What is the required thickness for the first layer of gypsum plaster over metal lath?

The required thickness for the first layer of gypsum plaster over metal lath is indeed 1/2 inch. This thickness is specified to ensure adequate adhesion and coverage over the lath, providing a stable and durable base for subsequent layers of plaster. A 1/2 inch thickness allows the plaster to bridge the gaps between the lath, ensuring that the plaster bonds effectively while also minimizing the risk of cracking or other issues related to inadequate thickness. Additionally, this thickness contributes to the overall performance and finish quality of the plaster system, aligning with industry standards for residential construction.

Other thicknesses listed, while they may sometimes be suitable for different applications or materials, do not meet the standard requirements for the first layer of gypsum plaster over metal lath as specified in building codes and best practices for construction.