Ever wondered why so much has been written about rack flue spaces in all the major fire sprinkler system standards? Flue spaces are clear vertical lines of sight from the floor up to the ceiling within rack storage areas. There are two types of flue spaces – Transverse flues and longitudinal flues. Transverse flues are the spaces between rows of storage in the direction of loading. Longitudinal flues are the spaces between rows of storage perpendicular to the direction of loading with a width up to 24 in. (600 mm). Longitudinal flues greater than 24 in. (600 mm) wide are treated as aisles.