Lathing and Plastering (C-35) · Lesson 2 of 6

Reading Plaster Mix Proportions by Coat

"Reading Plaster Mix Proportions by Coat" is lesson 2 of 6 in the Lathing and Plastering (C-35) Learning Hub. It opens with "The Code Prints the Mix as Parts by Volume" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. The Code Prints the Mix as Parts by Volume
  2. Portland or Blended Cement Row by Row
  3. Footnote b: Never Leaner in the Second Coat
  4. Convert Parts to a Sand Range for a Batch
  5. Gypsum Plaster Is Counted Per 100 Pounds of Neat Plaster

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Check the lime" through "Brown coat range and footnote b".

What you'll be able to do

  • Explain the main idea of "Reading Plaster Mix Proportions by Coat" in plain language you can use on the exam.
  • Apply this step with confidence: "Footnote b: Never Leaner in the Second Coat".
  • Apply this step with confidence: "Convert Parts to a Sand Range for a Batch".
  • Apply this step with confidence: "Gypsum Plaster Is Counted Per 100 Pounds of Neat Plaster".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Plaster Mix Proportions: Reading the Table by CoatTABLE R702.1(3): PORTLAND OR BLENDED CEMENT(parts by volume)COATCEMENTLIMESAND per sumFirst13/4 - 1 1/22 1/2 - 4Second13/4 - 1 1/23 - 5Finish11 1/2 - 21 1/2 - 3Sand is counted per the SUM of the separatevolumes of cementitious materials (cement + lime).Footnote b: second-coat sand proportion must bethe same as or greater than the first coat.Plastic or masonry cement: no added lime (CBC 2512.2)BATCH CHECK (cubic feet)4 cu ft cement + 4 cu ft limelime = 1 part (range 3/4 - 1 1/2): OKcementitious sum = 8 cu ftSCRATCH COATsand 2 1/2 to 4 x 8 = 20 to 32 cu ftcrew uses 28 cu ft = 3.5 parts: OKBROWN COATtable range 3 to 5 x 8 = 24 to 40 cu ftbut not less than 3.5 parts (fn b):28 to 40 cu ft allowed24 cu ft (3.0 parts) FAILS footnote bGYPSUM PLASTER, THREE-COAT WORK ON LATH (Table R702.1(2))Max damp loose sand per 100 lb neat plaster: first coat 2 cu ft, second coat 3 cu ft300 lb neat plaster = 3 x 100 lb: first coat 3 x 2 = 6 cu ft; second coat 3 x 3 = 9 cu ftTotal sand, two base coats = 6 + 9 = 15 cu ft maximumFootnote d: if one value is used for both coats, 2.5 cu ft per 100 lb (300 lb: 7.5 cu ft each coat)Source: 2025 CRC Tables R702.1(2) and R702.1(3), footnotes; 2025 CBC §2512.2CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: The Code Prints the Mix as Parts by Volume

The 2025 California Residential Code (CRC) Table R702.1(3), "Cement Plaster Proportions, Parts by Volume," gives one row per coat (first, second, finish) and per cement type. Cementitious materials are listed in columns: portland cement or blended or hydraulic cement, plastic cement, masonry cement (Type M, S or N), and lime. The last column is the "volume of aggregate per sum of separate volumes of cementitious materials," so the sand is counted against cement and lime added together.

Step 2: Portland or Blended Cement Row by Row

With 1 part portland or blended cement, the printed lime range is 3/4 to 1 1/2 parts in the first coat, 3/4 to 1 1/2 parts in the second coat, and 1 1/2 to 2 parts in the finish coat. The aggregate range is 2 1/2 to 4 for the first coat, 3 to 5 for the second coat and 1 1/2 to 3 for the finish coat. Plastic cement and masonry cement rows use 1 part of that cement with no lime column entry and the same aggregate ranges. The 2025 California Building Code (CBC) 2512.2 adds that where plastic cement or masonry cement is used, additional lime or plasticizers shall not be added.

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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