Ahhh, got the answer right here for ya.
The walls that are ajoining the attached house to the man cave (garage) have to be sheetrocked AND fire taped. So the ceiling on my garage in the new house and a portion of the front wall in the garage is sheetrocked. Either ceiling has to be or the front wall has to be all the way to the top of the rafters..
I priced out the two for finishing the garage and decided to go with OSB. I went with the 7/16 from Menards here when it was on sale, like 5.98 a sheet, when sheetrock was 10+ a sheet for the saem thickness... Plus I like the fact that it will hold up a lot better than sheetrock in the garage, and won't have to tape and mud... and like they said, if you think it looks redneck paint it white!
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5/8 sheetrock has to be used on the attached wall to make the one hour fire wall.