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.