Tile (C-54) · Lesson 4 of 7

Backer Board and Wet-Wall Tile Heights

"Backer Board and Wet-Wall Tile Heights" is lesson 4 of 7 in the Tile (C-54) Learning Hub. It opens with "What the Code Names and What It Does Not" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. What the Code Names and What It Does Not
  2. Approved Backers: CRC Table R702.4.2 and CBC Table 2509.2
  3. Water-Resistant Gypsum Backing Has Limits
  4. How High the Nonabsorbent Surface Goes
  5. Toilet Rooms Outside Dwelling Units

The worked example in the course applies this lesson to a realistic job in 4 steps, from "Find the row height" through "Pick the backer".

What you'll be able to do

  • Explain the main idea of "Backer Board and Wet-Wall Tile Heights" in plain language you can use on the exam.
  • Apply this step with confidence: "Water-Resistant Gypsum Backing Has Limits".
  • Apply this step with confidence: "How High the Nonabsorbent Surface Goes".
  • Apply this step with confidence: "Toilet Rooms Outside Dwelling Units".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Backer Board and Wall Tile HeightSHOWER WALL ELEVATION (not to scale at the floor)DRAIN INLET72 in4-1/4 in tile, 1/8 in joints17 rows = 74-1/4 inWALL HEIGHT CHECKRow n top = 4.375 n - 0.125 in16 rows = 69.875 in; 17 rows = 74.25 inCRC R327.2: top at 72 in or more above floor 16 rows: 69.875 is under 72: FAILCBC 1210.2.4: 72 in above the drain inlet floor at wall 1-5/8 in above inlet, so top at 70.375 in or more above floor 16 rows: 69.875 is under 70.375: FAIL17 rows (74.25 in) meets both: PASSTub and shower wall tile backer must be on the code list: 4 materials in CRCTable R702.4.2, 3 materials in CBC Table 2509.2Water-resistant gypsum backing board: not where there is direct water exposure2025 CRC R327.2, R702.3.7.1, Table R702.4.2; 2025 CBC 1210.2.4, 2509.2, 2509.3CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: What the Code Names and What It Does Not

2025 CRC Section R702.4.1 says ceramic tile surfaces shall be installed in accordance with ANSI A108.1, A108.4, A108.5, A108.6, A108.11, A118.1, A118.3, A136.1 and A137.1. The California code only names those standards; it does not reproduce their text. This lesson teaches what the California code itself prints: which backer boards are allowed behind wall tile in tub and shower areas, and how high the nonabsorbent wall finish must go.

Step 2: Approved Backers: CRC Table R702.4.2 and CBC Table 2509.2

CRC R702.4.2: backers for wall tile in tub and shower areas, and wall panels in shower areas, shall be of materials in Table R702.4.2, installed in accordance with the manufacturer's recommendations. The four listed are glass mat gypsum backing panel (ASTM C1178), fiber-reinforced gypsum panels (ASTM C1278), nonasbestos fiber-cement backer board (ASTM C1288 or ISO 8336, Category C), and nonasbestos fiber mat-reinforced cementitious backer units (ASTM C1325). CBC Section 2509.2 and Table 2509.2 list three of them: glass mat gypsum (C1178), fiber-cement (C1288 or ISO 8336 Category C) and fiber-mat reinforced cementitious units (C1325), so fiber-reinforced gypsum (C1278) appears in the CRC table but not in the CBC table.

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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