The single most critical first step in sprinkler design is placing the protected space in the correct hazard class, because that one decision drives everything else — design density, operating area, water supply duration and sprinkler type. This guide tabulates the four main hazard groups of TS EN 12845 and EN 12845-2 (LH, OH, HHP, HHS), the building types that fall into each, and the new FH conversion matrix, sector by sector.
1. The Four Main Hazard Groups
- Light Hazard (LH): low fire load and combustibility; no single compartment larger than 126 m², with a fire resistance of at least 30 minutes (clause 6.2.2).
- Ordinary Hazard (OH1–OH4): moderate fire load; commercial, industrial and service occupancies.
- High Hazard Process (HHP1–HHP4): high fire load with rapid spread; manufacturing and process areas rather than storage.
- High Hazard Storage (HHS1–HHS4): storage exceeding the Ordinary Hazard limits (clause 6.2.3, Table 1), with significant vertical fire spread risk.
2. Light Hazard (LH) and Building Types
In LH the design density is 2.25 mm/min over an operating area of 84 m², with a maximum coverage of 21 m² per sprinkler. Dry systems are not permitted in LH — where a dry system is required, at least OH1 criteria apply. Table A.1 lists only three types: certain areas of schools and other educational establishments, certain areas of offices (both subject to clause 6.2.2) and prisons. Note: hospitals and hotels are OH1 in Table A.2; Annex A contains no separate LH exception for patient rooms or hotel bedrooms.
3. Ordinary Hazard (OH1–OH4) and Building Types
Across OH1–OH4 the density is fixed at 5.0 mm/min; what changes is the assumed area of operation, which ranges from roughly 72 m² to 360 m² depending on the group and whether the system is wet or dry. Storage within an OH area is tightly limited — under clause 6.2.3 the protection must be at least OH3, the storage height is capped between 1.2 m and 4.0 m depending on the category (Table 1), a single block may not exceed 50 m² and needs a clearance of at least 2.4 m around it; storage in an OH4 area is treated as HHS. Building types by sector (Annex A, Table A.2; the table gives the minimum class):
| Sector | OH group | Building / occupancy types |
|---|---|---|
| Glass and ceramics | OH3 | Glass factories |
| Chemicals | OH1 | Cement works |
| Chemicals | OH2 | Photographic film factories |
| Chemicals | OH3 | Dyers works, soap factories, photographic laboratories, paint application shops using water-based paint |
| Engineering | OH1 | Sheet metal product factories |
| Engineering | OH2 | Metal working |
| Engineering | OH3 | Electronics factories, radio equipment factories, washing machine factories, car workshops |
| Food and beverage | OH2 | Abattoirs and meat factories, bakeries, biscuit factories, breweries, chocolate and confectionery factories, dairies |
| Food and beverage | OH3 | Animal fodder factories, corn mills, dehydrated vegetable and soup factories, sugar factories |
| Food and beverage | OH4 | Alcohol distilleries |
| Service and social | OH1 | Hospitals, hotels, libraries (excluding book stores), restaurants, schools and offices (see 6.2.2) |
| Service and social | OH2 | Physical laboratories, laundries, car parks, museums |
| Service and social | OH3 | Small broadcasting studios, railway stations, plant (technical) rooms, farm buildings |
| Service and social | OH4 | Cinemas and theatres, concert halls, tobacco factories, film and TV production studios |
| Paper | OH3 | Book binding, cardboard and paper factories (paper machine halls are HHP2) |
| Paper | OH4 | Waste paper processing |
| Retail and office | OH1 | Data processing / computer rooms (excluding tape storage), offices (see 6.2.2) |
| Retail and office | OH3 | Department stores, shopping centres |
| Retail and office | OH4 | Exhibition halls (excessive clearance to be taken into account) |
| Textiles and clothing | OH2 | Leather goods factories |
| Textiles and clothing | OH3 | Carpets (excluding rubber and foam plastics), cloth and clothing, fibre board, footwear (excluding plastics and rubber), knitting, linen, mattresses (excluding foam plastics), sewing, weaving, woollen and worsted mills |
| Textiles and clothing | OH4 | Cotton mills, flax and hemp preparation plants |
| Timber | OH3 | Woodworking, furniture (without foam plastics), furniture showrooms, upholstery (without foam plastics) factories |
| Timber | OH4 | Saw mills (also listed as HHP2 in Table A.3), plywood factories |
4. High Hazard Process (HHP1–HHP4)
HHP covers high fire load manufacturing processes rather than storage. Design densities run from 7.5 to 12.5 mm/min over areas of roughly 260–325 m². HHP4 hazards are usually protected by deluge systems, which are outside the scope of the standard (clause 6.2.4.1, NOTE). Classes per Annex A, Table A.3:
| Process class | Building / facility types |
|---|---|
| HHP1 — 7.5 mm/min | Floor cloth and linoleum; resin, lamp black and turpentine; rubber substitute; wood wool; match manufacture; paint application shops using solvent; refrigerator factories; printing works; cable, injection moulding and plastic goods (non-foam) factories for PP/PE/PS or similar other than OH3; rubber goods and synthetic fibre (excluding acrylic) factories; rope factories; carpets with unexpanded plastics; footwear including plastics and rubber |
