Q: What is the correct thickness of stucco?

A: Stucco thickness depends on the backup system and on whether or not lath is present. In ASTM C 926, the Standard Specification for Application of Portland Cement-Based Plaster, thicknesses are provided for scratch, brown, and finish coats.

Over frame construction, lath must be used. Over solid substrates—which include concrete masonry, cast-in-place concrete, and precast concrete—lath is sometimes used. When lath is present, three-coat plaster is recommended. Note that frame construction—metal or wood studs—may or may not have sheathing present, but that plaster thickness is independent of sheathing. With lath, total plaster thickness is 7/8 in.

Three-coat work can also be specified for solid plaster bases without metal lath. The correct thickness is then 5/8 in.

Two-coat applications are only for use over solid plaster bases without metal lath. For unit masonry, that thickness is ½ in. For cast-in-place or precast concrete, the thickness for two-coat work is 3/8 in. These are direct-applied systems, meaning that there is no metal lath involved.

It is important to note that the committee in charge of ASTM C 926, the reference document for this application, has decided to keep the term “nominal thickness” in the title of the table for two- and three-coat work. This term takes into account that walls are built to certain tolerances and may not be exactly plumb or plane. The reference to a nominal thickness allows for small variations from an exact dimension. The intent of the specified thickness is to provide a reasonable system that, over many years, has proven itself to be weather resistant and durable. Local building officials should be consulted for further information about variations from the specified thickness.

See www.astm.org for information about ordering a copy of ASTM C 926, which contains the recommendations for both vertical and horizontal plaster thickness over metal plaster bases and solid plaster bases.