The supporting construction shall be protected to afford the required fire-resistance rating of the horizontal assembly supported.
Exception: In buildings of Type IIB, IIIB or VB construction, the construction supporting the horizontal assembly is not required to be fire-resistance-rated at the following:
- Horizontal assemblies at the separations of incidental uses as specified by Table 509 provided the required fire-resistance rating does not exceed 1 hour.
- Horizontal assemblies at the separations of dwelling units and sleeping units as required by Section 420. 3.
- Horizontal assemblies at smoke barriers constructed in accordance with Section 709.