R804.3.7 Ceiling and Roof Diaphragms
Ceiling and roof diaphragms shall be in accordance with this section.
At gable endwalls a ceiling diaphragm shall be provided by attaching a minimum 1/2-inch (12.7 mm) gypsum board or a minimum 3/8-inch (9.5 mm) wood structural panel sheathing, that complies with Section R803, to the bottom of ceiling joists or roof trusses and connected to wall framing in accordance with Figures R804.3.7.1(1) and R804.3.7.1(2), unless studs are designed as full height without bracing at the ceiling. Flat blocking shall consist of C-shaped or track section with a minimum thickness of 33 mils (0.84 mm). For a gypsum board sheathed ceiling, the diaphragm length shall be in accordance with Table R804.3.7.1. For a wood structural panel sheathed ceiling, the diaphragm length shall be not less than 12 feet (3658 mm) for building widths less than 36 feet (10 973 mm), or not less than 14 feet (4267 mm) for building widths greater than or equal to 36 feet (10 973 mm).
The ceiling diaphragm shall be secured with screws spaced at a maximum 6 inches (152 mm) o.c. at panel edges and a maximum 12 inches (305 mm) o.c. in the field. The required lengths in Table R804.3.7.1 for gypsum board sheathed ceiling diaphragms shall be permitted to be multiplied by 0.35 if all panel edges are blocked. Multiplying the required lengths in Table R804.3.7.1 for gypsum board sheathed ceiling diaphragms by 0.9 shall be permitted if all panel edges are secured with screws spaced at 4 inches (102 mm) o.c.
|EXPOSURE CATEGORY||ULTIMATE DESIGN WIND SPEED (mph)|
|Roof pitch||Building endwall width (feet)||Minimum diaphragm length (feet)|
|3:12 to 6:12||24 - 28||20||22||28||32|
|> 28 - 32||22||28||32||38|
|> 32 - 36||26||32||38||44|
|> 36 - 40||30||36||44||50|
|6:12 to 9:12||> 24 - 28||22||26||32||36|
|> 28 - 32||26||32||38||44|
|> 32 - 36||32||38||44||52|
|> 36 - 40||36||44||52||60|
|9:12 to 12:12||> 24 - 28||26||30||36||42|
|> 28 - 32||30||36||42||50|
|> 32 - 36||36||42||50||60|
|> 36 - 40||42||50||60||70|
For SI: 1 inch = 25.4 mm, 1 pound per square foot = 0.0479 kPa, 1 mile per hour = 0.447 m/s, 1 foot = 304.8 mm, 1 mil = 0.0254 mm.
- Ceiling diaphragm is composed of 1/2-inch gypsum board (min. thickness) secured with screws spaced at 6 inches o.c. at panel edges and 12 inches o.c. infield. Use No. 8 screws (min.) where framing members have a designation thickness of 54 mils or less and No. 10 screws (min.) where framing members have a designation thickness greater than 54 mils.
- Maximum aspect ratio (length/width) of diaphragms is 2:1.
- Building width is in the direction of horizontal framing members supported by the wall studs.
- Required diaphragm lengths are to be provided at each end of the structure.
- Multiplying required diaphragm lengths by 0.35 is permitted if all panel edges are blocked.
- Multiplying required diaphragm lengths by 0.9 is permitted if all panel edges are secured with screws spaced at 4 inches o.c.
- To determine the minimum diaphragm length for buildings with ceiling heights of 9 feet or 10 feet values in the table above shall be multiplied by 1.15.