[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

A distribution warehouse will store cartoned plastic goods in double-row steel racks up to 7.5 m high, under a roof at 9.5 m. The architect wants to avoid in-rack sprinklers because the racks will be reconfigured.

• How do I decide between ceiling-only protection and in-rack sprinklers?
• What are the main steps under EN 12845 and NFPA 13?
• What clearances and flue spaces must the rack layout keep?

[MEP Center Engineering]

Start from the commodity classification and the storage height. Under both EN 12845 and NFPA 13, ceiling-only protection is allowed up to limits that depend on commodity, storage height and ceiling height. Beyond those limits you need in-rack sprinklers, or ceiling-level ESFR/CMSA sprinklers listed for that commodity and geometry. Plastic goods at 7.5 m under 9.5 m are usually near or beyond the ceiling-only limits, so check ESFR early.

Step 1: Classify

• EN 12845: high-hazard storage (HHS) with the commodity in Category I to IV, and the storage configuration (ST1 to ST6). Plastics often push goods to Category III or IV.
• NFPA 13: Class I to IV or Group A plastics (expanded or unexpanded, cartoned or exposed). Cartoned unexpanded Group A plastic is a common worst case.
• Get the classification from the owner in writing. It drives everything else.

Step 2: Choose the scheme

1. Ceiling-only density/area: look up the design density and area of operation for the class, height and configuration. If the tables allow it, this is the simplest.
2. Ceiling plus in-rack: required above the ceiling-only limits. It is less sensitive to roof height but fixed to the rack layout.
3. ESFR (or NFPA CMSA): ceiling-only for high storage, but strict about roof slope, obstructions, clearance and ceiling height. See ESFR Sprinklers: Quick Rules. Given the architect’s wish, this is often the best fit.

Worked hydraulic illustration

Suppose the applicable table gives 12.5 mm/min over 260 m², with 9 m² per sprinkler. (These values are for illustration only.)

• Flow per sprinkler: 12.5 × 9 = 112.5 L/min.
• Pressure at K80: (112.5 / 80)² = 1.98 bar. At K115: (112.5 / 115)² = 0.96 bar. A larger K-factor lowers the pump head.
• Sprinklers in the area: 260 / 9 = 28.9, so 29. Minimum ceiling flow: 29 × 112.5 = 3 263 L/min, before hydraulic imbalance, in-rack demand and hose allowance.

The real demand is always higher than this minimum, because remote sprinklers run at higher pressure. Use full hydraulic calculation and add hose streams to size the tank and pump: Fire Water Tank Sizing.

Rack layout rules to put on drawings

• Clearance between the sprinkler deflector and the top of storage: at least 1 m under EN 12845 for storage. NFPA 13 requires 457 mm (18 in) for standard sprinklers and 914 mm (36 in) for ESFR and CMSA.
• Flue spaces (NFPA 13, double-row racks): transverse flues about 75 mm (3 in), longitudinal flue about 150 mm (6 in). Solid shelves or flue blockage change the protection scheme.
• A maximum storage height line on the elevation, and “no storage above this line” signage.

Common mistakes

• Designing for a commodity class lower than what will actually be stored.
• Choosing ESFR and then adding ducts and lights that obstruct it.
• Ignoring the roof height. Ceiling-only criteria depend on it, not only on storage height.

Code differences: EN 12845 vs NFPA 13: Key Differences.

For deeper study: Fire Protection Design course.

Check the edition of the code that applies to your project.

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