EN 12845 HHS (High Hazard Storage) Design

High Hazard Storage (HHS) covers the storage of goods where the limits given in Clause 6.2.3 are exceeded. An HHS design follows from the intersection of three inputs: the category of the goods, the storage configuration and the storage height. These three inputs are carried into Table 4 (ceiling protection only) or Table 5 (with in-rack sprinklers). Source: TS EN 12845+A2:2026, Clause 6.2.4.2, EN p.32 and Clause 7.2, EN p.36-37.

The decision flow

Clause 6.3.1 (EN p.33): the total fire hazard of the goods stored is a function of the combustibility of the materials stored, including their packaging, and of the storage configuration. To determine the design criteria required in the case of goods stored, the procedure shown in Figure 2 is followed.

The order is as follows:

1. The category of the goods is determined: the methodology of Annex B (material factor and storage configuration) and the alphabetical product list of Annex C. B.3.1 (EN p.129): where an appropriate category is also given in Table C.1, the higher of the two values is used.
2. The storage configuration is determined: ST1-ST6 in Clause 6.3.2.
3. The storage height is compared with Table 1. If it is below the limit, OH3 protection may be sufficient; if it is exceeded, the design moves to HHS.
4. The configuration limitations of Table 2 are checked for HHS.
5. The density and area are read from Table 4; where the height limit is exceeded, in-rack sprinklers are added in accordance with 7.2.3 and the ceiling criteria are taken from Table 5.

Step 1: Category (Annex B and Annex C)

Annex B (normative, EN p.127) requires the material to be analysed to determine a material factor and, where necessary, that factor to be modified according to the configuration of the goods in order to determine the category. Where no modification is required, the material factor is the sole determinant of the category.

Source: Clauses B.2.2 - B.2.5, EN p.127-129.

Table B.1: Categories as a function of the storage configuration (EN p.129)

Clause B.3.2 (EN p.130) gives the detail of the row "exposed plastic container with non combustible content": this applies only to plastic containers holding a non combustible liquid or solid in direct contact with the container. Category I is containers with non combustible liquid; Category II is small (≤ 50 l) containers with non combustible solid; Category III is large (> 50 l) containers with non combustible solid. The examples given are plastic bottles of soft drinks or liquids with an alcohol content below 20 % and plastic buckets and drums containing inert powder such as talc. Note: metal parts in plastic storage boxes do not fall into this configuration.

Annex G.8.2 (EN p.151) sets a separate provision: polypropylene and polyethylene storage containers are classified as HHS Category IV.

Step 2: Storage configuration (Clause 6.3.2, EN p.33)

• ST1: free standing or block stacking;
• ST2: single row post pallets with aisles not narrower than 2.4 m;
• ST3: multiple row (including double row) post pallets;
• ST4: pallet racks (beam pallet racks);
• ST5: solid or slatted shelf racks 1 m wide or narrower;
• ST6: solid or slatted shelf racks wider than 1 m and not wider than 6 m.

Table 2 (EN p.34) gives, for each configuration, the maximum storage block area, the width of the aisles separating the rows and the minimum separation clearance around the storage block. In HH, for example, the maximum block area for ST1 and ST3 is 150 m² with a surrounding clearance of 2.4 m; for ST2 the area is unlimited and the aisle is 2.4 m or wider. For ST4 without intermediate level in-rack protection the area is unlimited and the aisle is 1.2 m or wider.

The footnotes to Table 2 tie in the hydraulic assumptions: where the aisle is narrower than 1.2 m, in-rack protection in three racks is assumed to be hydraulically involved; between 1.2 m and 2.4 m, two racks; at 2.4 m or wider, one rack (see 7.2.3.3). Footnote c limits the relevant protection method to risks where the ceiling sprinklers are less than 4 m above the highest level of the goods stored.

For ST6, HH protection is used in OH. Where it is not possible to install intermediate level sprinklers in ST6 storage, longitudinal and transverse full height bulkheads are fitted in each rack; these are constructed to EN 13501-1 Euroclass A1, A2 or the national equivalent (Table 2 footnote h).

Step 3: The OH3 limit in Table 1 (EN p.32)

Footnote a: where the storage heights exceed the values in the table, HHS protection is used, see 6.2.4.2 and 7.2. Footnote b: ST6 storage is limited to gondola shelving of 1.2 m total width with a solid central barrier extending from the base to the top of the gondola shelving; all other ST6 storage requires HHS protection in accordance with 7.2.

This table shows that there is no single "4 metre" threshold: Category IV goods reach the limit at 1.2 m even in ST1.

Step 4: Density and area from Table 4 (ceiling protection only, EN p.38)

Clause 7.2.2.1 (EN p.36): for buildings exceeding 12 m, the approval authorities are consulted.

Clause 7.2.2.2: Table 4 specifies the appropriate design density and area of operation for various storage types that can be protected by roof or ceiling protection alone, according to the category and the maximum permitted storage height. The storage heights in the table are considered to be the maximum values for effective sprinkler protection where sprinklers are located only at the roof or ceiling. Where the heights exceed this limit, intermediate level in-rack sprinklers are provided in accordance with 7.2.3.