| HHP2 — 10.0 mm/min | Fire lighter manufacture; tar distilling; depots for buses, unladen lorries and railway carriages; candle wax and paraffin manufacture; paper machine halls; carpets including rubber and foam plastics; saw mills; chipboard manufacture (additional object protection may be needed); paint, colour and varnish manufacture |
| HHP3 — 12.5 mm/min | Cellulose nitrate manufacture; car and lorry tyres; manufacture of material factor m3 foam plastics, foam rubber and foam rubber goods (excluding m4) |
| HHP4 — deluge | Firework manufacture. EN 12845 gives no density/area point for this class; such hazards are normally protected by deluge systems, which are outside the scope of the standard. |
5. High Hazard Storage (HHS1–HHS4) and Commodity Categories
In HHS the hazard is set by the commodity category (I–IV), which represents combustibility including packaging and pallet, together with the storage configuration (ST1–ST6). The category is derived from the material factor (M) in Annex B, based on plastics content (Figure B.1), then adjusted for the storage configuration in Table B.1; where the product also appears in Table C.1, the higher of the two categories applies:
| Material Factor | Definition and examples |
|---|---|
| Material Factor 1 (low plastics) | Unexpanded plastic below 5% by mass (including the pallet) and expanded plastic below 5% by volume. Examples: boxed metal tools, sacked powdered food, canned goods, leather articles. |
| Material Factor 2 (medium plastics) | More plastic than MF1; the limits are read from the diagram in Figure B.1. Examples: electrical equipment with plastic parts, synthetic fabrics, wood or metal furniture with plastic seats, cables on reels. |
| Material Factor 3 (high — unexpanded) | Predominantly unexpanded plastic. Examples: plastic brief cases, personal computers, car batteries without electrolyte, unexpanded plastic cups and cutlery. |
| Material Factor 4 (very high — expanded) | More than 40% expanded plastic by volume. Examples: foam mattresses, EPS packaging, foam upholstery. |
Final HHS categories for commonly stored goods (Annex C):
| Commodity | HHS category | Technical condition |
|---|---|---|
| Metal products | Category I | Metal parts and tools, unpackaged or in timber/cardboard packaging |
| Beer | Category I | Containers in wooden crates are Category II |
| Canned food | Category I | Tinned goods in cardboard boxes and trays |
| Books and paper sheets | Category II | Books; paper sheets stored horizontally |
| Cardboard (flat) | Category II | All types of cardboard stored flat (corrugated rolls stored vertically are Category IV) |
| Timber furniture | Category II | Standard timber furniture without foam upholstery |
| Electrical domestic appliances | Category III | Predominantly metal appliances with no more than 5% plastic by mass are Category I; all others Category III |
| Paper rolls (≥ 5 kg/100 m²) | Category III | For example newsprint, rolls stored vertically (Category II when stored horizontally) |
| Plastic footwear | Category III | Shoes with more than 5% plastic by mass |
| Synthetic fabric | Category III | Rolled synthetic fabric stored flat |
| Foam mattresses | Category IV | Mattresses with expanded plastic (other mattresses are Category II) |
| Paper rolls (< 5 kg/100 m²) | Category IV | For example tissue paper, rolls stored vertically (Category III when stored horizontally) |
| Rubber tyres (horizontal) | Category IV | Tyres stored horizontally; covered in Annex P |
| Empty cartons (assembled) | Category III | Assembled empty heavy board cartons (lighter board is Category II) |
| Paraffin wax | Category IV | Wax (paraffin); finished candles are Category III |
| Wood wool | Category IV | Baled wood wool |
6. The New FH Conversion Matrix and ESFR/CMSA Status
The FH classes are set out in Annex A of EN 12845-2:2024 and are taken from the draft prEN 12845-1; EN 12845:2015+A2:2026 does not change the LH/OH/HHP/HHS classes. The conversion from the former classes is given in Table A.1. Under clause 6.2, in non-storage occupancies ESFR and CMSA are permitted only in FH1 and FH2 (designed on an HHS1–HHS3 basis) and are prohibited in FH3 to FH5:
| Former class | New FH | ESFR / CMSA | Criterion |
|---|---|---|---|
| LH and OH1 | FH 1 | Permitted | Certain areas of schools and offices, prisons (LH); hospitals, hotels, restaurants, libraries (OH1). Where ESFR/CMSA is used, the design follows HHS1–HHS3 criteria. |
| OH2 and OH3 | FH 2 | Permitted | Car parks, laundries, bakeries, abattoirs (OH2); woodworking, paper factories, shopping centres (OH3). Again on HHS1–HHS3 criteria. |
| OH4 and HHP1 | FH 3 | Prohibited | Cinemas and theatres, exhibition halls, waste paper processing (OH4); cable factories, printing works, paint application shops with solvent (HHP1). |
| HHP2 | FH 4 | Prohibited | Paper machine halls, saw mills, paint and varnish manufacture, bus depots. |
| HHP3 | FH 5 | Prohibited | Cellulose nitrate, car and lorry tyres, foamed plastics manufacture. |
HHP4 (firework manufacture) does not appear in Table A.1.
