Warehouse fire

A warehouse fire is a fire in a building used to store goods, often in large undivided spaces with goods stacked on pallets or in racks to heights of several metres. The combination of a large floor area, a high fire load and tall vertical fuel surfaces makes such fires grow quickly and burn intensely. Warehouse fires are relatively rare, but in an analysis of fire investigations in Greater London from 1996 to 2000, warehouses had the largest mean fire damage area of all occupancies studied.

Design guidance for warehouses therefore deals with the growth of the fire in the stored goods, the heat it can release, and its control by sprinklers, smoke venting and the fire service. In New Zealand, Verification Method C/VM2 (second edition, 2025) uses an ultra-fast design fire with a peak of 50 MW for storage 3.0 to 5.0 m high. Higher racks get an even faster growth. In the United States, sprinkler criteria for storage are based on full-scale tests and were merged into NFPA 13 from its 1999 edition.

Characteristics of warehouse fires

A small room of about 200 m² behaves roughly as one well-mixed fire zone. In a warehouse of more than about 1,000 m², the fire is more complex and its burning rate is not uniform. It is expected to travel around the space and to burn with varying intensity. The intensity depends on the distribution of fuel, the geometry, the combustibility of the goods and the ventilation through roof vents, make-up air and, eventually, a failed roof.

In most warehouse designs the roof has no fire rating and is expected to fail early. Before it fails, heat and smoke are held in and the walls are exposed more intensely; after it fails, the hot gases escape and flame radiation dominates. The fire behaviour of metal-faced sandwich panels cannot be predicted from small-scale cone calorimeter tests, so the usual correlation with the full-scale room test does not apply to them.

Large-scale data are scarce. Fire experiments at warehouse scale are uncommon, and taking such a building to burnout in a test is almost unheard of. The International Fire Engineering Guidelines note that the usual relations for compartment temperatures come from rooms of about 10 m², sometimes up to 50 m². If simultaneous burning is assumed in a large enclosure, the predicted temperatures are unrealistically high. As a first approximation, the floor can be divided into grids of 10 to 50 m², whose heat release rates are added with time delays for the spread from grid to grid.

Stored goods present large vertical surfaces, which favour fire growth far faster than the fast design fire used for most buildings. Standard t-squared growth curves assume horizontal spread in all directions and do not suit fires that also spread upwards, as in high racks. The point of ignition matters too. In one comparison, igniting a rack at its front face delayed the fire events by about 470 s compared with ignition in the centre of the flue spaces, the gaps between the stored loads. Wooden pallets become a fire hazard mainly when idle pallets are stacked many units high.

In a high distribution centre, the hot smoke layer can ignite goods stored high in the racks. Most materials ignite at about 200 to 500 °C, so a smoke layer above 200 °C with goods in it gives a chance of such secondary fires. In the London fire investigation data, a fire damage area of 100 m² was used as the criterion for flashover in large spaces. Of 20 warehouse fires, 20.8 % exceeded this area, and the mean damage area was 170 m².

Fire load

The fire load density of storage depends on the height of the goods. In the International Fire Engineering Guidelines, the storage values from Swiss surveys of 1967 to 1969 are given per metre of storage height and must be multiplied by that height. Examples are 600 MJ/m² per metre for storage at a box manufacturer, 1,700 at a carpet manufacturer and 4,200 at a cardboard manufacturer. For occupancies with very different goods, such as industrial buildings, the coefficient of variation is 50 to 80 % of the average, and isolated peaks can reach 2.5 times the average.

New Zealand's C/VM2 (second edition, 2025) prescribes a design fire load of 1,200 MJ/m² for storage up to 3.0 m high, other than foamed plastics. For storage of combustible materials above 3.0 m, it prescribes 800 MJ/m² per metre of height, with a minimum of 2,400 MJ/m². A store of 6 m high goods therefore has a design fire load of 4,800 MJ/m². In Australia, the 2020 data sheets for the Fire Safety Verification Method assign a very high fire load distribution to storage buildings, with a mean of 1,000 MJ/m², but explicitly exclude high-rack storage from it.

