EN 12845 Ordinary Hazard (OH1-OH4) Design

Ordinary Hazard is the class of TS EN 12845+A2:2026 most often used in practice: the design density of all four groups is the same, and they differ only in the area of operation. That apparently simple structure opens out into a difference of up to five times along the water supply chain. Source: TS EN 12845+A2:2026, Clause 6.2.3, Table 3, Table 6 and Annex A.

Definition and the four groups

Clause 6.2.3 (TS p.34 / EN p.31): OH covers occupancies of medium fire load and medium combustibility where combustible materials are processed or manufactured. Annex A is consulted for examples. Ordinary Hazard is divided into four groups: OH1, OH2, OH3, OH4.

The same clause permits storage in OH areas, but requires three conditions together:

• a) The protection throughout the room shall be designed to at least OH3.
• b) The maximum stacking heights shown in Table 1 shall not be exceeded.
• c) The maximum storage area of a single block shall not exceed 50 m², with a clearance of at least 2.4 m around the block.

Where the area is classified as OH4, or where condition b) or c) cannot be satisfied, the storage in the area shall be treated as HHS (see Clause 6.2.4.2).

Table 1 (TS p.35 / EN p.32) gives the maximum stacking heights for OH3 protection: in ST1 free standing or block stacking, Category I 4.0 m, Category II 3.0 m, Category III 2.1 m, Category IV 1.2 m; in configurations ST2-ST6, 3.5 / 2.6 / 1.7 / 1.2 m respectively.

Design criteria: Table 3

Table 3 (Clause 7.1, TS p.39 / EN p.36):

The dry/alternate areas are exactly 1.25 times the wet areas: 72 x 1.25 = 90; 144 x 1.25 = 180; 216 x 1.25 = 270. A dry or alternate installation is not permitted in OH4, where the design moves to HHP1.

Clause 6.1 (TS p.34 / EN p.31) resolves the mixed case: where areas of different hazard classification are in open communication, the higher design criteria shall be extended into the area of lower classification by at least two rows of sprinklers.

Pre-calculated OH: reading Table 6 correctly

Table 6 (Clause 7.3.1, TS p.42 / EN p.39) consists of four data columns and gives two operating points in each row: (flow / pressure at the control valve set) and (maximum demand flow rate / pressure at the control valve set). The dry classes are grouped in the same row as the next wet class up.

NOTE: ps is the static head loss, in bar, due to the height of the highest sprinkler in the relevant arrangement above the 'C' gauge of the control valve set. Clause 7.3.1: the pressure loss due to friction and static head shall be calculated separately between the water supply and each control valve set.

On the pump side, Table 16 (Clause 10.7.1, TS p.65 / EN p.62) gives nominal and characteristic values in three height bands with the same grouping; for example, for OH1 wet with h ≤ 15 m, nominal 1.2 bar / 900 l/min and characteristic 2.2 bar / 540 l/min and 2.5 bar / 375 l/min.

Water volume from Table 9 (TS p.50 / EN p.47): 55 m³ for OH1 wet at h ≤ 15 m; 105 m³ for OH1 dry and OH2 wet; 135 m³ for OH2 dry and OH3 wet; 160 m³ for OH3 dry and OH4 wet. Where a reduced capacity tank is used, the Table 11 floors (TS p.52 / EN p.49) are 10, 20, 30 and 50 m³ respectively. The duration under Clause 8.1.1 is 60 minutes in OH.

Annex A: which occupancy is in which group?

Table A.2 (TS p.127-128 / EN p.124-125) lists the occupancies sector by sector. The grouping below follows the column arrangement of the table:

OH1: Cement works; Sheet metal product factories; Hospitals; Hotels; Libraries, excluding book stores; Restaurants; Schools (see 6.2.2); Offices (see 6.2.2); Data processing - computer room, excluding tape storage.

OH2: Photographic film factories; Metal working; Abattoirs, meat factories; Bakeries; Biscuit factories; Breweries; Chocolate factories; Dairies; Confectionery factories; Laboratories (physical); Laundries; Car parks; Museums; Leather goods factories.

