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Chapter 25 — GYPSUM PANEL PRODUCTS AND PLASTER

Section 2508 — GYPSUM CONSTRUCTION

2025 California Building Code (Title 24, Part 2) · 2025 edition · updated 2026-07-29 · California

Italicized text is a California amendment to the model code, as printed in the official publication.

2508.1 General.

Gypsum panel products and gypsum plaster construction shall be of the materials listed in Tables 2506.2 and 2507.2. These materials shall be assembled and installed in compliance with the appropriate standards listed in Tables 2508.1 and 2511.1.1 and Chapter 35.

TABLE 2508.1—INSTALLATION OF GYPSUM CONSTRUCTION

MATERIAL STANDARD
Gypsum panel products GA-216; ASTM C840
Gypsum sheathing and gypsum panel products ASTM C1280; GA-253
Gypsum veneer base ASTM C844
Interior lathing and furring ASTM C841
Steel framing for gypsum panel products ASTM C754; C1007

2508.2 Limitations.

Gypsum wallboard or gypsum plaster shall not be used in any exterior surface where such gypsum construction will be exposed directly to the weather. Gypsum wallboard shall not be used where there will be direct exposure to water or continuous high humidity conditions. Gypsum sheathing shall be installed on exterior surfaces in accordance with ASTM C1280 or GA-253.

2508.2.1 Weather protection. Gypsum wallboard, gypsum lath or gypsum plaster shall not be installed until weather protection for the installation is provided.

2508.3 Single-ply application.

Edges and ends of gypsum panel products shall occur on the framing members, except those edges and ends that are perpendicular to the framing members. Edges and ends of gypsum panel products shall be in moderate contact except in concealed spaces where fire-resistance-rated construction, shear resistance or diaphragm action is not required.

2508.3.1 Floating angles. Fasteners at the top and bottom plates of vertical assemblies, or the edges and ends of horizontal assemblies perpendicular to supports, and at the wall line are permitted to be omitted except on shear resisting elements or fireresistance-rated assemblies. Fasteners shall be applied in such a manner as not to fracture the face paper with the fastener head.

2508.4 Adhesives.

Gypsum panel products secured to framing with adhesives in ceiling assemblies shall be attached using an approved fastening schedule. Expandable foam adhesives for fastening gypsum wallboard shall conform to ASTM D6464. Other adhesives for the installation of gypsum wallboard shall conform to ASTM C557.

2508.5 Joint treatment.

Gypsum panel product fire-resistance-rated assemblies shall have joints and fasteners treated.

Exceptions: Joint and fastener treatment need not be provided where any of the following conditions occur:

  1. Where the gypsum panel product is to receive a decorative finish such as wood paneling, battens, acoustical finishes or any similar application that would be equivalent to joint treatment.
  2. On single-layer systems where joints occur over wood framing members.
  3. Square edge or tongue-and-groove edge gypsum board (V-edge), gypsum panel products, gypsum backing board or gypsum sheathing.
  4. On multilayer systems where the joints of adjacent layers are offset.
  5. Assemblies tested without joint treatment.

2508.6 Horizontal gypsum panel product diaphragm ceilings.

Gypsum panel products shall be permitted to be used on wood joists to create a horizontal diaphragm ceiling in accordance with Table 2508.6.