The total energy in a store varies greatly with what is stored. The C/VM2 commentary compares 1,000 m³ of different goods: low-density polyurethane foam contains about 1.3 TJ, MDF board 11.5 TJ and rolled polyethylene about 20 TJ. The book Introduction to Fire Safety Engineering gives the example of a shed with 3,500 m³ of stored goods with a heat of combustion of 23 MJ/kg and a density of 135 kg/m³. This contains 10,867,500 MJ, equivalent to about 572,000 kg of wood.

Fire growth and heat release

Design fires for warehouses usually describe the growth of the fire with a t-squared curve, in which the heat release rate increases with the square of the time since ignition. The growth coefficient expresses how quickly the fire grows; for a fast fire, for example, it is 0.047 kW/s²:

$$\dot{Q}=\alpha t^2$$

Symbol	Description	Unit
$\dot{Q}$	heat release rate	kW
$\alpha$	fire growth coefficient	kW/s²
$t$	time from effective ignition	s

For an ultra-fast fire, α is 0.188 kW/s², which means the fire reaches 1,055 kW after 75 s. C/VM2 uses this value for buildings that can store goods to a height of 3.0 to 5.0 m, with a peak of 50 MW. The growth stops at that peak, at flashover, at the limit set by the available ventilation or, for some calculations, at sprinkler activation. For goods stored above 5.0 m, C/VM2 uses a cubic growth that also increases with the storage height, so the t-squared curve is not valid for high racks. The commentary explains that storage up to 5.0 m is treated as ultra-fast following NFPA 72 and NFPA 204, and that growth increases further with taller vertical surfaces.

As a worked example, take a store with goods stacked 4 m high. With α = 0.188 kW/s², the heat release rate after 2 min is 0.188 × 120² = 2,707 kW, or about 2.7 MW. After 5 min it is 0.188 × 300² = 16,900 kW. The 50 MW peak is reached after √(50,000 / 0.188) = 516 s, or about 8.6 min. For comparison, the general design fire for most other buildings in C/VM2 grows at 0.0469 kW/s² to 20 MW, which takes 653 s. The book uses slightly different reference values: it defines the growth time as the time to reach 1 MW, 75 s for ultra-fast growth.

Design fires of New Zealand's C/VM2 (2025): storage 3.0 to 5.0 m high (ultra-fast, peak 50 MW) and most other buildings (peak 20 MW). Based on Ministry of Business, Innovation and Employment (2025b), p. 20.

Measurements show how fast warehouse goods burn. According to NUREG-1805, most warehouse materials in a table taken from NFPA 72 fall in the fast or ultra-fast range. Stacks of wood pallets 3 m high reach about 1,055 kW in 80 to 110 s, and vertical rolls of paper 6 m high in only 15 to 28 s. Wood pallets stacked to 3 m release about 7 MW, and polystyrene jars in cartons stacked 4.9 m high about 30 to 40 MW.

Peak heat release rates per unit floor area of stored commodities, analysed by Heskestad from Factory Mutual tests, range widely. Cardboard cartons with a metal liner stacked 2.95 m high gave about 1,490 to 1,680 kW/m², while polyethylene bins in cardboard cartons stacked 4.51 m high gave 28,900 kW/m². These values cannot be scaled: they may be applied conservatively to lower stacks, but not extrapolated to higher ones. Factory Mutual classifies stored goods for sprinkler protection by how much heat they release in the presence of water. Raising the share of polystyrene in cardboard boxes from about 15 % to 100 % increases the heat release rate exponentially and raises the water demand.

Peak heat release rate per unit floor area of stored commodities from Factory Mutual tests, compared with 250 kW/m² for a developed fire in an average building. Based on Heskestad (1997), as cited in Babrauskas (2002b), p. 3-18, and Van Rossum (2027), §4.5.