Frequently Asked Questions
What are the hazard classes in EN 12845?
There are four main groups: Light Hazard (LH), Ordinary Hazard (OH1-OH4), High Hazard Process (HHP1-HHP4) and High Hazard Storage (HHS1-HHS4). The class is determined by fire load, the type, quantity and geometry of combustible material, and the expected rate of fire spread. It directly dictates design density, area of operation, water supply duration and sprinkler type.
What are the design criteria for Light Hazard?
Light Hazard uses a design density of 2.25 mm/min over an operating area of 84 square metres, with a maximum coverage of 21 square metres per sprinkler. No single compartment may exceed 126 square metres, and it must have a fire resistance of at least 30 minutes. Dry systems are not permitted in Light Hazard; where one is needed, at least OH1 criteria apply. Table A.1 lists certain areas of schools and offices (see 6.2.2) and prisons. Hospitals and hotels are OH1.
What is the difference between the OH groups, and how much storage is allowed?
Across OH1 to OH4 the density stays fixed at 5.0 mm/min; what changes is the assumed area of operation, which grows from roughly 72 to 360 square metres depending on group and on whether the system is wet or dry. Storage inside an OH area needs protection of at least OH3 and is restricted in height (1.2 to 4.0 m by category), single block area (50 square metres) and clearance around the block (2.4 m). If those limits are exceeded, the storage must be treated as High Hazard Storage.
Why is HHP4 protected by deluge rather than wet pipe sprinklers?
The only occupancy in the HHP4 column of Table A.3 is firework manufacture. The note to clause 6.2.4.1 states that HHP4 hazards are usually protected by deluge systems, which are outside the scope of the standard, so EN 12845 gives no density/area design point for HHP4. Sprinklers opening one at a time may not keep pace with a very fast fire; a deluge system discharges through all open nozzles at once.
How is the HHS commodity category determined?
Annex B of EN 12845 determines a material factor from 1 to 4 based on the plastics content of the goods including packaging and pallet (Figure B.1). MF1 is low plastics such as metal goods and canned food; MF2 is moderate plastics; MF3 is predominantly unexpanded plastic; MF4 is more than 40% expanded plastic by volume, such as foam mattresses and EPS packaging. The material factor is then adjusted for the storage configuration in Table B.1, and where the product also appears in Table C.1 the higher category applies, giving the final HHS category.
How do the new FH classes map onto the former EN 12845 classes, and where is ESFR/CMSA prohibited?
Under EN 12845-2 Table A.1, LH and OH1 become FH1 and OH2 and OH3 become FH2; in both, ESFR and CMSA are permitted but must be designed on HHS1-HHS3 criteria. OH4 and HHP1 become FH3, HHP2 becomes FH4 and HHP3 becomes FH5; in all three, ESFR and CMSA are prohibited in non-storage occupancies. In those FH3 to FH5 process areas conventional CMDA density/area design is used.
Do hospitals and hotels count as Light Hazard?
No. EN 12845 Table A.2 lists hospitals and hotels as OH1, and Annex A contains no separate Light Hazard exception for wards or bedrooms. Service areas such as laundries (OH2) and plant rooms (OH3) fall into higher Ordinary Hazard groups, so hospital and hotel projects are designed as Ordinary Hazard.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App Store
MEP Calc — 110+ Engineering Calculators
MEP Calc bundles 110+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
Download MEP Calc on the App StoreTS EN 12845:2015+A2:2026, Fixed Firefighting Systems — Automatic Sprinkler Systems — Design, Installation and Maintenance (Annexes A, B and C). EN 12845-2:2024, Design and Installation of ESFR and CMSA Sprinklers. Values given here are for information; the full text of the current standard governs any actual design.