The first and most critical decision on a sprinkler project: which hazard class does the building fall into?
On a shopping centre project, the architectural team had started the retail floors as OH-1; when the hydraulic calculation arrived it was clear that the supermarket, the perfumery and an electronics store had all been lumped into one class. The density does not come out right, the pump falls short, and you end up adding deluge afterwards. In EN 12845 the hazard class decision is the multiplier that governs everything else; start it wrong and correcting the hydraulics means enlarging every distribution main.
EN 12845 divides occupancies into three main classes: Light Hazard (LH), Ordinary Hazard (OH) and High Hazard (HH). OH splits into four sub-groups (OH1–OH4), and HH into two branches — HHP for process and HHS for storage — each with four sub-categories of its own. Thirteen combinations in total, and picking the right one is the decision on the first line of the calculation.
LH — Light Hazard
LH applies to areas of low fire load and low combustibility, in compartments with at least 30 minutes of fire resistance, where no compartment exceeds 126 m². The standard is firm: exceed the 126 m² threshold or lack the fire-resisting compartment, and you must move up to OH1 automatically. The annex lists only three occupancy types:
- Certain schools and educational establishments
- Certain offices — not vast open-plan floors
- Prisons
Design parameters: 2.25 mm/min over an 84 m² area of operation, K57 sprinklers, wet or pre-action systems only — no dry or alternate. In practice LH is an exception in a European standard; most office projects fall to OH1 once the fire load is calculated.
OH — Ordinary Hazard
OH covers areas where combustible material of medium fire load and medium combustibility is processed or manufactured. All four sub-groups share the same 5.0 mm/min density, but their areas of operation — and therefore total flows — differ:
| Class | Density (mm/min) | Area — wet (m²) | Area — dry (m²) |
|---|---|---|---|
| LH | 2.25 | 84 | not permitted |
| OH1 | 5.0 | 72 | 90 |
| OH2 | 5.0 | 144 | 180 |
| OH3 | 5.0 | 216 | 270 |
| OH4 | 5.0 | 360 | not permitted → use HHP1 |
Note: there is no place for OH4 on a dry or alternate system. If a dry system is required, you must move up to HHP1. And even on a wet system, OH4's 360 m² area of operation brings 1800 L/min of pre-calculated flow — at which point most projects go either to full hydraulic calculation or to smaller OH3 zones.
Can storage sit within an OH area? Yes, with conditions: protection at least OH3, height within the tabulated limits (4.0 m free-standing stack and 3.5 m on racking for category I), each block under 50 m², and a 2.4 m clear aisle around the block. Fail any one of those three and you move automatically to high hazard storage.
HHP — High Hazard Process
HHP covers manufacturing processes of high fire load and high combustibility, capable of rapid spread or intense fire development. Across the four sub-groups the density rises while the area stays constant:
- HHP1: 7.5 mm/min over 260 m²
- HHP2: 10.0 mm/min over 260 m²
- HHP3: 12.5 mm/min over 260 m²
- HHP4: normally protected by deluge, and outside the scope of EN 12845 — fireworks and nitrocellulose manufacture, for example.
On a dry system the area rises to 325 m². That HHP4 sits outside the scope is an important trap: when a fireworks, fuse or cellulose nitrate plant comes along, you have to say EN 12845 alone is not sufficient and bring in a deluge or foam designer.
HHS — High Hazard Storage
HHS covers all storage exceeding the height limits OH3 permits, or classified as OH4. The four categories are set by the combination of material factor (M1–M4) and storage configuration (ST1–ST6). The design values are not in one table; density is read for the height, configuration and category combination — typically between 7.5 and 30 mm/min, calculated over 260 m².
Field examples: which occupancy falls where
A summary of the most frequently encountered occupancy types from the classification annex:
| Occupancy | Class |
|---|---|
| Offices (qualifying areas) | LH |
| Hospitals, hotels, restaurants, libraries (excluding book stores) | OH1 |
| Car parks, museums, laboratories, laundries | OH2 |
| Shopping centres, shops, cinemas, theatres, concert halls | OH3 |
| Furniture manufacture (excluding foam plastics) | OH3 |
| Cotton weaving, wicker preparation, match manufacture | OH4 |
| Saw mills | OH4 |
| Printing works | HHP1 |
| Solvent-based paint spraying shops | HHP1 |
| Tyre manufacture, bus garages | HHP2 |
| Paint and varnish manufacture, paper machine halls | HHP2 |
| Cellulose nitrate production, foam plastics manufacture | HHP3 |
| Fireworks and fuse manufacture | HHP4 (outside scope) |
| Plastic pallet and foam plastic rack storage | HHS3–HHS4 |
Two lessons from the table. First, "office" alone is not LH — the standard says certain areas, and an office on a data centre floor is OH1. Second, there are seven steps between OH1 and HHP3; a classification error changes not only the flow but the system type, the K-factor, the pipe sizes and the pump selection together.
