An attic is a concealed combustible space generally. There is not a distinction. If you have a shallow attic like you say, it is not uncommon to fill the space with noncombustible insulation so that you can avoid sprinklers.
There are many reasons why an R-2 occupancy 4 stories or less would require an NFPA 13 system. A few of them are the architect needed the area increase to get the building size he wanted, or maybe the municipality doesn't give a credit for fire flow if the building is protected by a 13R system vs 13.
This is something I have seen many contractors over the years have an issue with. Building and Fire code first. Sure, NFPA 13R says for residential buildings 4 stories or less, but that doesn't mean any residential building 4 stories or less. 13R is a lesser system than 13 and has good application. But, the building / fire code must be consulted first.
Travis Mack, SET, RME-G,
MEPCad, Inc
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