I have recently undertaken a couple of fire reports for two or three level residential dwellings where a lower floor or part of a lower floor is to be a separate minor dwelling firecell (granny flat). C/AS1 quite correctly addresses the firecell separation and egress requirements (the egress requirements could do with some work).
The part of C/AS1 I cannot see the rationale for is 5.1.1 b). If we have say a single two or three storey residence the only external wall fire separation requirements are for external walls within 1.0 m from the relevant boundary.
If we provide a separate firecell on a lower level (reducing the fire exposure to 1 or more boundaries) then C/AS1 requires all external walls (excluding windows) within 5.0 m from the relevant boundaries to be fire rated. For most modern properties this means that 2 or 3 of the external walls require fire rating. When you take into consideration the stability of the external walls then this generally means that floors affecting the stability of the fire rated external walls need to be fire rated as well.
In fact is some situations most of the dwelling structure is fire rated as it affects the stability of the fire rated external walls.
My question to the fire engineering community is: Does anyone know what problem is the C/AS1 5.0 m external wall fire rating requirement trying to solve?