Where the annex leaves room
The classification annex is normative but gives minimum classes: if your occupancy is not listed, or is used more severely than the listed case, you have the authority to move up. The standard is explicit that the lists are a minimum classification. So a restaurant is normally OH1, but if you are designing sprinklers around 50 kg LPG cylinders in an open kitchen, drawing that kitchen as OH3 is the correct decision. A note under the tables also matters: where an OH1 or OH2 occupancy contains a paint shop or similar high fire load area, that area is treated as OH3.
Comparison with NFPA hazard classes
Concepts map roughly between NFPA 13 and EN 12845, but the numbers differ:
| EN 12845 | NFPA 13 counterpart | Typical occupancy |
|---|---|---|
| LH (2.25 mm/min × 84 m²) | Light Hazard (4.1 mm/min × 139 m²) | Offices, prisons |
| OH1 (5.0 × 72) | Ordinary Hazard Group 1 (6.1 × 139) | Hotels, hospitals |
| OH2–OH3 (5.0 × 144–216) | Ordinary Hazard Group 2 (8.2 × 139) | Shopping centres, car parks |
| OH4 (5.0 × 360) | At the Extra Hazard 1 boundary | Cotton weaving, timber |
| HHP1–HHP3 (7.5–12.5 × 260) | Extra Hazard Groups 1–2 (12.2–16.3 × 232) | Printing works, tyre plants |
| HHS1–HHS4 | NFPA 13 storage chapters | High-bay racking, plastics, rubber |
The philosophies differ: NFPA prefers a higher density over a smaller area, EN 12845 a lower density over a larger area. The total flows for EN OH3 and NFPA OH-2 end up close (about 1100 versus 1140 L/min), but the hydraulic balance is different — less water per point, over a wider assumed spread.
Common errors
- Assuming a data centre is OH1: a computer room excluding tape storage is OH1; with tape or card storage it rises to OH2, and the additional data centre provisions come into play.
- Treating a car park as LH: car parks are listed as OH2 and, because of plastic bumpers and fuel load, are never LH.
- Assuming HHP4 is in scope: the standard says plainly that HHP4 is protected by deluge and lies outside EN 12845. Writing "HHP4 to EN 12845" in a specification is technically inconsistent.
- Drawing a dry OH4 system: the tables direct you to HHP1 instead. Miss the automatic upgrade and continue with K115 pipework, and the hydraulics fail.
- One class for a mixed occupancy: in a production building the office, workshop and storage areas each fall into a different class. Where areas are open to each other, the higher class must extend two rows of sprinklers into the lower.
Turkish context
BYKHY references either EN 12845 or NFPA 13 directly for sprinkler design. In practice most office, retail and hotel projects in Turkey run on EN 12845, while logistics warehouses and large production plants drift towards FM Global or NFPA 13, because that is what the insurer asks for. The hazard class decision is not the engineer's alone; the insurer, the authority and usually the client's risk manager need to sit at the table. Before the classification starts, the answer to "who is signing off against which standard" should be settled.
Frequently asked questions
What is the practical difference between OH3 and OH4?
The design density is 5.0 mm/min for both; the area of operation is 216 m² for OH3 and 360 m² for OH4. A wet system is mandatory for OH4 — dry and alternate are not permitted, and HHP1 must be used instead.
Is a hospital LH or OH1?
OH1. LH applies only to certain schools, certain offices and prisons; where occupants are bed-bound and evacuation times are long, OH1 is the accepted minimum.
Which class does a plastic pallet factory fall into?
Plastics manufacturing is listed as HHP2. The area where the plastic is stored can come out as HHS3 or HHS4 through the material factor — production and storage are separate classes.
Can I store goods within an OH classification?
Yes, subject to conditions: protection at least OH3, storage height within the tabulated limits, each block under 50 m², and 2.4 m clear around the block. Otherwise you move to high hazard storage.
Are NFPA Ordinary Hazard and EN 12845 OH the same?
They correspond roughly but not exactly. NFPA splits into OH-1 and OH-2; EN 12845 into four groups. The density-area combinations differ too, so read both tables before comparing.
Frequently Asked Questions
What separates OH3 from OH4?
Both use 5.0 mm/min, but the area of operation is 216 m² for OH3 and 360 m² for OH4. A dry OH4 system is not permitted; HHP1 must be used instead.
Is an office always Light Hazard?
No. LH requires 30 minute fire resisting compartments no larger than 126 m². Most offices fall to OH1 once the fire load is assessed.
Can storage sit inside an OH area?
Yes, at OH3 or above, within the tabulated height limits, with blocks under 50 m² and 2.4 m clear around each block.
Is HHP4 covered by EN 12845?
No. HHP4 occupancies such as fireworks manufacture are protected by deluge systems and fall outside the scope.
SprinkCalc — Fire Sprinkler Design Across Three Standards
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MEP Calc — 86+ Engineering Calculators
MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
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.