What is Stucco Lath? Wire Mesh, WRB, and ASTM C1063 Rules

If stucco is the skin of your home, the Lath is the skeleton. Without it, the heavy cement cladding would simply slide off the wall. Lath refers to the critical reinforcement and moisture management layer—comprising metal wire, waterproof paper, and flashings—that attaches the stucco to the wood framing.

When homeowners observe major cracking or bulging stucco, it is rarely a failure of the cement itself; it is almost always a failure of the lath installation. This guide breaks down the components of a code-compliant lathing system according to ASTM C1063 standards.

1. The Moisture Barrier: Grade D Paper

Before any metal goes up, the substrate must be waterproofed. Stucco is porous and acts as a reservoir. The paper behind it is the final line of defense protecting your wood studs from rot.

• The Material: We typically use 60-Minute Grade D building paper (often asphalt-impregnated) or an approved synthetic house wrap.
• The Two-Ply Rule: California building codes generally require two distinct layers of paper over wood-based sheathing. This creates a "bond breaker"—a microscopic air gap between the layers. The cement bonds to the outer layer, while the inner layer remains free to drain water down to the weep screed.
• Lapping Rules: The paper must be installed "shingle style" (bottom up) with a minimum 2-inch horizontal overlap and 6-inch vertical overlap.

2. The Reinforcement: Wire Mesh

Cement cannot adhere to smooth building paper or plywood. It needs a physical matrix to grip. That is the job of the wire mesh.

• Three-Coat Wire: We use heavy-duty 17-Gauge Woven Wire (often colloquially called chicken wire) or expanded metal lath. This robust steel mesh supports the massive weight of a 7/8" thick traditional stucco wall (10-12 lbs per square foot).
• One-Coat Wire: A lighter 20-Gauge woven wire is often used over foam board for proprietary "one-coat" systems, as the fiberglass-reinforced cement is thinner and lighter.

3. Metal Flashings: The Transitions

Lath isn't just wire; it includes the rigid metal trims that define the edges of the cladding and manage water.

• Weep Screed: Installed at the bottom foundation line. It holds the stucco off the ground and provides weep holes for water to drain out of the wall cavity.
• Plaster Stop (Casing Bead): A J-shaped metal strip installed around doors, windows, and rooflines to create a clean, straight edge where the stucco terminates.
• Corner Aid: A reinforced metal nose installed on outside corners to create a straight, impact-resistant vertical line and prevent edge chipping.

4. Fasteners: Securing the Skeleton

The lath must be securely fastened to the structural wood framing (the studs), not just the plywood sheathing.

We use wide-crown (1-inch) galvanized pneumatic staples or specialized furring nails. The fasteners must hit the studs (typically 16 inches on center) and provide enough penetration to hold the massive weight of the wet cement. The wire itself must also be overlapped (typically a minimum of 1 inch on all sides) and "tied" together to create a continuous steel membrane across the entire house.