ST1–ST6 storage configurations, commodity categories, and design density by height band in high-hazard storage.
On a sprinkler replacement at a logistics centre, when the client insisted "the warehouse is OH3, 5 mm/min is enough", it was time to open the classification table. Pallet height was 5.2 m and the goods were polyethylene containers in cartons. That is no longer OH; it is high-hazard storage. The density rises to 12.5 mm/min, the area of operation to 260 m², and the hydraulic demand roughly triples. The high-hazard storage provisions are the framework that takes over wherever a storage fire will not fit inside the OH classes.
When HHS applies
EN 12845 handles storage in two ways. Small quantities below the tabulated height limits can stay within the OH classes — a Category I commodity in an ST1 arrangement counts as OH up to 4.0 m, for instance. Exceed that limit, or use an ST4 configuration, and the table sends you straight to HHS. In short: high-hazard storage applies once storage height passes a given threshold, where the rack type is ST6 (shelving wider than 1 m), or where Category II or above is block-stacked or on pallet racking.
HHS divides into four sub-classes — HHS1 to HHS4 — reflecting the commodity fire load category (I to IV). The categories are given commodity by commodity in the classification annexes; the examples we meet most often:
- Category I: glass, ceramics, metal containers, predominantly non-combustible goods on cardboard or timber pallets.
- Category II: dry foodstuffs, textile rolls, leather goods, timber furniture carcasses.
- Category III: small containers of flammable liquids, paper rolls, limited plastic content.
- Category IV: expanded plastics (EPS, PUR), aerosols, liquids in PE or PP containers, rubber.
Storage configurations ST1 to ST6
The standard divides storage arrangements into six types:
- ST1: free-standing or block stacking — pallets on the floor or stacked, with no racking.
- ST2: single-row post pallets, at least 2.4 m apart.
- ST3: multiple- or double-row post pallets.
- ST4: palletised racking — conventional beam pallet racking, or "selective racking".
- ST5: solid or slatted shelving 1 m wide or less — gondola or small shelving.
- ST6: solid or slatted shelving wider than 1 m but not exceeding 6 m. Because it blocks smoke and water vertically, it almost always requires in-rack sprinklers.
ST6 is a special case: only gondola shelving up to 1.2 m total width with a solid barrier down the centre can stay within the OH classes; otherwise HHS protection is mandatory.
Design density by height band
A single table governs HHS design where protection is by ceiling sprinklers alone, with no in-rack heads. It maps category (I to IV) against storage height to a design density. The values worth committing to memory:
| Density (mm/min) | ST1 — Cat. I | ST1 — Cat. II | ST1 — Cat. III | ST1 — Cat. IV | Area of operation |
|---|---|---|---|---|---|
| 7.5 | 5.3 m | 4.1 m | 2.9 m | 1.6 m | 260 m² |
| 10.0 | 6.5 m | 5.0 m | 3.5 m | 2.0 m | 260 m² |
| 12.5 | 7.6 m | 5.9 m | 4.1 m | 2.3 m | 260 m² |
| 15.0 | — | 6.7 m | 4.7 m | 2.7 m | 260 m² |
| 17.5 | — | 7.5 m | 5.2 m | 3.0 m | 260 m² |
| 20.0 | — | — | 5.7 m | 3.3 m | 300 m² |
| 25.0 | — | — | 6.7 m | 3.8 m | 300 m² |
| 30.0 | — | — | — | 4.4 m | 300 m² |
The logic is clear: as the category hardens or the height rises, density climbs. The area of operation is 260 m² at 17.5 mm/min and below, rising to 300 m² at 20 mm/min and above. ST4 palletised racking allows lower heights for the same density rows — Category I at 7.5 mm/min reaches 5.3 m in ST1 but only 4.7 m in ST4. ST2 and ST3 track ST4. ST5 and ST6 fall outside the table below 12.5 mm/min; those configurations cannot be served by low-density ceiling protection and start from the middle of the range.
Where a dry or alternate system is selected, the area of operation is increased by 25 %: a 260 m² wet system becomes 325 m². In HHS that usually pushes the pump up a size, so a dry system should be a last resort.
