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6" nominal transverse flue width

Question asked by on Nov 11, 2020

If a rack storage array can guarantee well-defined, vertically-aligned 3-in. transverse flue spaces (e.g., through physical flue stops, etc.), does this meet the intent of what NFPA 13 is ultimately (and technically) after when they are requiring a “nominal” 6-in. transverse flue? 


Assuming that the 6-in. “nominal” flue width is intended to maintain a net 3-in. clear flue, allowing things like rack upright cross members to safely be within the 6-in. nominal flue without issue; and making accommodations for the “random variations in the width of the flue spaces or in their vertical alignment” that may occur in actual storage practices (see NFPA 13-2019 Section Since this interpretation agrees with how FM data sheet 8-9 defines flue spaces, is the FM Global approach ultimately the intent of NFPA 13?


In other words, did the 6-in. nominal width requirement get drafted that way to incorporate a “safety factor” on what is deemed to be a minimum acceptable net transverse flue width when the flues are well-defined and vertically-aligned?