Buildings of Type III construction are typically constructed
with both combustible and noncombustible
materials. The exterior walls are required to be non-
combustible with load-bearing exterior walls required
to have a minimum 2-hour fire-resistance rating (see
Table 601). Exterior nonload-bearing walls are not
required by Table 601 to have a fire-resistance rating
but must comply with the provisions of Table 705.5.
The elements within the perimeter established by the
exterior walls (i.e., floors, roofs and walls) are permitted
to be of combustible materials. An example of a
typical building of Type III construction is a structure
having its exterior walls constructed of concrete,
masonry or other approved noncombustible materials,
but with wood-framed floor, interior wall and roof construction
(see Commentary Figure 602.3). The structural
members of a building of Type IIIB construction
are not required to have a fire-resistance rating, with
the exception of the exterior load-bearing walls.
Although fire-retardant-treated wood (FRTW) does
not meet the specifications of the code as a noncombustible
material, it is permitted as a substitute for noncombustible
materials for framing and sheathing within
exterior wall assemblies of Type III construction. While
the exterior walls are permitted to be either nonload
bearing or load bearing, to apply the allowance for
FRTW, the required fire-resistance rating of the exterior
wall must be not greater than 2 hours. FRTW is
required to comply with the provisions in Section
2303.2.
