Lathing and Plastering (C-35) · Lesson 5 of 6

Plaster in Fire-Rated Assemblies: Equivalents and Reinforcement

"Plaster in Fire-Rated Assemblies: Equivalents and Reinforcement" is lesson 5 of 6 in the Lathing and Plastering (C-35) Learning Hub. It opens with "Fire Tests Set the Thickness; Measure From the Lath" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Fire Tests Set the Thickness; Measure From the Lath
  2. Plaster Equivalents
  3. Over 1 Inch Needs a Second Lath Layer
  4. Plaster as a Substitute for Concrete Protection
  5. Reading a Plaster Item in Table 721.1(2)

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Find the equivalent" through "Match to a listed assembly".

What you'll be able to do

  • Explain the main idea of "Plaster in Fire-Rated Assemblies: Equivalents and Reinforcement" in plain language you can use on the exam.
  • Apply this step with confidence: "Over 1 Inch Needs a Second Lath Layer".
  • Apply this step with confidence: "Plaster as a Substitute for Concrete Protection".
  • Apply this step with confidence: "Reading a Plaster Item in Table 721.1(2)".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Plaster in Fire-Rated Assemblies: EquivalentsEQUIVALENT PLASTER THICKNESS (to scale)1/2 in unsanded (neat) gypsum plaster3/4 in one-to-three gypsum sand plaster1 in Portland cement sand plasterRULES IN CBC SECTION 719719.1 Measure from the face of the lath(gypsum lath or metal lath)719.4 Over 1 in thick: add a lath layerembedded 3/4 in min from the outer surface(not for solid partitions or per fire tests)719.5 Plaster may replace 1/2 in of thepoured concrete protection; keep 3/8 in ofconcrete in floors, 1 in in columnsConcrete base prepared per 2510.7WORKED CHECKSSpec: 1/2 in neat gypsum. Cement sand substitute = 1 in, not more than 1 in: no extra lath layer.Crew applies 1 1/4 in cement plaster: over 1 in, so add lath embedded 3/4 in from the outer face.Column needs 1 1/4 in poured concrete (assumed): 1 in must remain, so plaster replaces 1 1/4 - 1 = 1/4 in.Table 721.1(2) Item 15-1.2: 2x4 studs 16 in o.c., metal lath, 3/4 in cement plaster each side,6d common nails 7 in o.c. bent over (1 in min penetration), mix 1:4 scratch / 1:5 brown by volume.Rating 1 hour; minimum finished thickness face to face 5 3/8 in.Always confirm the exact assembly in the listed table or fire test.Source: 2025 CBC §719.1-719.5; 2025 CBC Table 721.1(2), Item 15-1.2CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Fire Tests Set the Thickness; Measure From the Lath

The 2025 California Building Code (CBC) section 719.1 says the minimum thickness of gypsum plaster or portland cement plaster used in a fire-resistance-rated system shall be determined by the prescribed fire tests. The plaster thickness is measured from the face of the lath where the plaster is applied to gypsum lath or metal lath. That is why a rated plaster job cannot be priced from the general minimums alone; you read the listed assembly and then measure from the lath face.

Step 2: Plaster Equivalents

CBC 719.2 says that for fire-resistance purposes 1/2 inch of unsanded gypsum plaster is deemed equivalent to 3/4 inch of one-to-three gypsum sand plaster or 1 inch of portland cement sand plaster. In other words, as the sand goes in, the thickness needed to give the same protection goes up. This lets a plasterer replace one listed plaster with another when the code equivalent allows it.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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