In NFPA 13 the first design decision is not the sprinkler type or the pipe size but the hazard classification. Density, area of operation, sprinkler spacing, hose allowance, water supply duration, and therefore the pump and tank size, all flow from that one decision. A space that is classified wrongly is unprotected, however carefully the calculation is done.
This guide covers NFPA 13's non-storage classification end to end: the logic of the five classes, typical examples, the density/area curves, hose allowance and duration, how to handle mixed occupancies and when a space crosses into the storage rules. For a quick first pass you can use the NFPA 13 hazard classification tool, but the final call is always a reasoned engineering judgement.
The logic behind classification: three questions
NFPA 13 classifies a space by looking not at the building itself but at the combustible contents and the activity inside it. Three questions are asked of every area:
- Quantity: How much combustible material is there? Furniture and paper, or production raw material, work in progress and packaging stockpiles?
- Combustibility and heat release: Does the material burn slowly (timber furniture, paper) or quickly with a high heat release (foamed plastics, solvents, hydraulic oil)?
- Stockpile height and liquids: How high is material stacked? Are there flammable or combustible liquids, dust or lint present?
The answers lead to one of five classes: Light Hazard (LH), Ordinary Hazard Group 1 (OH1), Ordinary Hazard Group 2 (OH2), Extra Hazard Group 1 (EH1) and Extra Hazard Group 2 (EH2). These classes apply only to non-storage occupancies; rack and pile storage are handled through a separate commodity classification.
The five classes and typical examples
| Class | Essence of the definition | Typical examples |
|---|---|---|
| LH | Low quantity and combustibility of contents, low expected heat release. | Offices (including data processing), dwellings, hotels and dormitories, classrooms, hospitals and nursing homes, places of worship, museums, libraries other than large stack rooms, restaurant seating areas, theatres and auditoriums excluding stages. |
| OH1 | Low combustibility, moderate quantity; stockpiles not above 2.4 m (8 ft); moderate heat release. | Car parks and showrooms, bakeries, beverage and dairy production, canneries, electronics assembly, glass manufacturing, laundries, restaurant service and kitchen areas. |
| OH2 | Moderate to high quantity and combustibility; stockpiles of moderate heat release up to 3.7 m (12 ft), high heat release up to 2.4 m. | Shops and retail sales floors, machine shops and metalworking, printing and publishing, textile and leather goods manufacture, woodworking and wood product assembly, dry cleaners, cereal and feed mills, repair garages, stages, large library stack areas, exterior loading docks. |
| EH1 | Very high quantity and combustibility; dust, lint or similar material can cause rapid fire spread; little or no flammable or combustible liquid. | Plywood and particleboard manufacture, sawmills, rubber processing and vulcanising, areas using combustible hydraulic fluids, die casting and metal extrusion, upholstering with plastic foams, textile opening and blending. |
| EH2 | Moderate to substantial amounts of flammable or combustible liquids, or extensive shielding of combustibles. | Flammable liquid spraying, dip painting and varnishing, solvent cleaning, plastics manufacture, open oil quenching, asphalt saturating, assembly lines for modular buildings with finished combustible interiors. |
These example lists are not a binding catalogue; the standard itself places them in an explanatory annex. Two facilities with the same name can fall into different classes because of different materials and stockpile heights. Rather than saying "a print works is OH2", ask "does this print works use low flash point inks, and how high are the paper reels stored?" Printing with low flash point inks, for instance, moves up to EH1.
Density/area curves
For hydraulically calculated systems each class has a density/area curve. Any point on the curve may be chosen: a higher density over a smaller area or a lower density over a larger one. The table gives the end points of each curve; intermediate values are read from the curve.
| Class | Start point | End point |
|---|---|---|
| LH | 4.1 mm/min × 139 m² (0.10 gpm/ft² × 1500 ft²) | 2.8 mm/min × 279 m² (0.07 × 3000 ft²) |
| OH1 | 6.1 mm/min × 139 m² (0.15 × 1500) | 4.9 mm/min × 372 m² (0.12 × 4000) |
| OH2 | 8.1 mm/min × 139 m² (0.20 × 1500) | 6.9 mm/min × 372 m² (0.17 × 4000) |
| EH1 | 12.2 mm/min × 232 m² (0.30 × 2500) | 10.2 mm/min × 465 m² (0.25 × 5000) |
| EH2 | 16.3 mm/min × 232 m² (0.40 × 2500) | 14.3 mm/min × 465 m² (0.35 × 5000) |
In practice most projects are calculated at the start of the curve (smallest area), because that point usually gives the lowest total flow. Once area adjustments come into play (below), however, the starting area grows and another point on the curve may become more economical. Whichever point is chosen should be stated plainly in the calculation report.
