Use of plywood for interior of garage

[Jim Guss]

Hello Codeman,

I own a raised ranch type home. There are bedrooms above the garage and two walls share the interior living space. I know that code requires 5/8 drywall for the ceiling and at least the two walls that share the living space. I believe that since there is living space above the garage that the other two walls require 5/8 drywall as well but I am not sure.

I want to remove the existing drywall due to damage and replace. What I want to know is can use 1/2 plywood directly on the studs and then attach exterior grade fiber cement 4x8 sheets on top of the plywood. I only want to do this for the walls and keep 5/8 drywall on the ceiling. This in my opinion would still give me the fire resistance they are looking for. Do you know if this would be considered acceptable? I live in Illinois...specifically Cook County.

Thanks,
Jim

[Jerry Peck - Codeman]

I will give what I know is typical around the country, and contact someone who is much more familiar with Cook County codes than I am.

First, the walls and structure supporting the living space above the garage would need to have 1/2" drywall on them on the garage side. Cook County may require 5/8" Type X on the wall and structure supporting the living space above - I will let my contact from Cook County address that.

Second, the ceiling would need to have 5/8" Type X drywall on the garage side.

Replacing the 1/2" drywall (or 5/8" Type X drywall as required) with plywood (or OSB, or any wood surface) would be a definite no go.

I am forwarding this question to a person more knowledgeable about the codes in Cook County.

[markus]

Hi Jim, there are a few options as an answer to your question. Under Section 4(13-96-290)a of the City of Chicago Code, 1 hour fire separation is required. There are also various requirements for the door as well.
Depending on whether you are in the City or suburbs there will be some variance in the requirements. Most suburbs in Cook County are under some version/year of the IRC, a few are still under UBC, and I ran into a few a couple of years ago that were working on going from BOCA (think that was it) to the IRC. As you can tell there isn't absolute uniformity in adoption. You could contact your local building dept. and ask which Code they have adopted. For your purposes however, I would go with the 1 hour rating as a baseline.
Putting plywood under drywall in garages is a common practice by woodworkers, handyman, weekend DIY warriors. It gives them backing in order to hang stuff pretty much anywhere and keeps drywall from breaking upon typical impact. This installation has been approved by City inspectors when covered with 5/8" drywall.
You stated your desire to use 'fiber cement board'. I'm guessing you are talking about either durock, wonderboard or hardie backer board. These would be an option but you would need to verify fire rating per thickness of each board. It's sunday morning so I haven't had a chance to look this up, from what I remember 1/2 cement board was compliant. I vaguely remember that there was an issue with hardie board though. A critical factor in all of this would be that all seams would have to be fully mudded and taped to have a complete fire rating.
If damage resistance is what you are looking for, you may want to consider impact resistant drywall. USG makes such a product and it works really great. I've used it on jobs in the past. It will definitely take a beating. Taping out seams would also be easier. The big box stores could probably order it for you. However you would be best off finding a local drywall supply house, they tend to stock it. There are various in the City and suburbs. These places typically sell quantity to tradesman but will sell to homeowners no problem.
I hope that helps. Feel free to ask further questions.
Markus