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ASTM C933
Standard Specification for Welded Wire Lath
Overview
ASTM C933 covers metal plaster base used for reinforcing portland cement-based plaster (stucco) and adhered stone veneer. The product is manufactured from steel wires that are resistance-welded to form a grid pattern. It can be supplied flat or self-furring (with crimps or dimples that hold it off the substrate) and with or without absorbent backing paper.
Key Specifications
- Wire dimensions and spacing to create uniform grid pattern
- Zinc coating (galvanized) for corrosion resistance
- Self-furring depth: nominal 1/4 inch for proper plaster embedment
- Optional backing paper for enhanced weather resistance
Critical Full Embedment
The primary function of lath is reinforcement, similar to rebar in concrete. It provides tensile strength to the brittle cementitious matrix, controlling shrinkage cracking and distributing stresses. Full embedment within the plaster scratch coat is essential. If lath is pressed tight against the weather-resistive barrier, plaster cannot flow behind it, resulting in poorly reinforced cladding prone to cracking and failure.
Self-Furring Advantage
Specifying self-furring welded wire lath is a significant quality assurance measure. Built-in furring crimps automatically create the necessary 1/4-inch space between lath and substrate, making proper embedment much easier compared to flat lath requiring separate furring devices. The rigidity and flat profile offer advantages over woven wire or expanded diamond mesh - more stable, less prone to sagging, easier to achieve true and plumb plaster plane. This is particularly beneficial in seismic regions or high-wind areas where the lath contributes to overall cladding strength and resilience.