds weighs 2.1 pounds, all these compromises play into a lower sheet price; enticing the dealer to stock-up.
AMICO has developed a packaging method to carry the third party identification all the way down to individual bundles that are supplied to the job site (Photo 4). This method is not proprietary so we encourage all lath manufacturers to proudly display their third party test data and identify the weights of the laths being supplied. Whatever method is chosen, the intent is to provide full disclosure to the dealer and contractor they are purchasing lath sheets with confidence that it is produced to proper dimensions and weights defined in the ASTM C-847-10 standard.
Other laths like pure zinc alloy (a softer metal than galvanized steel) and “non-metallic scrims” are being advertised as acceptable laths for stucco and stone as well. While these alternate products may carry an ICC-ES report, are they recognized by the building code as well? Do they meet or exceed ASTM C-847-10 physical properties and recognized by ASTM C-1063-08 as acceptable materials for installation in Portland-cement based plaster systems? Without this documentation the contractor is taking a calculated risk in terms of liability for long-term performance.
Lastly, the EMLA saw the need to publish transverse loading values using expanded metal lath on three-coat stucco. 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 a IES accredited lab in Los Angeles and asked the Technical Services Information Bureau (a division of the Western Wall & Ceiling Contractors Association) to assist by having the local union build the 4-foot-by-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. Photos 6 and 7 are the test panels being constructed, using typical wall systems we see used with lath and plaster. These systems include:
2.5 Self-furring, expanded metal lath in a three-coat stucco system over exterior gypsum sheathing attached to 6-inches x 54 mil x 50 ksi min. (16 gauge) metal studs spaced at 16-inches on center with #8 modified truss screws.
2.5 Self-furring, expanded metal lath in a three-coat stucco over exterior gypsum sheathing attached to 2x6 wood studs (No. 2, SPF or So. Pine) spaced at 16-inches on center with #8 modified truss screws.
3.4 Self-furring, expanded metal lath in a three-coat stucco over exterior gypsum sheathing attached to 3⅝-inches x 33 mil x 33 ksi min. (20 gauge) metal studs spaced at 16-inches on center with #8 modified truss screws.
3.4 Self-furring, expanded metal lath in a three-coat stucco over exterior gypsum sheathing attached to 2x6 wood studs (No. 2, SPF or So. Pine) spaced at 16-inches on center with #8 modified truss screws.
The test panels cured last fall. Then they were subjected to the transverse load testing criteria outlined in AC 191 - Acceptance Criteria for Metal Plaster Bases (lath), Section 4.0 test methods. Once this data is compiled, the EMLA will share the results as they see fit with various ASTM committees and code bodies to establish a benchmark for stucco or stone installation design.
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 can also be noted on markings and or tags on each bundle shipped to your project; providing additional assurance that one is in fact installing code-compliant lath.