The sprinkler hazard class is the risk band a building is placed in before a sprinkler system is designed, and it fixes how much water the system must deliver over how large an area. Light Hazard, four Ordinary Hazard classes and eight High Hazard classes split between processes and storage. For a warehouse the class follows what is stored, how it is packaged and how high it is stacked, which means it can be wrong for a new tenant while the building itself has not changed.
What a sprinkler hazard class is
It is a classification, not a product. Before anybody selects a sprinkler head, a designer places the occupancy into one of the bands defined by the European sprinkler standard, and that band determines the whole hydraulic design that follows. The standard sets three tiers. Light Hazard covers occupancies with a low combustible load and low combustibility, typically offices, schools and hospitals. Ordinary Hazard covers most commercial and light industrial use and is split into four sub-classes, OH1 to OH4, running from the lightest processes upward. High Hazard is split in two directions: High Hazard Process for manufacturing risks, HHP1 to HHP4, and High Hazard Storage for warehousing, HHS1 to HHS4.
The distinction that matters most on an industrial estate is the one between process and storage. A hall can be a modest process risk and a severe storage risk at the same time, because the two are measured differently. Storage concentrates fuel vertically, and a rack full of goods behaves nothing like the same goods spread across a shop floor. That is why the storage classes exist as their own family rather than as a higher rung on the ordinary ladder.
The sprinkler hazard class is also not the same thing as the sprinkler technology. ESFR heads, in-rack sprinklers and control-mode heads are answers to a design problem. The class is the statement of the problem.
The two numbers the class actually sets
Every class resolves into a design density in millimetres per minute and an area of operation in square metres. The density is how much water must fall on each square metre; the area of operation is the size of the patch the system must be able to supply at that density simultaneously. Multiply them and you have the flow the water supply has to sustain, which is the number that sizes the tank, the pump and the incoming main.
For the lighter classes the published figures are these. Light Hazard is designed at 2,25 mm per minute over 84 square metres. OH1 is 5,0 mm per minute over 72 square metres, OH2 the same density over 144, OH3 over 216 and OH4 over 360. The density stops rising across the ordinary band and the area grows instead, which is worth noticing: moving from OH1 to OH4 multiplies the demanded flow fivefold without changing a single sprinkler head.
The class reaches further down into the installation than most occupiers expect. The standard sets a minimum pressure at the hydraulically most unfavourable sprinkler of 0,70 bar in Light Hazard, 0,35 bar in Ordinary Hazard and 0,50 bar in both High Hazard families except for in-rack sprinklers, which need 1,00 bar for a K 115 head and 2,00 bar for a K 80 head. It sets minimum pipe diameters too: 20 mm in Light Hazard, 20 mm for an ordinary or high hazard horizontal or upright pipe serving one sprinkler with a K-factor of 80 or less, and 25 mm for everything else. A change of class is therefore never a paperwork change. It is a tank, a pump and a pipe network.
Why a storage class follows the goods, not the hall
Three things drive a High Hazard Storage class, and only one of them belongs to the landlord. The first is what the goods are. The standard groups commodities by how they burn, and treats plastics separately according to whether they are foamed or unfoamed and whether the packaging around them is cardboard. Foamed plastic in a cardboard carton is a different fire from the same weight of tinned food. The second is the storage configuration: free-standing stacks, palletised blocks and racking with open channels between the loads all behave differently. The third is the height at which the goods sit.
Only the third is visible in a building specification, and that is the trap. A hall marketed with a stated clear height and a stated sprinkler system does not thereby have a stated hazard class for your goods. The class was fixed for the commodities the designer was told about. A tenant who moves in with a heavier commodity category, or who racks to a greater height than the design assumed, has taken the building outside its own design basis without touching the structure.
The reverse case is just as common and quietly expensive: a hall designed for a severe storage class carries a water supply, a tank and a pump sized for goods the new tenant does not store, and that capital sits in the rent.
Where the class appears in a Slovak project
The European standard is adopted nationally as STN EN 12845+A1 with the classification number 92 0408, published in December 2021 under the title Stabilne hasiace zariadenia, automaticke sprinklerove systemy, navrhovanie, instalovanie a udrzba. It becomes binding on a particular building through the fire safety route rather than by itself: the technical requirements for fire safety in the construction and use of buildings, set out in Decree 94/2004 Z. z. of the Ministry of the Interior, are what put a fire suppression obligation on the project in the first place.
Since 1 April 2025 there is a second part in the Slovak catalogue, STN EN 12845-2 under the same classification number, covering the design and installation of ESFR systems. The split is useful for occupiers to know about, because it separates the two questions cleanly. Part 1 is where the risk is classified. Part 2 is one of the ways of answering it.
In practice the class is fixed at design stage, it is embedded in the fire safety documentation that supports the occupancy permit, and it is not revisited when a tenant changes. The document that records it is usually the fire safety solution in the project documentation, not the lease, which is why the question has to be asked rather than read.
Why the sprinkler hazard class belongs in a site appraisal
Because it decides whether a building can hold your goods, and no other line in the specification answers that question. Clear height tells you how high the racking can be. Floor loading tells you what the slab carries. Pallet positions tell you how many units fit. None of them tells you whether the suppression system was designed for what you intend to put in them, and that is the constraint that binds first.
Four questions settle it, and all four can be asked before heads of terms. What hazard class was the system designed to, in the standard’s own notation? What design density and area of operation follow from it? What storage height and what commodity categories did the designer assume? And is the protection at roof level only, or does it include in-rack sprinklers, which is the answer that most often distinguishes a hall that can take high racking from one that cannot.
The answers also travel straight to the insurer. The party that holds goods in a Slovak warehouse answers for damage to them without any weight-based ceiling, so the fire scenario is priced rather than capped. Where that ceiling stops applying, and why the same pallet is capped on the lorry and uncapped in the shed, is worked through in our post on CMR liability and the warehouse. A building whose hazard class matches the intended commodity is not a compliance detail in that conversation. It is the premium.
Frequently Asked Questions
Is the hazard class the same as the sprinkler type?
No. ESFR is a sprinkler technology, and since 1 April 2025 it has its own part of the Slovak standard, STN EN 12845-2. The hazard class is the risk band assigned under part 1 before any head is selected. A building can be in the same class with two entirely different sprinkler solutions.
Can a tenant move the building into a higher hazard class?
Not by asking. The class is a property of the design, and raising it means raising the water supply behind it: a larger tank, a stronger pump, sometimes new pipework and in-rack sprinklers. It is a capital works decision, so it belongs in the negotiation rather than in the fit-out.
Does higher racking automatically mean a higher class?
Height is one of three inputs, alongside the commodity itself and the storage configuration. Raising the stack of a light commodity may leave the class untouched, while stacking foamed plastics at a modest height can move it several steps. The combination decides, not the height alone.
Who assigns the class?
The fire safety designer, on the basis of the intended use declared for the building, and the result travels with the project documentation into the occupancy permit. The insurer will form its own view and can price against a stricter reading than the minimum the standard allows.
What does the class cost if it is wrong?
Rarely a fine, usually a refusal. The realistic outcomes are an insurer declining the risk or pricing it punitively, a landlord refusing the intended use, or an expensive upgrade before occupation. All three are avoidable by asking the four questions above during the shortlist rather than after signing.
Weighing up a hall against the goods you intend to store in it? Send us the commodity profile: what is stored, how it is packaged, and to what height. We will tell you which hazard class it implies and whether the building in front of you was designed for it.