Sprinkler protection

Classifying the occupancy and the stored commodity is the most critical step in sprinkler design. If the hazard is underestimated, the fire can overpower the sprinklers. This step is described under sprinkler hazard classification. Sprinkler design for fire suppression is largely empirical. The criteria for rack storage and for large-drop and early suppression sprinklers are based on full-scale tests, while most other criteria rest on experience and judgement.

Ordinary sprinklers are designed to control a fire. An ESFR sprinkler, short for early suppression fast response, combines a fast response with large water drops and a high velocity, so that it can extinguish a high-challenge fire. Its installation rules are strict, and in-rack sprinklers are a separate type placed within the racks. The variables of storage protection include the water application rate, the response time index, the temperature rating and in-rack protection, and the storage arrangement matters as well. Water from sprinklers also wets the goods, which helps to stop the fire from jumping across the aisles of a warehouse.

Sprinklers in storage buildings perform slightly below average. The Australian data sheets give a reliability of 84 %, an efficacy of 97 % and an effectiveness of 83 % for storage buildings. United States data for 2010 to 2014 show that wet-pipe sprinklers in warehouses, excluding cold storage, operated in 84 % of fires and were effective in 82 %.

Smoke and heat venting

Smoke and heat exhaust ventilation in a warehouse aims to support escape and firefighting, so that firefighters can enter and see the seat of the fire, and to reduce property damage. A typical arrangement is a large space with vents in the roof, often divided by draft curtains into separate smoke reservoirs. Without venting, smoke fills such a space quickly. Cooper considers goods that reach about 1 MW in 100 s, in a warehouse 6 m high with a floor area of 1,500 m². He calculates that the smoke layer becomes untenable at 183 °C after about 350 s. At that moment its lower edge is 2.7 m above the floor, and after about 600 s the layer fills the whole warehouse at about 430 °C.

Cross-section of a racked warehouse with a fire in one rack: smoke layer, roof vents, draft curtains above the aisles, ceiling and in-rack sprinklers. Based on Cooper (2002b), pp. 3-219 and 3-237, and Iqbal and Salley (2004), p. 10-3.

Roof vents rely mainly on the buoyancy of the smoke. Strong wind can create suction over the roof and increase the extraction, but the vent may then draw in clear air from below the smoke layer, so that a critical vent area exists. Cooper gives a worked example for a 5 m × 5 m fire in a warehouse with a ceiling at 9 m and draft curtains reaching down to 6 m. To keep the smoke above the bottom of the curtains, each curtained space needs a total vent area of 18.6 m².

The combination of vents and sprinklers was long controversial, and no generally accepted design guidelines existed when Cooper's chapter was written. A review of 34 position papers, as cited by Cooper, concluded that venting does not harm sprinkler performance. It has no effect on the activation of the first sprinklers or on the total number activated, although a vent directly above the fire may slightly delay the first sprinklers. The review also advised placing draft curtains in aisles rather than over storage. The Australian data sheets warn, on the other hand, that a vent may remove hot gases before sprinklers activate, which could compromise early suppression systems.

Sprinkler spray can drag the smoke layer down and mix it with the air below, an effect called smoke logging. Once this happens, the benefits of venting are lost, even with very large vents. In the Netherlands, the book describes the criteria of the standard NEN 6093 for fire service intervention in a space with smoke and heat exhaust. Visibility up to a height of 1.8 m must be at least 100 m, and radiation from the smoke layer at that height at most 3.5 kW/m². The layer temperature may be at most 200 °C if goods are stored in the smoke layer and 500 °C otherwise.