25-6 2025 CALIFORNIA BUILDING CODE

on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

GYPSUM PANEL PRODUCTS AND PLASTER

TABLE 2508.6—SHEAR CAPACITY FOR HORIZONTAL WOOD-FRAME GYPSUM PANEL PRODUCT DIAPHRAGM CEILING ASSEMBLIES

MATERIAL THICKNESS OF
MATERIAL
(MINIMUM) (inches)		 SPACING OF
FRAMING MEMBERS
(inches)		 SHEAR VALUEa, b
(PLF OF CEILING)		 MIMIMUM FASTENER SIZE
Gypsum panel product 1/2 16 o.c. 90 5d cooler or wallboard nail; 15/8-inch
long; 0.086-inch shank;15/64-inch headc
Gypsum panel product 1/2 24 o.c. 70 5d cooler or wallboard nail; 15/8-inch
long; 0.086-inch shank;15/64-inch headc
For SI: 1 inch = 25.4 mm, 1 pound per foot = 14.59 N/m.
a. Values are not cumulative with other horizontal diaphragm values and are for short-term wind or seismic loading. Values shall be reduced 25 percent for normal loading.
b. Values shall be reduced 50 percent in Seismic Design Categories D, E and F.
c. 11/4-inch, No. 6 Type S or W screws are permitted to be substituted for the listed nails.

2508.6.1 Diaphragm proportions. The maximum allowable diaphragm proportions shall be 1 [1] / 2 :1 between shear resisting elements. Rotation or cantilever conditions shall not be permitted.

2508.6.2 Installation. Gypsum panel products used in a horizontal diaphragm ceiling shall be installed perpendicular to ceiling framing members. End joints of adjacent courses of gypsum board shall not occur on the same joist.

2508.6.3 Blocking of perimeter edges. Perimeter edges shall be blocked using a wood member not less than 2-inch by 6-inch (51 mm by 152 mm) nominal dimension. Blocking material shall be installed flat over the top plate of the wall to provide a nailing surface not less than 2 inches (51 mm) in width for the attachment of the gypsum panel product.

2508.6.4 Fasteners. Fasteners used for the attachment of gypsum panel products to a horizontal diaphragm ceiling shall be as defined in Table 2508.6. Fasteners shall be spaced not more than 7 inches (178 mm) on center at all supports, including perimeter blocking, and not more than [3] / 8 inch (9.5 mm) from the edges and ends of the gypsum panel product.

2508.6.5 Lateral force restrictions. Gypsum panel products shall not be used in diaphragm ceilings to resist lateral forces imposed by masonry or concrete construction.

2508.6.6 Diaphragm ceiling connection to partitions. [DSA-SS & DSA-SS/CC and OSHPD 1, 1R, 2, 4 & 5] Gypsum board shall not be used in diaphragm ceilings to resist lateral forces imposed by partitions. Connection of diaphragm ceiling to the vertical lateral force resisting elements shall be designed and detailed to transfer lateral forces.

Exception: [OSHPD 2] Single-story Type V skilled nursing or intermediate care facilities utilizing wood-frame or light-steel-frame construction.

