Plaster and Stucco Base Coats, Grounds, and Lath
This domain covers the wet side of the trade: gypsum and cement base coats, the grounds that set thickness, and the lath that gives the plaster its key.
Three-coat work
Scratch coat is pressed through the lath to form keys, then scored, about 6 mm (1/4 in.) thick. Brown coat straightens the wall and brings it out to the grounds, about 10 mm (3/8 in.). Finish coat is thin, roughly 1.5-3 mm, and goes on after the brown coat has set. Total thickness over metal lath is commonly 19-22 mm (3/4-7/8 in.).
Mixes and materials
- Gypsum plaster is gauging plaster mixed with lime putty; follow the manufacturer's ratio and allow proper slaking. Perlite-gypsum products such as Structo-lite are lightweight base coats; Keene's cement gives a hard, dense finish.
- Lime putty must be soaked and aged; unaged putty causes popping and an unsound finish.
- Cement plaster and stucco use portland cement, lime and graded sand, with a richer mix in the scratch coat than in the brown.
Lath and grounds
Metal lath is lapped about 13 mm at sides and 25 mm at ends, fastened about 150 mm (6 in.) on centre, and must be self-furring over solid backing so plaster can key behind it. Gypsum lath is applied with joints broken and fastened per the manufacturer. Grounds, wood grounds at the base, plaster grounds at openings, casing beads and screeds establish the finished plane. Install them first and plaster to them; they are the reference for thickness and for trim.
Worked example
A wall over self-furring lath has 6 mm furring, 6 mm scratch and 10 mm brown. Where should the grounds sit?
Reasoning: 6 + 6 + 10 = 22 mm to the brown coat face. The finish coat adds about 2 mm, so the grounds are set to give the specified total of roughly 22-25 mm from the substrate, and trim is set to that plane. If grounds are set from the framing face without allowing for furring, every jamb and base will be out of line.
Traps
Applying brown coat to a scratch coat that has not taken its initial set; over-troweling and burning the finish; skipping scratch scoring so keys fail; wrong lath lap or fastening; plastering onto a green brown coat; no curing in dry heat.