According to the U.S. Fire Administration there are two types of flue spaces for warehouse racking. The requirements for flue space maintenance depending on the height of the racks but the purpose remains the same. The purpose of flue spaces is to allow vertical fire spread to reach ceiling sprinklers and for sprinkler discharge to reach the warehouse racking without too many obstructions.
The two types of flue spaces are:
Transverse flue space – As one faces the rack storage array, it is the space between commodities on the racks and between the racks themselves.
Longitudinal flue space – If one were to walk to the end of an aisle and look along double or multiple-row long dimension of the racks (back-to-back) this is the longitudinal flue space.
For additional information please refer to the International Fire Code®; NFPA 13, Standard for the Installation of Sprinkler Systems; or NFPA 230, Standard for the Fire Protection of Storage.