Download PDF version (48.7k) Log In or Register to view the full article as a PDF document.
Q.Are there any code-approved sheathing materials for a garage under a living space other than the typical 5/8-inch drywall? My clients complain that drywall doesn't hold up well in our environment, so I'm wondering if fire-retardant plywood would be a good substitute if the joints are properly prepared.

A.Lynn Underwood, an engineer, licensed contractor, and building code official in Norfolk, Va., responds: There are two approaches to code compliance: prescriptive and performance. The prescriptive approach is like reading a cookbook: Follow the recipe spelled out in the code book — in this case, the 2006 IRC — and you won't have any trouble with your local code official. Among other conditions, Section R309.2 of the IRC requires that "garages beneath habitable rooms shall be separated from all habitable rooms above by not less than 5/8 inch (15.9 mm) Type X gypsum board or equivalent."

The performance approach to code compliance is trickier. In the code language above, the key word is "equivalent," which means that materials that can be shown to perform equal to or better than 5/8-inch Type X gypsum board may also be allowed. For example, according to Table 720.1(3) in the IBC (items 21 and 22), two layers of 5/8-inch Type X drywall are needed to attain a one-hour fire-resistance rating. Thus, it's reasonable to suppose that one layer of 5/8-inch Type X drywall — the thickness required by the IRC's Section R309.2 — has a fire-resistance rating of approximately 30 minutes.

According to the ICC evaluation report of a major manufacturer, its FRT plywood has a 30-minute rating for both surface-burning and flame-spread characteristics (per ASTM E84), but that is not a fire-resistance rating (per ASTM E119 or IBC 703.3). What I'd recommend is installing the FRT over the 5/8-inch Type X drywall on the ceiling; since only regular 1/2-inch drywall (or the equivalent) is required on garage separation walls, I would think that FRT plywood sheathing would be an acceptable alternative there.

Another durable finish option might be cement board, such as Durock or HardiBacker, but that would depend on its evaluation report and acceptance by your local code official.