Why the garage gets its own table
Table R302.6 of the 2018 IRC lists four garage situations, each with its own board. The exam tests whether you pick the right row.
The four rows of Table R302.6
- Separation from the residence and attics: not less than 1/2-inch gypsum board or equivalent, applied to the garage side.
- Habitable rooms above the garage: not less than 5/8-inch Type X gypsum board or equivalent on the garage ceiling.
- Structure supporting the floor/ceiling assembly used for the separation: not less than 1/2-inch gypsum board or equivalent.
- Garage less than 3 ft from a dwelling unit on the same lot: not less than 1/2-inch gypsum board or equivalent on the interior side of the exterior walls within that area.
Traps the table is built to catch
- Plain 1/2-inch board under a bedroom or bonus room is the classic error. A habitable room above means 5/8-inch Type X. 1/2-inch Type X misses the 5/8-inch thickness, and two layers of 1/2-inch regular board is not the material named.
- The garage wall to the house needs only 1/2-inch board. The 5/8-inch Type X figure applies only where habitable rooms sit above. 3/8-inch board is below the minimum.
- A 1-hour rated assembly is not the garage separation. That rating belongs to exterior walls by fire separation distance in Table R302.1(1).
- The ceiling membrane alone does not finish the job. A dropped beam and posts carrying the ceiling joists must be wrapped in at least 1/2-inch board. Leaving them bare is a listed common error, and matching the 5/8-inch Type X ceiling is more than the table requires.
Detached garages close to the house
A detached garage is not automatically exempt. If it sits less than 3 ft from the dwelling on the same lot, board the interior side of only the exterior walls within that 3-ft area. Boarding every exterior wall applies the rule beyond its zone, and putting 5/8-inch Type X on the ceiling borrows the habitable-room row.