ST1: free standing or block stacking

ST2 single row post pallets and ST4 pallet racks

ST3 multiple row post pallets and ST5 with ST6 solid or slatted shelf racks

ST3 together with ST5 and ST6 form a single combined group of rows in Table 4. Consequently the 7.5 and 10.0 mm/min values for ST5 and ST6 are present in the table.

Note to Table 4: dry and alternate systems should not be used in high hazard storage, particularly with the more combustible products (the higher categories) and with higher storage. Where it is nevertheless necessary to install a dry or alternate system, the area of operation should be increased by 25 %. Arithmetically: 260 × 1.25 = 325 m² and 300 × 1.25 = 375 m².

Excessive clearance and in-rack sprinklers

Clause 7.2.2.3 (EN p.36-37): the values in Table 4 are used to determine the clearance. Where the clearance, that is the distance between the storage height and the ceiling sprinkler deflector, exceeds 4 m, one of the following options is applied:

• Option 1: the density is increased by 2.5 mm/min for the first metre of excess; for each subsequent metre the water density is increased by 1 mm/min; a minimum of K115 is used.
• Option 2: intermediate level in-rack sprinklers are provided in accordance with 7.2.3.

The standard contains no provision for raising the category by one step because of the clearance.

Clause 7.2.3.2 (EN p.37): the design density for the roof or ceiling sprinklers shall be at least 7.5 mm/min over an area of operation of 260 m². Where goods are stored above the highest intermediate protection level, the design criteria for the roof or ceiling sprinklers are taken from Table 5.

Table 5: Design criteria for roof or ceiling sprinklers with in-rack sprinklers (EN p.39)

In Table 5 the heights are the vertical distance from the highest level of in-rack protection to the top of the storage.

In Table 5, ST4 and ST5/ST6 are repeated as separate blocks. The same note repeats that the area of operation is to be increased by 25 % in dry and alternate systems.

Clause 7.2.3.3 (EN p.37): for the purpose of hydraulic calculation, 3 sprinklers are assumed to operate simultaneously in the most remote position at each in-rack sprinkler level, up to a maximum of three levels. Where the rack aisles are 2.4 m or wider, one rack is included; where narrower than 2.4 m but equal to or wider than 1.2 m, two racks; where narrower than 1.2 m, three racks.

Clause 7.2.3.1 (EN p.37): where more than 50 intermediate level sprinklers are installed in racks, they shall not be fed from the same control valve set as the roof or ceiling sprinklers.

Pre-calculated or fully calculated

Clause 13.1 (EN p.82): pipe sizes are determined by either the pre-calculated system method or the fully calculated system method. The designer may choose between the two; full calculation is always used in the following two cases: arrangements with intermediate level HHS sprinklers; gridded or looped arrangements.

Section (4) of Table 7 (EN p.40) gives flows and pressures from 10.0 mm/min to 30.0 mm/min for pre-calculated systems using the pipe sizes of Tables 35 and 34 with K115 sprinklers:

In other words, the pre-calculated method is not limited to 12.5 mm/min.

Water volume and duration

Clause 8.1.1 (EN p.42): the minimum water supply duration for HHS is 90 minutes.

In pre-calculated systems Table 10 (EN p.48) is used; the values are shared with HHP: for 7.5 mm/min, wet 225 m³ and dry 280 m³; for 30.0 mm/min, wet 875 m³ and dry 1 090 m³. In calculated systems, in accordance with Clause 9.3.2.3, the minimum effective volume is found by multiplying the maximum demand flow by the duration.

Sprinkler selection and pressure

• Table 37a (EN p.104): for HHP and HHS ceiling or roof sprinklers, K 80, 115 or 160 up to 10 mm/min and K 115 or 160 above 10 mm/min. For HHS intermediate level sprinklers in high piled storage, conventional, spray and flat spray types with K 80 or 115. K factors of K200 and above are not covered by EN 12845-1; they are addressed in EN 12845-2 for ESFR and CMSA.
• Clause 13.4.4 (EN p.102): the minimum discharge pressure is 0.50 bar at HHP and HHS ceiling sprinklers, 1.00 bar at K115 in-rack sprinklers and 2.00 bar at K80 in-rack sprinklers.
• Table 38 (EN p.106): only Quick sensitivity is used for in-rack sprinklers.
• Clause 12.1.2 (EN p.70): in HHP and HHS a clear space of at least 1.0 m is maintained below the deflector.
• Clause 11.2.2 (EN p.67): it is strongly recommended that dry and alternate installations not be used in HHS applications.

