Interior plaster coats
IBC Section 2511.1 sets the minimum number of coats of interior gypsum or cement plaster by base:
- Over metal lath or wire fabric lath: at least three coats
- Over other bases: at least two coats
- One coat: allowed only for gypsum veneer plaster, or cement plaster approved for one-coat use
The trap is using two coats over metal lath. Two is the minimum over other bases; metal lath needs three. Four coats is not a code minimum.
Weep screed clearances
IBC Section 2512.1.2 requires the exterior plaster weep screed to sit at least:
- 4 in. above the earth, or
- 2 in. above paved areas
The common error is reversing them to 2 in. above earth and 4 in. above paving. Earth gets the larger clearance. Requiring 4 in. above paving ignores the reduced 2 in. clearance the code allows over paved areas, and 6 in. above earth does not appear in this section.
Weep screed material and location
Section 2512.1.2 also sets what the screed is and where it goes, per ASTM C926:
- Corrosion-resistant
- At least 0.019 in. thick (No. 26 galvanized sheet gage)
- A vertical attachment flange of at least 3½ in.
- Installed at or below the foundation plate line
Keep the numbers apart
This section carries several figures that are easy to cross:
- 0.019 in. is the minimum thickness. A 0.010 in. screed is too thin.
- 3½ in. is the flange. A 2 in. flange is too short; 2 in. is the paved-area clearance, not a flange size.
- The location is at or below the foundation plate line. A screed placed at or above the plate line is wrong, and so is one placed 4 in. above the plate line, which confuses the 4 in. clearance above earth with the location relative to the plate line.