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▸ Contents — 2025 California Building Code (Title 24, Part 2)
2025 California Building Code (Title 24, Part 2)
  1. Chapter 1 — ADMINISTRATION
  2. Chapter 2 — DEFINITIONS
  3. Chapter 3 — OCCUPANCY CLASSIFICATION AND USE
  4. Chapter 4 — SPECIAL DETAILED REQUIREMENTS BASED ON OCCUPANCY A…
  5. Chapter 5 — GENERAL BUILDING HEIGHTS AND AREAS
  6. Chapter 6 — TYPES OF CONSTRUCTION
  7. Chapter 7 — FIRE AND SMOKE PROTECTION FEATURES
  8. Chapter 7A — MATERIALS AND CONSTRUCTION METHODS FOR EXTERIOR W…
  9. Chapter 8 — INTERIOR FINISHES
  10. Chapter 9 — FIRE PROTECTION AND LIFE SAFETY SYSTEMS
  11. Chapter 10 — MEANS OF EGRESS
  12. Chapter 11 — RESERVED
  13. Chapter 11A — HOUSING ACCESSIBILITY
  14. Chapter 11B — ACCESSIBILITY TO PUBLIC BUILDINGS, PUBLIC ACCOMM…
  15. Chapter 12 — INTERIOR ENVIRONMENT
  16. Chapter 13 — ENERGY EFFICIENCY
  17. Chapter 14 — EXTERIOR WALLS
  18. Chapter 15 — ROOF ASSEMBLIES AND ROOFTOP STRUCTURES
  19. Chapter 16 — STRUCTURAL DESIGN
  20. Chapter 16A — STRUCTURAL DESIGN
  21. Chapter 17 — SPECIAL INSPECTIONS AND TESTS
  22. Chapter 17A — SPECIAL INSPECTIONS AND TESTS
  23. Chapter 18 — SOILS AND FOUNDATIONS
  24. Chapter 18A — SOILS AND FOUNDATIONS
  25. Chapter 19 — CONCRETE
  26. Chapter 19A — CONCRETE
  27. Chapter 20 — ALUMINUM
  28. Chapter 21 — MASONRY
  29. Chapter 21A — MASONRY
  30. Chapter 22 — STEEL
  31. Chapter 22A — STEEL
  32. Chapter 23 — WOOD
  33. Chapter 24 — GLASS AND GLAZING
  34. ▸ Chapter 25 — GYPSUM PANEL PRODUCTS AND PLASTER
    Overview
    1. Section 2501 — GENERAL
    2. Section 2502 — PERFORMANCE
    3. Section 2503 — INSPECTION
    4. Section 2504 — VERTICAL AND HORIZONTAL ASSEMBLIES
    5. Section 2505 — SHEAR WALL CONSTRUCTION
    6. Section 2506 — GYPSUM PANEL PRODUCT MATERIALS
    7. Section 2507 — LATHING AND PLASTERING
    8. Section 2508 — GYPSUM CONSTRUCTION
    9. Section 2509 — SHOWERS AND WATER CLOSETS
    10. Section 2510 — LATHING AND FURRING FOR CEMENT PLASTER (STUCCO)
    11. Section 2511 — INTERIOR PLASTER
    12. Section 2512 — EXTERIOR PLASTER
    13. Section 2513 — EXPOSED AGGREGATE PLASTER
    14. Section 2514 — REINFORCED GYPSUM CONCRETE
  35. Chapter 26 — PLASTIC
  36. Chapter 27 — ELECTRICAL
  37. Chapter 28 — MECHANICAL SYSTEMS
  38. Chapter 29 — PLUMBING SYSTEMS
  39. Chapter 30 — ELEVATORS AND CONVEYING SYSTEMS
  40. Chapter 31 — SPECIAL CONSTRUCTION
  41. Chapter 31A — SYSTEMS FOR WINDOW CLEANING OR EXTERIOR BUILDING…
  42. Chapter 31B — PUBLIC POOLS
  43. Chapter 31C — RADIATION
  44. Chapter 31D — FOOD ESTABLISHMENTS
  45. Chapter 31F — MARINE OIL TERMINALS
  46. Chapter 32 — ENCROACHMENTS INTO THE PUBLIC RIGHT-OF-WAY
  47. Chapter 33 — SAFEGUARDS DURING CONSTRUCTION
  48. Chapter 34 — RESERVED
  49. Chapter 35 — REFERENCED STANDARDS
  50. Appendix A — EMPLOYEE QUALIFICATIONS
  51. Appendix B — BOARD OF APPEALS
  52. Appendix C — GROUP U—AGRICULTURAL BUILDINGS
  53. Appendix D — FIRE DISTRICTS
  54. Appendix E — RESERVED
  55. Appendix F — RODENTPROOFING
  56. Appendix G — FLOOD-RESISTANT CONSTRUCTION
  57. Appendix H — SIGNS
  58. Appendix I — PATIO COVERS
  59. Appendix J — GRADING
  60. Appendix K — GROUP R-3 AND GROUP R-3.1 OCCUPANCIES PROTECTED B…
  61. Appendix L — EARTHQUAKE RECORDING INSTRUMENTATION
  62. Appendix M — TSUNAMI-GENERATED FLOOD HAZARDS
  63. Appendix N — REPLICABLE BUILDINGS
  64. Appendix O — PERFORMANCE-BASED APPLICATION
  65. Appendix P — SLEEPING LOFTS
  66. Appendix Q — EMERGENCY HOUSING

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