Drywall: Which Type the Code Actually Requires
Drywall: Which Type the Code Actually Requires
The difference between a board that is suitable for a location and one the code requires there, with the garage separation rules people most often get wrong.
Where the code REQUIRES a thickness (IRC R302.6)
| Garage side of a garage/dwelling wall | Not less than 1/2 in gypsum board |
| Garage ceiling, habitable room above | Not less than 5/8 in TYPE X |
| Garage ceiling, nothing above | 1/2 in is sufficient |
| Structure supporting the separation | Not less than 1/2 in, or equivalent |
The board types
| Regular | No fire or moisture rating. |
| Type X | Defined in ASTM C1396. 5/8 in, both sides of 2x4 studs at 16 in on center, gives a one-hour assembly. |
| Type C | A proprietary improvement on Type X, not itself an ASTM-defined type. Used where a tested assembly calls for it by name. |
| Mold and moisture resistant | Tested for mold growth. Not a fire rating and not a substitute for one. |
| Abuse and impact resistant | Classified under ASTM C1629. A durability rating, not a fire one. |
Two things worth knowing before you order
- Type X is a PERFORMANCE class, not a brand. Two makers' Type X are both Type X.
- A fire rating belongs to the whole ASSEMBLY, not the board. Swapping fastener spacing or stud size can void a tested design even with the right board on it.
The thickness requirements above come from the IRC, a model code. They bind you only once your state or county adopts that edition, and some jurisdictions amend them or lag by several editions. Confirm the adopted edition with your building department. Shaft wall assemblies, ceiling requirements tied to framing spacing, and tile backer in wet areas are governed by provisions not pulled here.
Sources: iccsafe.org · gypsum.org
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