Once ST4 pallet racking reaches 9 m in a logistics warehouse, ceiling protection is no longer sufficient and the in-rack requirement comes into play. Which level, which flue, which K-factor for which configuration — from a site perspective.
Open the rack protection section of the European standard and the same question arrives immediately: what are ST1 to ST6, in which configuration is ceiling protection enough, and at what point do in-rack sprinklers become mandatory? Let us work from a designer's site notebook rather than clause headings.
ST1–ST6, briefly
EN 12845 divides storage arrangements into six classes:
- ST1 — free standing or block stacking
- ST2 — single-row post pallets, aisles at least 2.4 m
- ST3 — multiple-row (including double-row) post pallets
- ST4 — beam pallet racking, the classic racking system
- ST5 — solid or slatted shelving, 1 m or narrower
- ST6 — solid or slatted shelving, wider than 1 m and up to 6 m
The classification carries no meaning on its own; the accompanying table ties each configuration's maximum block area, aisle width and in-rack requirement together.
Which configurations require in-rack protection
Read plainly:
- ST1, ST2: ceiling protection may be sufficient in OH or HH; in-rack is recommended but not mandatory.
- ST3 (multiple-row post pallets): a ceiling plus intermediate combination makes sense for HH; where the ceiling is more than 4 m above, in-rack must be used.
- ST4 (pallet racking): the most critical. As aisles narrow below 2.4 m, in-rack becomes mandatory; below 1.2 m, three racks enter the hydraulic calculation together.
- ST5: in-rack required where aisles are narrower than 1.2 m or height increases.
- ST6: in practice always requires in-rack. Apart from 1.2 m gondola shelving with a listed internal bulkhead, there is no alternative; otherwise it is treated as high-hazard storage.
The common rule: where ceiling sprinklers sit more than 4 m above the top of stored goods, in-rack protection is mandatory. That catches high-ceilinged warehouses even with modest stacks.
Horizontal spacing — the flue space rule
The standard requires in-rack sprinklers to be placed in the longitudinal (back-to-back) flue, and where possible at its intersection with the transverse flue. The two dimensions most often violated on site:
- Longitudinal flue ≥ 0.15 m — between two back-to-back pallets. Without pallet stops, the operator pushes the pallet to the back and the flue closes.
- Transverse flue ≥ 0.10 m — between side-by-side pallets.
- Deflector to top of goods: at least 0.10 m for flat spray heads and 0.15 m for others.
Maximum horizontal spacing between sprinklers is 3.75 m for Categories I and II and 1.9 m for Categories III and IV. The product of horizontal and vertical spacing may not exceed 9.8 m² and 4.9 m² respectively. More combustible goods mean closer sprinklers.
Vertical spacing — a level every 3.5 m
The distance from floor to the lowest intermediate level, and between levels, may not exceed 3.5 m or two tiers, whichever is smaller. The topmost level must be no more than one tier below the top of storage. Where ceiling sprinklers are not within 4 m of the top of goods, a level is placed at the top as well.
In practice, on conventional pallet racking with 1.5 m tiers, that means a sprinkler line beneath every second beam.
K-factor and sprinkler type
EN 12845 does not impose a single K-factor for in-rack heads, but the de facto design is:
- K80 — the typical in-rack choice, with standard flow-pressure characteristics.
- K115 — for plastic containers and empty plastic pallets, where a minimum operating pressure of 3 bar is required.
- Temperature: in-rack heads are quick response, typically 68–74 °C, while the associated ceiling heads may run 93–100 °C.
- Sensitivity: special or quick response is mandatory for plastic container storage.
Water shields — why they are mandatory
When a sprinkler at an upper level operates, it wets the bulb of the head below and destroys its thermal response. EN 12845 therefore requires a metal water shield at least 75 mm in diameter on every sprinkler inside racking, beneath perforated shelving, or mounted back to back. On an upright head the shield attaches to the body, not the deflector. This is one of the most commonly missed details in site inspection — in-rack heads installed without shields are outside the standard.
How many sprinklers open in the calculation
Where ceiling and in-rack protection combine, the hydraulic calculation is fully calculated throughout. On the in-rack side the assumptions are:
- Three sprinklers at each level operate at the hydraulically most remote position.
- No more than three levels together, so a maximum of nine in-rack sprinklers in the calculation.
- Aisle ≥ 2.4 m includes one rack; 1.2–2.4 m includes two racks; below 1.2 m includes three. So in a narrow-aisle ST4 the worst case brings 9 × 3 = 27 in-rack sprinklers into the calculation.
- Ceiling side: a minimum of 7.5 mm/min over 260 m², increased where goods sit above the topmost intermediate level.
- Minimum sprinkler pressure 2.0 bar.
- Above 50 in-rack sprinklers, they must be fed from a separate control valve set from the ceiling system.