OH3: Glass factories; Dyers works, soap factories; Photographic laboratories; Paint application shops with water based paint; Electronics factories; Radio equipment factories; Washing machine factories; Car workshops; Animal fodder factories; Corn mills; Dehydrated vegetable and soup factories; Sugar factories; Broadcasting studios (small); Railway stations; Plant (technical) room; Farm building; Book binding factories, cardboard factories, paper factories; Department stores, shopping centre; Carpet factories (excluding rubber and foam plastic); Cloth and clothing factories, fibre board factories; Footwear factories (excluding plastic and rubber); Knitting factories, linen factories; Mattress factories (excluding foam plastic); Sewing factories, weaving mills; Woolen and worsted mills; Woodworking factories; Furniture factories (without foam plastic); Furniture showrooms; Upholstery factories (without foam plastic).

OH4: Alcohol distilleries; Cinemas and theatres; Concert halls; Tobacco factories; Film and TV Production Studio; Waste paper processing; Exhibition halls (excessive clearance to be taken into account); Cotton mills; Flax preparation plants; Hemp preparation plants; Saw mills; Plywood factories.

The NOTE to Table A.2: where an OH1 or OH2 occupancy contains paint spraying or similar high fire load areas, those areas should be treated as OH3.

Two items frequently confused: prisons are in Table A.1, that is, they are LH; and match manufacturers are in the HHP1 column of Table A.3 (TS p.129 / EN p.126), not in OH4.

Sprinkler layout and type

Table 19 (Clause 12.2, TS p.73 / EN p.70): in OH, the maximum area per sprinkler is 12.0 m²; the maximum S and D in a standard layout is 4.0 m; in a staggered layout S is 4.6 m and D 4.0 m.

Table 20 (TS p.74 / EN p.71): in OH, the maximum area per sidewall sprinkler is 9.0 m²; the spacing between sprinklers along the wall is 3.4 m (which may be increased to 3.7 m where the fire resistance of the ceiling is not less than 120 minutes); the distance from a sprinkler to the end wall is 1.8 m.

Clause 12.1.2 a) (TS p.73 / EN p.70): in LH and OH, a clear space of at least 0.3 m below the deflector for flat spray sprinklers and at least 0.5 m in other cases.

Table 37a (TS p.107 / EN p.104): in OH the design density is 5.0 mm/min; the sprinkler types are conventional, spray, ceiling, flush, flat spray, recessed, concealed and sidewall; the nominal K-factor is 80 or 115.

Clause 14.2.2 (TS p.107 / EN p.104): ceiling, flush, recessed and concealed sprinklers shall not be installed in OH4, HHP or HHS areas.

Clause 14.2.3: sidewall sprinklers shall not be installed on HH installations (other than for the protection of corridors, cable ducts and columns), in OH storage areas or above suspended ceilings; they may be installed only beneath flat ceilings.

Pipe sizing

On pre-calculated OH, range pipe diameters comply with Table 30 and distribution pipe diameters with Table 31 (Clause 13.3.4.1, TS p.91 / EN p.88). For example, on all other range pipes, 25 mm feeds 3, 32 mm 4, 40 mm 6 and 50 mm 9 sprinklers.

The design point is found from Table 26 (TS p.89 / EN p.86): where the number of sprinklers on one distribution pipe in a room is more than 16, at the 17th sprinkler in a two end-side layout; where it is more than 18, at the 19th sprinkler in other layouts.

Clause 13.3.4.2 (TS p.92 / EN p.89): the diameters between the design point in the most remote area and the control valve set are calculated such that the total friction loss at a flow of 1 000 l/min does not exceed 0.5 bar. Clause 13.3.4.4 states that this may be increased by 50 per cent of the excess pressure available where OH3/OH4 and HHP/HHS areas on the system are connected to a common water supply, and gives an example for OH3: 1.4 bar required at the control valve set plus 1.2 bar from the height difference, total 2.6 bar; 6.0 bar available at the flow appropriate to HH; the excess pressure that may be used is 50 % x (6.0 - 2.6) = 1.7 bar; the pipework is sized for a loss of 0.5 + 1.7 x (1 000 / 1 350)² = 1.43 bar.

