AskCodeMan

Does the residential code allow for plywood to be used as an interior finish instead of gypsum board? I assume it is not allowed where 1 hour separation is required (ceiling of garage with living space above). Thanks.

The residential code does not really get into interior finishes, other than minimum 1/2" gypsum board on the garage side of the walls and ceiling separating the garage from the house and its attic, or, as you mentioned, 5/8" Type C on the ceiling if there is living space above the garage (not really a 1 hour rated ceiling and not addressed as such, just a requirement for 5/8" Type C on the ceiling (and protecting the supporting structure with 1/2" gypsum board).

The only thing I can think of right now which may affect the use of plywood on the interior of the living might be air infiltration / exfiltration and the R-rating for walls with plywood/OSB on them - but that could be addressed with vapor barriers / retarders installed behind the plywood / OSB.

Of course, though, one would not want to use plywood / OSB on walls at wet areas.