Adjustments that change the area of operation
- Dry pipe and double-interlock pre-action systems: because water arrives late, the area of operation is increased by 30% without changing the density.
- Sloped ceilings: where the pitch exceeds 2 in 12 (about 16.7%), hot gases travel up the slope and the area is increased by 30%.
- Quick-response (QR) sprinklers: in wet-pipe LH and OH areas with a ceiling below a set height (6.1 m / 20 ft), the area may be reduced by a percentage that depends on ceiling height (up to 40%), subject to a floor on the reduced area.
- High-temperature sprinklers in EH: in extra hazard areas the area may be reduced by up to 25% when high-temperature rated sprinklers are used, but not below a stated minimum area.
- Unsprinklered combustible concealed spaces: where such voids exist, a larger minimum area of operation applies to the space beneath.
When several adjustments apply at once, how they combine must follow the relevant clause of the standard. Confirm these percentages and limits against the current edition of NFPA 13; clause numbers and some thresholds have moved between editions.
Sprinkler spacing depends on the class too
The hazard class sets not only how much water is delivered but how densely sprinklers are placed. Typical limits for standard spray pendent and upright sprinklers:
| Class | Maximum coverage per sprinkler (hydraulically calculated) | Maximum spacing between sprinklers |
|---|---|---|
| LH | 20.9 m² (225 ft²) — may be lower depending on ceiling type | 4.6 m (15 ft) |
| OH1 / OH2 | 12.1 m² (130 ft²) | 4.6 m (15 ft) |
| EH1 / EH2 | 9.3 m² (100 ft²) — 8.4 m² (90 ft²) at lower densities | 3.7 m (12 ft) |
For extended coverage sprinklers these values follow the listing. To check a layout, try the sprinkler spacing and layout calculator.
Hose allowance and water supply duration
Density × area gives only the sprinkler demand. The water supply must also deliver a hose allowance for fire service and internal hose use, for the same duration.
| Class | Inside + outside hose allowance (total) | Water supply duration |
|---|---|---|
| LH | 380 L/min (100 gpm) | 30 min |
| OH1 / OH2 | 950 L/min (250 gpm) | 60–90 min |
| EH1 / EH2 | 1900 L/min (500 gpm) | 90–120 min |
Where a range is given, the lower value is generally accepted when waterflow alarm and supervisory signals are transmitted to a constantly attended location; otherwise the upper value is used. The inside hose allowance forms part of the total, and where hose stations are fed from the sprinkler system the calculation should show where that flow is drawn from. Duration and total flow set the tank volume directly; the fire water tank sizing calculator handles that step.
Quick example (OH2): 8.1 mm/min × 139 m² ≈ 1126 L/min theoretical sprinkler flow. Hydraulic imbalance and minimum pressures typically push the real figure 10–20% higher; take ~1300 L/min. Adding the hose allowance gives 1300 + 950 = 2250 L/min. Over 90 minutes the effective tank volume is ≈ 203 m³. Had the same area been wrongly taken as OH1 (6.1 mm/min, 60 min), the answer would have been about 120 m³ — a tank some 40% too small.
How to classify mixed occupancies
Real buildings are rarely one class. A factory may have an office floor (LH), a production hall (OH2), a spray booth (EH2) and a dispatch area (OH2 or storage) side by side. The approach:
- Classify each area on its own contents. Giving the whole building a single class is either needlessly expensive or unsafe.
- Check for physical separation. Where walls, full-height partitions or beams deep enough to hold heat divide the areas, each zone can be designed to its own criteria.
- Without separation, carry the higher hazard over. In open-plan transitions the higher hazard design criteria are extended a set distance beyond the boundary (of the order of 4.6 m / 15 ft in NFPA 13). Confirm the distance in the current edition.
- Run a separate hydraulic calculation for each class. The water supply is governed by the area that produces the highest total demand (sprinklers plus hose, over the correct duration). That is not always the highest density area: a remote, elevated OH2 zone can be harder to satisfy than a small EH1 zone next to the pump.