The continuation of Clause 13.3.4.1: where range pipes run longitudinally beneath roofs sloping more than 6°, the number of sprinklers on a range pipe shall not exceed six.

Minimum pressure, Clause 13.4.4 (TS p.105 / EN p.102): 0.35 bar in OH; but it applies together with the pressure required to provide the density, whichever is the greater. As arithmetic: a K80 sprinkler at 0.35 bar gives 80 x √0.35 = 47.3 l/min; over the maximum 12.0 m² permitted by Table 19 that amounts to only 3.94 mm/min. For the 5.0 mm/min required by Table 3, 60 l/min is needed over 12.0 m², which with the relationship Q = K√P in Clause 14.3 means (60 / 80)² = 0.56 bar. In OH, therefore, the density requirement is what governs in practice.

The other limits specific to OH

• Table 17 (Clause 11.1.3, TS p.70 / EN p.67): in OH, including any LH sprinklers within it, the maximum protected area per control valve set is 12 000 m² (except as permitted in Annex D and Annex F).
• Annex D (TS p.139 / EN p.136): zoning applies only to wet pipe OH installations. On zoned installations, the protected area of a single wet control valve set on any one floor shall not exceed 12 000 m² and the total protected floor area it controls shall not exceed 120 000 m²; the protected floor area per zone shall not be greater than 6 000 m²; zoned installations shall contain no hazard greater than OH3; car parks and goods unloading and storage areas shall be on a separate, unzoned installation.
• Annex E (TS p.142 / EN p.139): high rise sprinkler systems shall comply with the requirements for OH3 protection; special fire engineering solutions are required for high rise systems whose hazard is greater than OH3.
• Clause 9.6.1 c) (TS p.59 / EN p.56): a pressure tank counts as a single water supply only in LH and OH1. Clause 9.5.3 (TS p.57 / EN p.54): the minimum water volume in a pressure tank used as a single supply is 23 m³ for OH1; in a duplicate supply it is 15 m³ for LH and all OH groups.
• Clause 17.1.9 (TS p.116 / EN p.113): copper pipes may be used only on LH, OH1, OH2 and OH3 wet systems, downstream of steel pipe.
• Table 39 (Clause 15.4, TS p.111 / EN p.108): the drain valve and pipe for an OH installation are a minimum of 50 mm.
• Clause 20.1.4 b) (TS p.121 / EN p.118): at least 24 spare sprinklers are kept for OH installations.
• Table 15 (TS p.65 / EN p.62): in OH, HHP and HHS the priming tank is at least 500 litres and the priming pipe at least 50 mm.
• Clause 5.4 (TS p.33 / EN p.30): protection in concealed spaces is designed to OH1 in all cases where the main hazard class is not LH.
• Clause 8.1.1 (TS p.45 / EN p.42): the OH water supply duration is 60 minutes.

Field note: the difference between OH1 and OH3 looks small in a single row, but the design flow rises from 360 l/min to 1 080 l/min and the tank volume from 55 m³ to 135 m³; that is why the classification decision is the most expensive decision in the project.

Frequently Asked Questions

What are the OH1-OH4 design criteria?

Table 3 (Clause 7.1, TS p.39 / EN p.36): the design density for all four groups is 5.0 mm/min; the wet or pre-action area of operation is 72 m² for OH1, 144 m² for OH2, 216 m² for OH3 and 360 m² for OH4. On a dry or alternate system the areas become 90, 180 and 270 m² respectively; a dry or alternate installation is not permitted in OH4, where HHP1 is used instead.

May goods be stored in an OH area?

Clause 6.2.3 (TS p.34 / EN p.31) permits it on three conditions: the protection throughout the room shall be designed to at least OH3; the maximum stacking heights in Table 1 shall not be exceeded; and the maximum storage area of a single block shall not exceed 50 m², with a clearance of at least 2.4 m around the block. Where the area is classified as OH4, or where condition b or c cannot be satisfied, the storage in the area is treated as HHS.

