R302.3

Section R302.3 is titled 'Two-family dwellings' and covers the requirements for providing a fire-resistance rated assembly between units of a two-unit building. The hourly rating of the assembly, standard to which the assembly must have been tested, and how the fire-resistance rated assembly must be installed relative to other elements of the building is established. The purpose of this section is to slow the spread of fire. This section has 2 exceptions which allows for a reduction in the hourly rating where the building contains fire sprinklers installed in accordance with a specific code, and offers an alternative construction method to allow a wall to not need to extend through the attic space. The purpose of these exceptions is to allow for practical alternatives where risk offsetting systems or construction methods are installed.

The model shows the x-rayed skin of 2 two-family dwellings with the fire-resistance rated assemblies highlighted. One building has the units side by side, and the other has the units stacked one on top of the other. Notice that the stacked building must provide vertical assemblies, as well as horizontal, in order to provide protection between the 1st floor unit and the stairwell to the 2nd floor unit.