Installation of Lath, Plaster, and Stucco Systems
This domain tests traditional and modern plaster and stucco: lath selection, fastening, coat proportions, curing, and movement control. The exam expects you to know why each coat exists, not just how thick it is.
Core rules and concepts
- Lath types: gypsum lath, metal lath (diamond or expanded), rib lath, paper-backed lath, and self-furring lath. Self-furring lath holds the plaster away from a solid substrate so it can key. Metal lath is fastened along supports and between them at the manufacturer's spacing, commonly about 150 mm o.c. along supports and 200 mm o.c. between, with correct laps.
- Three-coat work: scratch coat, brown coat, and finish coat. The scratch coat keys into the lath; the brown coat builds thickness and levels the surface; the finish coat provides texture and appearance. Typical total thickness is about 19 mm over metal lath, with the scratch and brown coats doing the structural work.
- Mix and cure: use the specified cement, lime, and sand proportions. Do not add extra cement to speed setting. Moist-cure between coats and protect from rapid drying, wind, and direct sun.
- Accessories: casing bead, corner bead, grounds, control joints, and weep screed. Control joints accommodate movement and are required at substrate changes and at intervals set by the system.
- Veneer plaster: applied to a special base board in a thin coat; it is not the same as conventional three-coat plaster.
Worked example
A scratch coat is specified at roughly 10 mm over self-furring metal lath on masonry. The mix is cement, lime, and sand in the specified ratio, applied with enough pressure to key through the lath, then scored. It is moist-cured before the brown coat. The reasoning is that the scratch coat provides mechanical key and the curing period develops strength; skipping either produces cracking or delamination.
Traps that cost marks
Installing lath with insufficient lap; omitting the weep screed; letting coats dry too fast; adding cement to a mix without approval; and forgetting control joints at substrate changes.