This process produces flat expanded metal lath sheets. To create &#;self-furring&#; lath, the flat sheets continue through an additional process, where either dimples or V-grooves are embossed into the face of the lath sheet. These provide 6.4 mm (¼ in.) of furring, measured from the back of the dimple or groove to the face of the lath strands. This furring process is also rolled with the equipment and wears over time&#;this can result in furring falling below the 6.4 mm required in ASTM C (specifically in Table 3, Footnote C). The change is monitored weekly by the manufacturer such that when it falls below minimum levels, the tooling is replaced.
Code-required properties
Characteristics of code-compliant expanded metal lath are specified in ASTM C847-10, which defines lath materials, dimensions (length, width, and expanded thickness), weights (expressed in lb/sy), and allowable tolerances. There are various manufacturing characteristics to satisfy these criteria, such as the base thickness of the galvanized steel, the configurations of the diamond cutters, and the amount of &#;stretch&#; the manufacturer applies to the base galvanized coil. The various U.S. lath manufacturers configure design criteria differently, but share the common objective to produce lath that satisfies the required weight, dimensions, and other physical properties within the allowable tolerances.
ASTM C847 requires metal lath be composed of cold-rolled carbon steel sheet conforming to ASTM A653, Standard Specification for Steel Sheet, Zinc-coated (Galvanized) or Zinc Iron Alloy-coated (Galvannealed) by the Hot-dip Process. As also required by ASTM A653, hot-dipped galvanized lath shall have a minimum G-60 coating (representing a zinc content of 183 g/m2 [0.6 oz/sf]).
While the minimum galvanized protection specified in ASTM C847 is G-60, other materials are available to produce lath with enhanced corrosion protection. Examples in ascending order of corrosion protection are:
The selection of the lath material is often based on the environment, along with the plaster or stone application. For example, zinc alloy or stainless steel lath and accessories are preferred in coastal environments, whereas galvanized would be suitable in arid climates for a fraction of the price. It is important to note while zinc alloy offers greater corrosion resistance, the material is much softer than galvanized and stainless steel. Consequently, zinc lath may result in reduced shear and transverse load performance. It is for this reason the authors recommend the selection of stainless steel lath when enhanced corrosion protection is required.
Whichever material is selected for the lath, it is good practice to maintain it in the plaster accessories (e.g. control joints, casing beads, and weep screeds) and fasteners. Incompatible materials can invite galvanic corrosion or varying levels of corrosion protection that can adversely affect the service life of the building&#;s cladding.
Zinc alloy and stainless steel are not directly recognized by ASTM C847 or the building code. Moreover, they may not necessarily meet or exceed ASTM C847 physical properties or be recognized by ASTM C as acceptable materials for installation in portland-cement-based plaster systems. However, it stands to reason the harder, stronger alloy (e.g. SS304) will exceed the performance capabilities of galvanized steel, ultimately providing equivalent structural resistance and superior corrosion protection.
The use of laths comprised of these materials must be evaluated and deemed acceptable in the form of a code-compliance report (i.e. International Code Council Evaluation Service [ICC-ES] Report) that tests and certifies their performance as an alternate material. Without this documentation, the use and installation of these laths present designers and contractors with an assumed risk in terms of liability for long-term performance.
In addition to being knowledgeable in the material makeup of the lath discussed in this article, the specifier and installer should also be aware of physical characteristics of lath readily available in the marketplace, but potentially non-compliant with code and industry standards. Examples of these physical characteristics include lath weight, length, and width.
Wusheng Hardware Product Page
This intentional substandard quality is being marketed as &#;utility&#; or &#;nominal&#; and intentionally does not meet ASTM C847-10. Examples include:
The adverse effects of reduced galvanizing and lath weight are clear&#;less-than-minimum zinc coatings invite corrosion, while less-than-minimum lath weights can compromise the resistance against structural and wind loads. However, the effects of the reduced sheet lath length may not appear so obvious. Lath sheets are traditionally produced in mm (97 in.) lengths to accommodate framing member spacing of 406 or 610 mm (16 or 24 in.) on center (oc) with an extra 25 mm (1 in.) to provide the minimum 25 mm end lap stipulated by ASTM C. In other words, use of -mm (96-in.) length sheet lath over standard framing member spacing will not provide the specified lath lap, which, in turn, can result in cracking through the plaster finishes. In short, the compromises associated with utility lath can ultimately affect its long-term performance and longevity.