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; compartment limited in area, with a minimum fire resistance requirement.
- 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, 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. Typical buildings include school and university classrooms, offices without bulk archive storage, and prison cells. An important exception: hospital inpatient wards and hotel bedrooms can qualify as LH in isolation, but because of service areas such as kitchens and laundries the building as a whole is normally designed to Ordinary Hazard.
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 — for example in OH3 the storage height is capped between 1.2 m and 4.0 m depending on the commodity, block area is limited and a minimum aisle width is required. Building types by sector (Annex A):
| Sector | OH group | Building / occupancy types |
|---|---|---|
| Glass and ceramics | OH1 | Cement works |
| Glass and ceramics | OH2 | Glass factories |
| Glass and ceramics | OH3 | Ceramic kiln buildings, blast furnace halls, glass processing workshops |
| Chemicals | OH2 | Photographic laboratories |
| Chemicals | OH3 | Water-based paint works, soap factories, photographic film plants, dyeing operations |
| Engineering | OH1 | Sheet metal product factories |
| Engineering | OH2 | Metalworking shops, electronics and radio factories, appliance factories, vehicle repair workshops |
| Food and beverage | OH1 | Abattoirs, meat processing plants |
| Food and beverage | OH2 | Bakeries, dairies, chocolate and confectionery, breweries, dehydrated food |
| Food and beverage | OH3 | Animal feed, flour mills, biscuit plants, distilleries, sugar factories |
| Service and social | OH1 | Hospitals, hotels, libraries (excluding bulk book stores), restaurants, schools, offices |
| Service and social | OH2 | Physics laboratories, laundries, enclosed car parks, museums, small broadcast studios |
| Service and social | OH3 | Railway stations, plant rooms, agricultural buildings, cinemas and theatres, concert halls, film and TV studios, tobacco factories |
| Paper | OH3 | Bookbinding, carton factories, paper mills (excluding machine halls) |
| Paper | OH4 | Waste paper recycling and processing plants |
| Retail and office | OH2 | Computer rooms (excluding tape stores), standard commercial offices |
| Retail and office | OH3 | Department stores, shopping centres, exhibition halls |
| Textiles and clothing | OH2 | Leather goods and footwear (excluding plastics and rubber), garment sewing, knitwear factories |
| Textiles and clothing | OH3 | Carpets (excluding rubber and foam), cotton spinning, fabric and garment manufacture, flax and hemp, bedding (excluding foam), weaving, woollen mills |
| Timber | OH3 | Wood processing, sawmills, timber furniture (excluding foam), plywood, furniture showrooms, upholstery workshops |
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 is so severe that a deluge system is required instead of wet pipe sprinklers (Annex A):
| Process class | Building / facility types |
|---|---|
| HHP1 — 7.5 mm/min | Floor covering and linoleum, refrigerator manufacture, cable (excluding acrylic), printing works, empty vehicle and wagon stores |
| HHP2 — 10.0 mm/min | Firelighters, tar distillation and oil processing, wood wool, sawmills, particle board, solvent-based paint works, synthetic fibre (excluding acrylic), rope and twine |
| HHP3 — 12.5 mm/min | Cellulose nitrate, fireworks and pyrotechnics, resin and turpentine, rubber tyre manufacture and retreading, foamed plastics and rubber, wax and paraffin, matches, paper machine halls, PP/PE/PS injection moulding, combustible plastic raw material, plastic and rubber footwear |
| HHP4 — Deluge | Extremely rapid flame spread: chemical synthesis reactors, large flammable liquid tank areas, explosive dust and gas process buildings |
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 analysis in Annex B, based on plastics content:
| Material Factor | Definition and examples |
|---|---|
| Material Factor 1 (low plastics) | Unexpanded plastic below about 5% by mass and expanded plastic below about 5% by volume. Examples: boxed metal tools, sacked powdered food, canned goods, leather articles. |
| Material Factor 2 (medium plastics) | Roughly 5–25% plastic by mass. Examples: power tools with plastic housings, synthetic fabrics, timber furniture with plastic seats. |
| Material Factor 3 (high — unexpanded) | Predominantly unexpanded plastic. Examples: plastic handbags, computer casings, unexpanded plastic cups and cutlery. |
