Chapter 25 — GYPSUM PANEL PRODUCTS AND PLASTER
2509 Showers and Water Closets — 2025 California Building Code
California Building Code (Title 24, Part 2) · 2025 edition · updated 2026-09-25 · California
CBSC: official editions, supplements and errata
Italicized text is a California amendment to the model code, as printed in the official publication.
Cite as: 2025 California Building Code (Title 24, Part 2) § 2509 · Effective January 1, 2026 · Official source: Building Standards Commission
2509.1 Wet areas.¶
Showers and public toilet walls shall conform to Section 1210.2.
2509.2 Base for tile.¶
Materials used as a base for wall tile in tub and shower areas and wall and ceiling panels in shower areas shall be of materials listed in Table 2509.2 and installed in accordance with the manufacturer’s recommendations. Water-resistant gypsum backing board shall be used as a base for tile in water closet compartment walls when installed in accordance with GA-216 or ASTM C840 and the manufacturer’s recommendations. Regular gypsum wallboard is permitted under tile or wall panels in other wall and ceiling areas when installed in accordance with GA 216 or ASTM C840.
TABLE 2509.2—BACKERBOARD MATERIALS
| MATERIAL | STANDARD |
|---|---|
| Glass mat gypsum backing panel | ASTM C1178 |
| Nonasbestos fiber-cement backer board | ASTM C1288 or ISO 8336, Category C |
| Fiber-mat reinforced cementitious backer unit | ASTM C1325 |
2509.3 Limitations.¶
Water-resistant gypsum backing board shall not be used in the following locations:
- Over a vapor retarder in shower or bathtub compartments.
- Where there will be direct exposure to water or in areas subject to continuous high humidity.
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