Masonry Magazine February 2012 Page. 41
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. US. 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.
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READER SERVICE #173
February 2012
MASONRY 31