6.1 Light Hazard & Ordinary Hazard Groups 1 & 2
Key Takeaways
- NFPA 13 (2022) 4.3.1.1–4.3.1.2 occupancy classes exist only for sprinkler design, installation, and water supply — they are not IBC or NFPA 101 occupancy groups.
- Light Hazard (4.3.2 / 3.3.141.3) is low quantity and/or combustibility with low heat-release fires; Annex A.4.3.2 examples include offices, churches, educational, hospitals, museums, restaurant seating, and residential rooms.
- A restaurant is two classes: seating is Light Hazard; restaurant service areas (kitchens, dish rooms, serving lines) are Ordinary Hazard Group 1.
- OH1 stockpiles of low-combustibility contents stop at 8 ft (2.4 m); OH2 allows moderate-heat-release piles to 12 ft (3.7 m) and high-heat-release piles to 8 ft (2.4 m).
- The 2022 annex moved automobile parking garages (and, per NFSA, showrooms) from the old OH1 crib list to Ordinary Hazard Group 2 — do not protect a 2022 garage as OH1.
Why hazard class is a layout decision
This chapter is independent OpenExamPrep study material that helps learners study NICET Water-Based Systems Layout Level I Task 1.5.1 (System Layout, 53–63 percent of the exam). It is not an NFPA handbook and does not claim NICET, NFPA, or authority having jurisdiction (AHJ) approval.
A Level I layout technician who mis-classifies a room does not merely pick the wrong density on someone else's calculation sheet. Hazard class drives sprinkler spacing tables, design density, hose-stream allowance, water-supply duration, and the Section 4.4 riser-area limits — 52,000 ft² (4,830 m²) for light and ordinary hazard, 40,000 ft² (3,720 m²) for extra hazard and high-piled storage. Task 1.5.1 expects you to read the occupancy, walk the contents, and put the right class on each area of the drawing before pipe is sized.
NFPA 13 (2022) 4.3.1.1 states that occupancy classifications in this standard relate to sprinkler design, installation, and water supply requirements only. 4.3.1.2 adds that they are not a general classification of occupancy hazards. An International Building Code (IBC) Group B office, a Group E school, and a Group I-2 hospital can all be Light Hazard for sprinklers. They can also contain Ordinary Hazard kitchens, laundries, or shops. Do not copy the architect's occupancy-group letter onto the sprinkler legend.
Every occupancy definition in 3.3.141 is written as occupancies or portions of other occupancies. That phrase is the mixed-use rule. Classify by area contents, not by the name on the building permit.
Light Hazard (4.3.2)
Section 4.3.2 protects spaces with low quantity and combustibility of contents. Definition 3.3.141.3 adds that fires with relatively low rates of heat release are expected.
Annex A.4.3.2, extracted in NFPA 750 as [13:A.4.3.2], lists uses and conditions similar to: animal shelters; churches; clubs; eaves and overhangs of combustible construction with no combustibles beneath; educational; hospitals, including animal hospitals and veterinary facilities; institutional; kennels; libraries, except large stack rooms; museums; nursing or convalescent homes; offices, including data processing; residential; restaurant seating areas; theaters and auditoriums, excluding stages and prosceniums; and unused attics. The same annex note says typical library bookshelves about 8 ft (2.4 m) high, books stored vertically on end, with aisles wider than 30 in. (750 mm) in the extract, can still be Light Hazard, while stack areas akin to shelf or record storage should be treated as ordinary hazard.
Those lists are representative, not exhaustive. If a "museum gallery" is actually a scenery shop with solvent-soaked flats, you are no longer on the Light Hazard example. If an "office" is a plotter room stacked with foam-board crates, you classify the contents you see, not the door sign.
Worked case: the restaurant is two classifications
A 4,800 ft² restaurant named Harbor Grill is not automatically Light Hazard because "restaurant" sounds like assembly. Annex A.4.3.2 lists restaurant seating areas as Light Hazard. Ordinary Hazard Group 1 annex examples include restaurant service areas. The dining room with tables and chairs is Light Hazard. The kitchen, dish room, and service line are OH1. Draw a boundary at the contents change — typically the wall or pass-through that separates seating from cooking and warewashing — and label each zone. If the kitchen opens fully into the dining room with no separation, flag the shared volume as the more demanding contents and ask the designer of record before you pipe both rooms as Light Hazard.
Residential rooms under NFPA 13 (hotels, apartments, dormitories) sit on the Light Hazard annex list. That is an NFPA 13 occupancy-hazard call. It is not permission to use NFPA 13D or NFPA 13R. Those residential installation standards have their own occupancy limits, treated in Section 6.4.
