Backing is not required under which lath installations?

Backing is not required under which lath installations?

• A. Under metal lath and under paper backed wire fabric lath
• B. Under exterior lath only
• C. Under interior lath only
• D. Under plaster base only

Answer: (A)

Backing is a separate coat used when the substrate itself doesn’t provide a strong bond for the plaster. Some lath types are designed so the plaster can bond directly to them without an additional backing layer. Metal lath has a rigid mesh with ridges and openings that give the plaster a solid mechanical key, so the base coat can be applied directly to the lath. Paper-backed wire fabric lath already provides a built-in backing surface—the paper on the fabric—so the plaster can adhere without needing an extra backing coat. In contrast, other lath installations may require a backing to ensure proper adhesion when the lath or substrate doesn’t furnish a suitable key. Therefore, backing is not required under metal lath and under paper-backed wire fabric lath.

Backing is a separate coat used when the substrate itself doesn’t provide a strong bond for the plaster. Some lath types are designed so the plaster can bond directly to them without an additional backing layer. Metal lath has a rigid mesh with ridges and openings that give the plaster a solid mechanical key, so the base coat can be applied directly to the lath. Paper-backed wire fabric lath already provides a built-in backing surface—the paper on the fabric—so the plaster can adhere without needing an extra backing coat.

In contrast, other lath installations may require a backing to ensure proper adhesion when the lath or substrate doesn’t furnish a suitable key. Therefore, backing is not required under metal lath and under paper-backed wire fabric lath.