Special cases: Annex G

• G.8 polypropylene or polyethylene storage containers (EN p.151): these are classified as HHS Category IV. In ST4 pallet rack storage, the horizontal spacing of the in-rack sprinklers does not exceed 1.5 m and the vertical spacing 2 m; the ceiling sprinklers are of 'Special' sensitivity and the in-rack sprinklers of 'Special' or 'Quick' sensitivity. In all other storage the maximum storage height does not exceed 3 m, only non combustible pallets (for example steel pallets) are used, the stack height per pallet does not exceed 1 m, and the topmost storage container on each pallet is closed with a lid. A film forming foam suitable for sprinkler water is added in accordance with the supplier's recommendation.
• G.7.2 non-woven synthetic fabric, rack storage (EN p.151): in-rack sprinklers are used in accordance with the Category IV requirements; the ceiling sprinklers have a design density of at least 12.5 mm/min over 260 m².

Frequently Asked Questions

When does storage leave OH3 and become HHS?

TS EN 12845+A2:2026 Clause 6.2.3 (EN p.31): for material to be stored in occupancies classified as OH, three conditions shall be met: a) the protection throughout the room is designed to at least OH3; b) the maximum storage heights in Table 1 are not exceeded; c) the maximum storage area of a single block does not exceed 50 m² and there is a clearance of at least 2.4 m around the block. Where the area is classified as OH4, or where b) or c) cannot be met, the storage in the area is treated as HHS.

What are the maximum OH3 storage heights in Table 1?

Table 1 (EN p.32) gives separate values for free standing or block stacking (ST1) and for the other storage configurations (ST2-ST6): Category I 4.0 and 3.5 m; Category II 3.0 and 2.6 m; Category III 2.1 and 1.7 m; Category IV 1.2 and 1.2 m. Where the storage heights exceed these values, HHS protection is used in accordance with 6.2.4.2 and 7.2. In other words there is no single 4 m threshold; the threshold depends on the category and the configuration.

Are 7.5 and 10.0 mm/min values given in the table for ST5 and ST6?

Yes. In Table 4 (EN p.38), ST3 multiple row post pallets and ST5 and ST6 solid or slatted shelf racks form a single combined group of rows. The block starts at 7.5 mm/min (Category I 4.7 m, Category II 3.4 m, Category III 2.2 m, Category IV 1.6 m) and includes rows for 10.0 mm/min (Category I 5.7 m, Category II 4.2 m, Category III 2.6 m, Category IV 2.0 m), 12.5 mm/min (Category II 5.0 m, Category III 3.2 m, Category IV 2.3 m), 15.0 mm/min (Category IV 2.7 m) and 17.5 mm/min (Category IV 3.0 m). The area of operation is 260 m².

Up to which densities may the pre-calculated method be used in HHS?

Section (4) of Table 7 (EN p.40) gives flows and pressures from 10.0 mm/min to 30.0 mm/min for pre-calculated systems using the pipe sizes of Tables 35 and 34 with K115 sprinklers. Clause 13.1 (EN p.82) makes full calculation mandatory in only two cases: arrangements with intermediate level HHS sprinklers, and gridded or looped arrangements.

How many rack rows are included in the hydraulic calculation?

Clause 7.2.3.3 (EN p.37): where the rack aisles are 2.4 m or wider, only one rack is assumed to be involved; where narrower than 2.4 m but equal to or wider than 1.2 m, two racks; where narrower than 1.2 m, three racks. At each level, 3 sprinklers in the most remote position are assumed to operate simultaneously, up to a maximum of three levels.

References

• TS EN 12845+A2:2026 Clause 6.2.3, Table 1, Clause 6.2.4.2 (EN p.31-32)
• TS EN 12845+A2:2026 Clauses 6.3.1, 6.3.2, Figure 2 (EN p.33)
• TS EN 12845+A2:2026 Table 2 and its footnotes (EN p.34)
• TS EN 12845+A2:2026 Clauses 7.2.1, 7.2.2.1, 7.2.2.2, 7.2.2.3 (EN p.36-37)
• TS EN 12845+A2:2026 Clauses 7.2.3.1, 7.2.3.2, 7.2.3.3, 7.2.3.4 (EN p.37)
• TS EN 12845+A2:2026 Table 4 (EN p.38), Table 5 (EN p.39), Table 7 (EN p.40)
• TS EN 12845+A2:2026 Clause 8.1.1 (EN p.42)
• TS EN 12845+A2:2026 Clause 9.3.2.2, Table 10, Clause 9.3.2.3 (EN p.48)
• TS EN 12845+A2:2026 Clause 11.2.2 (EN p.67), Clause 12.1.2 (EN p.70)
• TS EN 12845+A2:2026 Clause 13.1 (EN p.82), Clause 13.4.4 (EN p.102)
• TS EN 12845+A2:2026 Table 37a (EN p.104), Table 38 (EN p.106)
• TS EN 12845+A2:2026 Annex B, Clauses B.2.2 - B.2.5 (EN p.127-129), Table B.1 (EN p.129), Clauses B.3.1, B.3.2 (EN p.129-130)
• TS EN 12845+A2:2026 Annex C (EN p.132)
• TS EN 12845+A2:2026 Annex G, Clauses G.7.2, G.8.1 - G.8.5 (EN p.151-152)

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