Ordinary Hazard Group 1 (4.3.3.1)
OH1 (3.3.141.4 / 4.3.3.1) is moderate quantity and low combustibility, with stockpiles of low-combustibility contents not exceeding 8 ft (2.4 m), and moderate heat-release fires expected.
Opened annex extracts (2019 A.4.3.3 numbering, still the working OH1 example set after the 2022 parking move) list uses similar to: bakeries; beverage manufacturing; canneries; dairy products manufacturing and processing; electronic plants; glass and glass products manufacturing; laundries; restaurant service areas; porte cocheres; and mechanical rooms.
Do not treat "bakery" as a building name. The retail cafe seating can be Light Hazard; the oven room and flour storage are OH1 if they match the definition. A mechanical room on an otherwise Light Hazard office floor is an OH1 island — pipe it as OH1 even if the rest of the floor is Light Hazard.
Stockpile height is a layout trip. Palletized copy paper stacked 9 ft in an "office" is no longer inside the OH1 8 ft stockpile limit for that contents story, and it may have left occupancy-hazard design for the storage rules in 4.3.1. Flag it. Do not hide 9 ft of combustibles behind an office label.
Ordinary Hazard Group 2 (4.3.3.2)
OH2 (3.3.141.5 / 4.3.3.2) is moderate to high quantity and combustibility. Stockpiles with moderate heat-release rates do not exceed 12 ft (3.7 m); stockpiles with high heat-release rates do not exceed 8 ft (2.4 m).
The 2022 annex change that Level I candidates miss: the National Fire Sprinkler Association and 2022-edition consulting notes record that automobile parking and showrooms, listed for decades under OH1, were reclassified to Ordinary Hazard Group 2. NFPA 750's extract of the NFPA 13 OH2 example list now includes automobile parking garages as item (2), along with agricultural facilities, barns and stables, cereal mills, ordinary chemical plants, confectionery products, distilleries, dry cleaners, and exterior loading docks (with a note that docks handling flammable liquids or used for storage should be protected as if the materials were stored in that configuration). Continuing OH2 annex items from the opened 2019 A.4.3.4 roster include repair garages, stages, textile manufacturing, wood machining, and wood product assembly.
A trainee who still quotes a pre-2022 crib card that says "parking garage = OH1" will undersize density and spacing on a 2022 job. Put OH2 on the garage, and keep the office floors above it Light Hazard if their contents match 4.3.2.
Mixed occupancies: classify each area
Walk this hospital floor: patient rooms and corridors (Light Hazard annex: hospitals), a dietary kitchen (OH1 restaurant service), a laundry (OH1 annex), and an enclosed loading dock (OH2 annex). Four labels, one building. If the laundry is not separated and its lint-laden air shares the patient-wing volume, flag the most demanding contents for that undivided space rather than averaging the classes.
Trainee checklist before you pick a head-spacing table:
- Ignore the building name. Read what burns in this room.
- Match the 4.3 body test (quantity, combustibility, heat release, stockpile height).
- Use Annex A.4.3 examples only as similar-use guidance.
- Split mixed occupancies at contents boundaries. If they share one open volume, flag the most demanding contents.
- If piles, racks, or plastics exceed miscellaneous/low-piled limits, leave this occupancy-hazard chapter and open Section 6.2.
| Class | 2022 body test (NFPA 13 4.3 / 3.3.141) | Stockpile cue | Opened annex examples (representative) |
|---|---|---|---|
| Light Hazard | Low quantity and/or combustibility; low heat-release fires | Not a storage class | Offices, churches, educational, hospitals, museums, restaurant seating, residential, unused attics |
| OH1 | Moderate quantity, low combustibility; moderate heat release | Low-combustibility piles ≤ 8 ft (2.4 m) | Bakeries, laundries, restaurant service, electronic plants, mechanical rooms, porte cocheres |
| OH2 | Moderate to high quantity and combustibility | Moderate HRR ≤ 12 ft (3.7 m); high HRR ≤ 8 ft (2.4 m) | Parking garages (2022), dry cleaners, repair garages, wood product assembly, cereal mills |
A new restaurant has a 2,200 ft² dining room of tables and chairs, separated by a gypsum wall and pass-through from a 900 ft² commercial kitchen and dish room. Using NFPA 13 (2022) occupancy-hazard practice, how should a Level I layout technician classify those two areas?
The architectural cover sheet labels a three-story clinic as IBC Group B. Patient exam rooms look like ordinary offices; a basement laundry matches the OH1 annex laundry example. Which statement is correct under NFPA 13 (2022) 4.3.1.1 and 4.3.1.2?
A 2022-edition parking garage occupies the first two levels of an otherwise Light Hazard office building. A pre-2019 crib sheet on the trainee's desk still lists automobile parking and showrooms under Ordinary Hazard Group 1. What is the correct 2022 layout call for the garage?