What is metal lath used for?

It is used as a mechanical key substrate for stucco, plaster, and similar trowel-applied finishes. The mesh openings allow the mortar scratch coat to curl behind the wire and lock into place, giving the cladding a strong mechanical bond rather than relying on adhesion alone.

How long does metal lath last behind stucco?

Galvanized metal lath in a properly detailed stucco assembly can last for many decades. Problems arise when water gets behind the stucco through cracks, failed sealant, or missing flashing. Once water reaches the lath, corrosion can develop within a few years in damp conditions.

Does replacing metal lath require a permit?

Stucco re-application over new lath is typically a permitted alteration because it involves the building envelope. Many jurisdictions require a permit for stucco re-cladding or full re-stucco projects, particularly when the underlying sheathing or weather-resistive barrier is also being replaced.

What does metal lath and stucco repair cost?

Patch repairs to a small area typically run $500 to $2,000 depending on access and how deep the damage goes. Full re-stucco of an exterior wall with new lath can cost $8 to $15 per square foot or more depending on the region and existing conditions.

Can I use fiberglass mesh instead of metal lath?

Fiberglass mesh tape is used in one-coat synthetic stucco and EIFS systems but is not a code-compliant substitute for metal lath in traditional three-coat portland cement stucco. Metal lath provides both mechanical keying and the structural rigidity that portland cement scratch coats need.

Metal Lath — Expanded Steel Mesh for Stucco and Plaster

A metal lath is an expanded or woven steel mesh used as a base coat substrate for stucco, plaster, and thin-set masonry cladding systems.

Metal Lath diagram — labeled parts and installation context

What It Is

Metal lath gives plaster and stucco something to grip. When a scratch coat is troweled over expanded diamond-mesh lath, the mortar is forced through the openings and curls around the wire on the back side, forming a mechanical key that holds the cladding to the wall. Without this key, plaster and stucco depend entirely on adhesion, which is unreliable over large areas and prone to delamination as the substrate expands and contracts.

Metal lath replaced wood lath in most commercial and institutional applications during the mid-twentieth century, and it is the standard reinforcing mesh referenced in ASTM C1063 and current building codes for three-coat portland cement stucco systems. It is distinct from fiberglass mesh tape used over rigid foam or gypsum sheathing in synthetic one-coat stucco and EIFS assemblies.

Standard diamond-mesh lath weighs approximately 2.5 pounds per square yard for the common 2.5-lb expanded type, and it is manufactured from galvanized or zinc-coated sheet steel to resist corrosion within the stucco assembly. Proper galvanizing is critical because moisture that migrates through the stucco will reach the lath, and uncoated steel corrodes rapidly in this environment.

Types

Diamond-mesh expanded lath is the most common type and is produced by slitting and stretching sheet steel into a pattern of interlocking diamond-shaped openings. It comes in sheets typically measuring 27 inches by 97 inches. Rib lath has longitudinal V-shaped ribs pressed into the sheet that increase stiffness, allowing it to span wider stud spacings of up to 24 inches. Hi-rib lath has deeper ribs and is used in ceiling and soffit applications where the lath must support its own weight plus the plaster before the plaster cures.

Self-furring lath has dimples stamped into it that hold the mesh approximately 1/4 inch away from the sheathing, ensuring plaster encapsulates the lath fully from behind without requiring separate furring nails. Welded wire lath uses heavier-gauge wires in a grid pattern and is used for special applications such as column wraps and heavily loaded veneer systems.

Where It Is Used

Metal lath is used behind three-coat stucco on wood-framed or steel-framed walls, on soffits and ceilings receiving plaster, around curved or irregular surfaces where rigid backer boards cannot conform, and in fireplace surrounds and column wraps receiving a masonry finish. It is fastened to the framing or sheathing with galvanized staples or roofing nails at 6-inch intervals along each stud.

In restoration work, metal lath is used to patch damaged sections of historic plaster walls and to reinforce new plaster applied over old substrates. It is also the standard reinforcing mesh in thin-stone veneer systems where manufactured stone is adhered to the scratch coat over the lath.

How to Identify One

Metal lath is visible during construction before the scratch coat is applied, appearing as a silver or gray mesh stretched taut over the wall framing. On a finished wall it is completely buried under the plaster or stucco layers and cannot be seen. Rust staining at stucco cracks or delaminated sections can indicate that moisture has reached ungalvanized or compromised lath behind the cladding.

When removing a section of failed stucco, the lath is revealed as a thin metal mesh embedded in the back of the scratch coat. Its condition -- intact versus corroded and crumbling -- determines whether the existing lath can be reused or must be replaced.

Replacement

Metal lath behind a failed stucco section must be replaced when it is corroded through, pulling away from fasteners, or missing entirely. The stucco must be removed back to clean lath or sheathing, the weather-resistive barrier inspected and repaired, new lath lapped at least 1 inch over existing sound lath and fastened per code, and the three-coat stucco system rebuilt with proper cure times between coats. Spot repairs that bridge over deteriorated lath will fail again because the new plaster has no sound mechanical key to grip.