Worked example — 9 m ST4 racking, Category III goods
Conditions: ST4 pallet racking, 9.0 m storage height, Category III (packaged cartons), 2.4 m aisles, 2.7 m rack width (double row), 1.5 m tiers (six tiers).
| Item | Value |
|---|---|
| Configuration | ST4, 2.4 m aisle → one rack in the calculation |
| Ceiling design density | 7.5 mm/min over 260 m² |
| Number of intermediate levels | 3, at roughly 3.0 m intervals |
| Horizontal spacing (Cat. III) | ≤ 1.9 m, product ≤ 4.9 m² |
| Rows (2.7 m rack) | 1 row (below 3.2 m) |
| K-factor / temperature | K80 / 68 °C quick response, with a 75 mm water shield |
| In-rack heads in the calculation | 3 levels × 3 sprinklers = 9 |
| Minimum sprinkler pressure | 2.0 bar |
The hydraulic calculation must satisfy the 260 m² ceiling area and the nine most remote in-rack heads simultaneously. In practice that comes out at 1500–1800 L/min plus the hose reel allowance, and pump selection follows from there.
ST5 and ST6 — the special case
Water cannot pass through solid or slatted shelving, so a sprinkler is placed above every shelf level, including the topmost where the ceiling is more than 4 m away:
| Shelf width s | Rows | Max. spacing along the row | Max. between rows |
|---|---|---|---|
| ST5: s ≤ 1.0 m | 1 | 2.8 m | — |
| ST6: 1.0 < s ≤ 3.0 m | 1 | 2.8 m | — |
| ST6: 3.0 < s ≤ 6.0 m | 2 | 2.8 m | 2.8 m |
ST6 wider than 6 m is outside the scope of the standard; in that case the racking is subdivided below 6 m with full-height longitudinal and transverse bulkheads of EN 13501-1 class A1 or A2.
Comparison with NFPA 13 storage in-rack
NFPA 13 defines in-rack levels and a mixed face, in-rack and sub-level logic. From a site perspective the differences:
- Ceiling density approach: NFPA 13 gives density-area tables by commodity class and storage height; EN 12845 does the same with fewer parameters.
- In-rack counts: NFPA commonly uses six to eight sprinklers per level over three levels; EN 12845 fixes 3 × 3 = 9.
- K-factor: K115 (K8.0 imperial) is the NFPA in-rack standard; EN 12845 accepts K80, rising to K115 for special commodities.
- Water shields: mandatory in both, at a comparable 75 mm.
- Flue space: NFPA requires 6 in (about 150 mm) longitudinal and 3 in (about 75 mm) transverse; EN 12845 requires 150 mm and 100 mm — close, but EN asks slightly more.
- The ESFR alternative: EN 12845-2 allows in-rack protection to be eliminated entirely with ESFR, as does NFPA 13.
Common field errors
- No pallet stops, allowing the longitudinal flue to close. Water then cannot reach the lower levels in a fire.
- Connecting in-rack heads to the same control valve set as the ceiling — outside the standard above 50 heads.
- Omitting water shields. The most frequent inspection finding.
- Shifting sprinklers away from the flue intersection because the pipe run cannot penetrate a rack beam — breaching the 4.9 m² spacing product.
- Building ST6 bulkheads from light gauge sheet; the standard requires EN 13501-1 class A1 or A2 fire resistance.
Turkish context
BYKHY states that one of the national or international standards must be applied to sprinklers in high-bay warehouses, leaving the choice to the investor — in practice NFPA 13 or EN 12845. European-funded logistics centres and automotive supplier warehouses more commonly use EN 12845 with its storage annexes, while American-origin third-party logistics operators favour NFPA 13. The insurer may add further requirements, so ask at the outset.
Frequently asked questions
Are in-rack sprinklers mandatory in every warehouse?
No. Ceiling protection can be sufficient for ST1 and ST2. For ST4, ST5 and ST6, in-rack becomes mandatory once storage height or clearance to ceiling exceeds the limits; ST6 requires intermediate levels in almost every case.
What is the minimum longitudinal flue?
0.15 m for the longitudinal (back-to-back) flue and 0.10 m for the transverse flue. Pallet stops must preserve that clearance.
What is the vertical spacing between in-rack levels?
No more than 3.5 m or two tiers, whichever is smaller. On typical pallet racking that puts a level roughly every 3 to 3.5 m.
How many in-rack sprinklers open in the calculation?
Three at each level, across a maximum of three levels — nine in total. Depending on aisle width, one, two or three racks are included hydraulically.
Which K-factor is used for in-rack heads?
K80 is the typical choice. Special commodities such as plastic containers require K115 at a minimum of 3 bar. All in-rack heads must be quick response with a 75 mm water shield.
SprinkCalc — Fire Sprinkler Design Across Three Standards
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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.