Area of operation and hydraulic demand
The HHS pre-calculated tables give roughly the following maximum demands:
- 7.5 mm/min → 2300 L/min wet, or 2900 L/min dry/alternate
- 10.0 mm/min → 3050 L/min wet, or 3800 L/min dry/alternate
Above these densities EN 12845 requires a full hydraulic calculation; the pre-calculated method is not adequate above 12.5 mm/min. On HHS3 and HHS4 projects we therefore always run a node-by-node calculation, checking the flow-pressure balance across the four most remote sprinkler groups. K-factors typically start at K115 for HHS1 and HHS2 and rise through K160, K200 and K240, with K280–K360 seen at ceiling level in high-bay warehouses.
When in-rack sprinklers become mandatory
Exceed the ceiling-only height limits and in-rack sprinklers must be added. The good news is that ceiling design density then drops to a fixed 7.5 mm/min. The bad news is that each in-rack group is calculated as a separate hydraulic segment, with sprinklers on three adjacent racks assumed to operate simultaneously. Where the number of in-rack sprinklers exceeds 50, a separate control valve is required — in practice a separate alarm valve set for the in-rack system.
Common field errors
- Pressure to downgrade the category. Even if the client says "that carton doesn't burn", PE film inside it lifts it to Category III. The commodity list is the first question to ask on site.
- Height is not the top pallet level. Height is measured from the storage surface to the top of the goods; covers, slip sheets and stretch film all raise it. Allow 100–200 mm of slack.
- Ignoring clearance. Where more than 1 m of clear height is left between roof and storage, either in-rack sprinklers or a category step up is required.
- Aerosols and lithium. These fall outside EN 12845; refer to the relevant FM Global data sheets or NFPA 30B and 855.
- K-factor selection. A high density combined with low ceiling pressure will not work with K115. To keep at least 2 bar at the design point, K200 or K240 is needed.
Comparison with NFPA 13
The EN 12845 HHS table corresponds to the storage chapters of NFPA 13. Roughly:
| EN 12845 | Approximate NFPA 13 equivalent |
|---|---|
| Category I | Class I commodities |
| Category II | Class II |
| Category III | Class III–IV (partly) |
| Category IV | Group A plastics |
| ST1 | Palletised / solid pile storage |
| ST4 | Single or double row rack |
| ST5/ST6 | Shelf storage |
NFPA 13 uses design curves rather than a height-density matrix, but the results are of the same order. Since EN 12845-2:2024 brought ESFR and CMSA design into the European standard, direct comparison with the NFPA storage tables has become easier.
The ESFR and CMSA alternative
EN 12845-1 describes protection by conventional and spray sprinklers only. ESFR and CMSA protection, which can avoid in-rack sprinklers entirely, is covered in Part 2. For logistics operators wanting to minimise in-rack pipework in high racking, the Part 2 route is a relief in both capital and operating terms.
Application in Turkey
BYKHY requires sprinklers in warehouses but gives no density-area table; in practice projects reference EN 12845 or FM Global. For the last five years, fire insurers have been asking for an FM Global commodity data sheet comparison before approving HHS3 and HHS4 projects at large e-commerce warehouses, so checking the HHS calculation against both EN 12845 and FM has become standard.
Frequently asked questions
What is the difference between HHS and HHP?
HHP covers manufacturing and process; HHS covers storage. HHP densities are fixed (7.5, 10.0 or 12.5 mm/min over 260 m²). HHS density runs from 7.5 to 30 mm/min by category and height, over 260 or 300 m².
Why is the HHS area 260 or 300 m²?
260 m² at 17.5 mm/min and below, 300 m² at 20.0 mm/min and above. Higher-density fires spread faster within the area, so the table enlarges it.
Why do ST1 and ST4 give different densities at the same height?
Vertical flue spaces in racking create a chimney effect, whereas a block stack burns from the surface. So Category I reaches 5.3 m at 7.5 mm/min in ST1 but stops at 4.7 m in ST4.
Can I use ESFR or CMSA for HHS?
EN 12845-1 describes only conventional and spray sprinklers. ESFR and CMSA designs sit in EN 12845-2:2024, with their own rules on clearance, K-factor and spacing.
What is the NFPA 13 equivalent?
The storage chapters covering Class I–IV commodities and Group A plastics. EN Categories I–IV do not map exactly onto NFPA Classes I–IV, but the fire loads are comparable — the format differs, the logic does not.
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Download MEP Calc on the App StoreBS EN 12845:2015+A1:2019 Fixed firefighting systems — Automatic sprinkler systems. EN 12845-2:2024 (CMSA & ESFR sprinkler systems). NFPA 13 Standard for the Installation of Sprinkler Systems. Turkish Regulation on Fire Protection of Buildings (BYKHY). FM Global Property Loss Prevention Data Sheet 2-0.