Explain flue space requirements for rack storage and how they interact with automatic sprinkler protection under NFPA 13.

Steps

1. Define transverse flue space: the gap between loads across the rack (side to side) and at rack uprights, required in single-row, double-row, and multiple-row racks to let heat and sprinkler water penetrate vertically through the array
2. Confirm NFPA 13's actual rule for rack storage: a nominal 6-inch transverse flue space between loads and at rack uprights must be maintained in single-, double-, and multiple-row racks, and NFPA 13 explicitly permits random variations in flue-space width and vertical alignment — it does not prescribe a minimum net width smaller than 6 in. or a fixed maximum horizontal spacing interval between flues
3. Do not attribute FM Global's numeric criteria to NFPA 13: FM Global Property Loss Prevention Data Sheet 8-9 (a separate, insurer-specific document) is the source of the commonly cited 3-inch minimum net transverse flue width permitted at reduced intervals (roughly every 4.5 ft when net width is under 6 in., relaxing to about every 9 ft at 6 in. or greater) — this is FM's own criteria, not an NFPA 13 provision
4. Define longitudinal flue space: the front-to-back gap between double/multiple rows; NFPA 13 does not always mandate one, but when provided, a minimum nominal 6 inches is commonly required to aid water penetration
5. Explain the safety consequence: closing up or blocking flue spaces (e.g., with oversized pallets or off-spec storage) can defeat the sprinkler design basis, potentially requiring additional in-rack sprinklers or reduced storage height to remain compliant
6. Recommend verifying flue-space compliance against the facility's actual approved sprinkler design and the governing document (NFPA 13 vs. FM Global DS 8-9, since an FM-insured property may be held to FM's own, different numeric criteria) rather than assuming one document's numbers apply under the other