- Do not miss small high-hazard pockets. An archive room, kitchen or server room within a large LH office floor is protected to its own class.
When does a space become "storage"?
This is the boundary where classification most often goes wrong. The OH1 and OH2 definitions already include stockpiles up to a certain height, so not every stack counts as storage. The road map, broadly:
- Where stockpiles exceed 3.7 m (12 ft) the area is designed to the storage chapters: commodity class (Class I–IV, Group A/B/C plastics), storage arrangement (solid pile, palletised, shelf, rack) and ceiling height are assessed together.
- Storage below 3.7 m is either "miscellaneous storage" or "low-piled storage". Miscellaneous storage is incidental to another occupancy, limited to a small share of the building area and capped in area per pile; where those conditions are met it can be protected to the surrounding class (usually OH2).
- Some commodities trigger stricter rules at lower heights. Group A plastics, tyres, roll paper and rack storage have lower thresholds. Where such materials are present, calling the area OH2 because "we are under 12 ft" is a mistake.
For the storage side see NFPA 13 commodity classification and the warehouse fire protection guide. For high-bay warehouses the ESFR sprinkler design guide offers a separate decision path.
Rough correspondence with EN 12845 and FM Global
Projects often move between standards. The classes do not map one to one; the table is a rough orientation only:
| NFPA 13 | EN 12845 (approx.) | FM Global DS 3-26 (approx.) |
|---|---|---|
| LH | LH (compartment size limited) or OH1 | HC-1 |
| OH1 | OH1–OH2 | HC-2 |
| OH2 | OH2–OH3 (some processes OH4) | HC-2 |
| EH1 / EH2 | OH4 or HHP1–HHP4 | HC-3 |
Because EN 12845 ties LH to compartment size and fire resistance, many offices that are LH under NFPA become OH1 under EN. Do not "translate" classes between standards; reclassify using the chosen standard's own table. See the NFPA 13 vs EN 12845 vs FM Global comparison and, for the EN side, the EN 12845 hazard class finder.
Checklist for the classification report
- Has the activity and material content of every area been defined in writing?
- Are stockpile heights based on the real and planned maximum on site?
- Are areas with flammable or combustible liquids, dust or lint marked separately?
- Where there is no separation, have the higher hazard criteria been carried beyond the boundary?
- Have the dry system, sloped ceiling, QR and concealed space adjustments been applied?
- Has a separate hydraulic calculation been run for each class and the governing demand identified?
- Are hose allowance and duration correct for the class?
- Has the owner been told in writing that future changes of use may alter the class?
The last point looks minor but is the source of the costliest problems: when plastic packaging is stacked 5 m high years later in a production hall designed as OH2, the system is compliant on paper and inadequate in reality. Reassessment after a change of use in an existing building should be part of the general workflow in the fire sprinkler system design guide.
Frequently Asked Questions
What NFPA 13 hazard class is an office building?
Usually Light Hazard (LH): 4.1 mm/min density over 139 m², a 380 L/min hose allowance and 30 minutes. Archive rooms, kitchens or stores within it are classified separately on their own contents.
Which point on the density/area curve should I choose?
Any point on the curve is valid. On most projects the start point (smallest area) gives the lowest flow, but once dry-system or sloped-ceiling adjustments enlarge the area, compare other points and record the choice in the report.
Does the density increase on a dry pipe system?
No. Under NFPA 13, dry pipe and double-interlock pre-action systems keep the same density and increase the area of operation by 30%.
Can any stockpile below 3.7 m be protected as OH2?
No. It must meet the miscellaneous storage conditions (incidental use, share of building area and per-pile area limits). Group A plastics, tyres, roll paper and rack storage have lower thresholds.
Can I convert an NFPA 13 class directly to EN 12845?
No. The classes correspond only roughly. Reassess the space with the chosen standard's own classification table; offices that are LH under NFPA, for example, are often OH1 under EN 12845.
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 StoreNFPA 13, Standard for the Installation of Sprinkler Systems (2022/2025) · BS EN 12845:2015+A1:2019 · FM Global DS 3-26. This guide is a general road map; percentages, thresholds and clause numbers vary between editions, and the binding text is the current standard and the authority having jurisdiction.