Which OH group covers restaurants, car parks and cinemas?

Table A.2 (TS p.127-128 / EN p.124-125): restaurants are in the OH1 column; car parks in OH2; cinemas and theatres and concert halls in OH4. The NOTE to the table requires paint spraying or similar high fire load areas in an OH1 or OH2 occupancy to be treated as OH3.

What is the minimum sprinkler discharge pressure in OH?

Clause 13.4.4 (TS p.105 / EN p.102): 0.35 bar in OH. However, the same clause states that this value applies together with the pressure required to provide the density in Clause 13.4.1, whichever is the greater. A K80 sprinkler gives 47.3 l/min at 0.35 bar; over the maximum 12.0 m² permitted by Table 19 that flow amounts to only 3.94 mm/min, whereas Table 3 requires 5.0 mm/min. For 5.0 mm/min over 12.0 m², 60 l/min is required, which means 0.56 bar.

What is the maximum area one control valve set may protect in OH?

Table 17 (Clause 11.1.3, TS p.70 / EN p.67): in OH, including any LH sprinklers within it, the maximum protected area per control valve set is 12 000 m², except as permitted in Annex D and Annex F. Annex D requires that on zoned wet OH installations the total protected floor area controlled by a single control valve set not exceed 120 000 m², and the area per zone not exceed 6 000 m².

What is done where different OH groups occur in the same building?

Clause 6.1 (TS p.34 / EN p.31): where areas of different hazard classification are in open communication, the higher design criteria shall be extended into the area of lower classification by at least two rows of sprinklers.

References

• TS EN 12845+A2:2026 Clause 5.4 (TS p.33 / EN p.30), Clause 6.1, Clause 6.2.3 (TS p.34 / EN p.31), Table 1, Clause 6.2.4.2 (TS p.35 / EN p.32)
• TS EN 12845+A2:2026 Clause 7.1, Table 3 (TS p.38-39 / EN p.35-36), Clause 7.3.1, Table 6 (TS p.42 / EN p.39)
• TS EN 12845+A2:2026 Clause 8.1.1 (TS p.45 / EN p.42)
• TS EN 12845+A2:2026 Table 9 (TS p.50 / EN p.47), Table 11 (TS p.52 / EN p.49), Clause 9.5.3 (TS p.57 / EN p.54), Clause 9.6.1 (TS p.59 / EN p.56)
• TS EN 12845+A2:2026 Table 15, Clause 10.7.1, Table 16 (TS p.65 / EN p.62)
• TS EN 12845+A2:2026 Clause 11.1.3, Table 17 (TS p.70 / EN p.67)
• TS EN 12845+A2:2026 Clause 12.1.2, Clause 12.2, Table 19 (TS p.73 / EN p.70), Table 20 (TS p.74 / EN p.71)
• TS EN 12845+A2:2026 Clause 13.3.2.3, Table 26 (TS p.89 / EN p.86), Clause 13.3.4.1, Table 30, Table 31 (TS p.91 / EN p.88), Clauses 13.3.4.2, 13.3.4.4 (TS p.92 / EN p.89)
• TS EN 12845+A2:2026 Clause 13.4.4 (TS p.105 / EN p.102)
• TS EN 12845+A2:2026 Clauses 14.2.2, 14.2.3, Table 37a (TS p.107 / EN p.104), Clause 14.3 (TS p.107-108 / EN p.104-105)
• TS EN 12845+A2:2026 Clause 15.4, Table 39 (TS p.111 / EN p.108)
• TS EN 12845+A2:2026 Clause 17.1.9 (TS p.116 / EN p.113)
• TS EN 12845+A2:2026 Clause 20.1.4 (TS p.121 / EN p.118)
• TS EN 12845+A2:2026 Table A.1 (TS p.127 / EN p.124), Table A.2 (TS p.127-128 / EN p.124-125), Table A.3 (TS p.129 / EN p.126)
• TS EN 12845+A2:2026 Annex D, Clauses D.2, D.3.1 (TS p.139 / EN p.136), Annex E, Clause E.2.1 (TS p.142 / EN p.139)

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