| Material Factor 4 (very high — expanded) | More than about 40% expanded foamed 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 and soft drinks | Category I | Glass or metal bottles in cardboard or timber cases |
| Canned food | Category I | Tinned goods in cartons or shrink-wrapped trays |
| Books and printed paper | Category II | Books, archives and paper stored flat |
| Cardboard (flat) | Category II | Palletised flat board, excluding corrugated board |
| Timber furniture | Category II | Standard timber furniture without foam upholstery |
| Electrical domestic appliances | Category III | Appliances with plastics content above roughly 5% by mass |
| Paper rolls (vertical, heavy) | Category III | Heavier grammage rolls (for example newsprint) stored on end |
| Plastic footwear | Category III | Shoes and boots with plastics content above roughly 5% by weight |
| Synthetic fabric | Category III | Rolled synthetic fabric stored flat |
| Foam mattresses and upholstery | Category IV | Expanded plastic mattresses, cushions and foam upholstery |
| Paper rolls (tissue) | Category IV | Light grammage rolls (tissue, toilet paper) stored on end |
| Rubber tyres (horizontal) | Category IV | Car and truck tyres stacked horizontally |
| Empty cartons (assembled) | Category III | Assembled empty heavy board cartons (lighter board is Category II) |
| Paraffin and candles | Category IV | Rapidly melting, high fire load paraffin and wax products |
| Baled wood wool | Category IV | Bales with vertical air paths and very rapid combustion |
6. The New FH Conversion Matrix and ESFR/CMSA Status
With the integration of EN 12845 and EN 12845-2, the traditional classes were mapped onto FH classes in order to prevent hydraulic under-design. 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 | FH 1 | Permitted | Light hazard buildings. Where ESFR/CMSA is used, the design is taken directly to HHS1–HHS3 criteria. |
| OH1 and OH2 | FH 2 | Permitted | Moderate hazards such as glass factories, vehicle workshops and bakeries, again on HHS1–HHS3 hydraulic criteria. |
| OH3 and OH4 | FH 3 | Prohibited | Timber and paper plants, shopping centres, waste paper facilities. ESFR/CMSA ceiling sprinklers are not permitted. |
| HHP1 and HHP2 | FH 4 | Prohibited | Cable factories, printing works, sawmills, solvent-based paint works. |
| HHP3 | FH 5 | Prohibited | Cellulose nitrate, pyrotechnics, rubber tyres, foamed plastics manufacture. |
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. The compartment area and minimum fire resistance are also limited. Dry systems are not permitted in Light Hazard; where one is needed, at least OH1 criteria apply. Typical occupancies are schools, offices, prison cells and hotel bedrooms.
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 is restricted in height, block area and aisle width. If those limits are exceeded, the design must be escalated to High Hazard Storage.
Why does HHP4 require a deluge system rather than wet pipe sprinklers?
HHP4 risks such as chemical synthesis reactors, large flammable liquid tank areas and explosive dust or gas process buildings develop extremely rapid flame spread. Sprinklers opening one at a time cannot keep pace with that growth rate, so an open-nozzle deluge system that discharges over the whole protected area simultaneously is required instead.
How is the HHS commodity category determined?
Annex B of EN 12845 uses a Material Factor analysis from 1 to 4 based on plastics content. MF1 is low plastics such as metal goods and canned food; MF2 is moderate plastics; MF3 is predominantly unexpanded plastic; MF4 is high expanded foam content such as foam mattresses and EPS packaging. The Material Factor is then combined with packaging, pallet type and storage configuration to give the final HHS category.
How do the new FH classes map onto the former EN 12845 classes, and where is ESFR/CMSA prohibited?
LH becomes FH1 and OH1-OH2 become FH2, and in both ESFR and CMSA are permitted but must be designed on HHS1-HHS3 criteria. OH3-OH4 become FH3, HHP1-HHP2 become FH4 and HHP3 becomes FH5, and in all three ESFR and CMSA are prohibited. In those FH3 to FH5 process areas only conventional CMDA density/area design may be used.
Do hospitals and hotels count as Light Hazard?
The bedrooms and wards themselves can meet Light Hazard criteria, but the building is rarely designed that way as a whole. Service areas such as kitchens, laundries, plant rooms and stores fall into Ordinary Hazard, so the practical outcome on most hospital and hotel projects is an Ordinary Hazard design with any genuinely light hazard zones treated as part of the same system.
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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.