Detection and other measures

Detection in high warehouses is a design problem of its own. In a design example for a furniture warehouse, Schifiliti and co-workers show that heat detectors must be spaced more closely as the distance from the fire to the ceiling grows. For a floor area of 5,000 m², about 48 fixed-temperature detectors or about 8 rate-of-rise detectors were needed to detect a fire of about 2,000 kW. For a storage room of 50 m × 75 m × 10 m, a projected beam smoke detector had to be set to respond at about 30 % total obscuration or less. Such detectors typically respond at 20 to 70 % total obscuration.

In New Zealand, C/VM2 (second edition, 2025) allows manual activation of the fire alarm in buildings such as warehouses, where the ceiling is high and the occupant load is low. The conditions are an average ceiling height of at least 5 m and fewer than 50 occupants who are awake and familiar with the building. The rule rests on 34 simulations with the Fire Dynamics Simulator. In these, the 12 m high building had the fastest ceiling jet, because the growth of a rack fire increases with rack height.

Fire spread can also be limited by the way goods are stored. The International Fire Engineering Guidelines mention changing from rack storage to palletised storage and storing items horizontally rather than vertically. Another approach is to lower the oxygen level, the principle behind an oxygen reduction system. Air contains 23.3 % oxygen by mass. Tewarson shows that flames of PMMA, a plastic, go out at an oxygen mass fraction of about 0.18 under normal conditions. When the fuel receives an external heat flux of 65 kW/m², it keeps burning down to 0.128.

Regulation and limits

Building codes set limits on large stores. In New Zealand, C/VM2 (second edition, 2025) sets conditions for unsprinklered firecells within 15 m of a relevant boundary that have a fire load above 20 TJ or a floor area above 5,000 m². When firefighters first apply water, the radiation at 1.5 m above the floor must not exceed 4.5 kW/m², and the smoke layer must be at least 2 m above the floor. The commentary explains that the 5,000 m² limit was based on 800 MJ/m² per metre of storage height and a storage height of 5.0 m. With these values, 5,000 m² corresponds to 20 TJ.

The commentary works this out for an unsprinklered warehouse of 1,200 m² and 8.0 m high, with goods stored to 6.0 m. With an ultra-fast fire to 50 MW and water applied after 1,200 s, the criteria were met with automatic make-up air and 22 roof vents of 2.0 m², 3.7 % of the floor area. The Building Code of Australia, as described in the commentary, requires sprinklers where more than 1,000 m³ of combustible goods are stored higher than 4.0 m. In the Netherlands, buildings with large fire compartments outside the scope of NEN 6060 are a typical field for fire safety engineering.

The data behind warehouse design are limited. The heat release rates per unit area apply only to the tested storage heights, the Australian growth rate statistics for storage rest on only six fires, and full-scale burnout tests are almost never done. A common misconception is that a store full of goods burns like an office with a higher fire load. In reality, tall vertical fuel surfaces make the growth much faster, and the fire spreads upwards through the racks rather than only across the floor.

See also

• Beam smoke detector
• Sprinkler-vent interaction
• Car park fire

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Babrauskas, V. (2002b). Heat release rates. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-1–3-37). National Fire Protection Association; Society of Fire Protection Engineers.
• Cooper, L. Y. (2002a). Compartment fire-generated environment and smoke filling. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-243–3-267). National Fire Protection Association; Society of Fire Protection Engineers.
• Cooper, L. Y. (2002b). Smoke and heat venting. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-219–3-242). National Fire Protection Association; Society of Fire Protection Engineers.
• Fleming, R. P. (2002). Automatic sprinkler system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-72–4-87). National Fire Protection Association; Society of Fire Protection Engineers.
• Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
• Kandola, B. S. (2002). Introduction to mechanics of fluids. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-1–1-26). National Fire Protection Association; Society of Fire Protection Engineers.
• Klote, J. H. (2002). Smoke control. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-274–4-291). National Fire Protection Association; Society of Fire Protection Engineers.
• Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-88–4-122). National Fire Protection Association; Society of Fire Protection Engineers.
• Schifiliti, R. P., Meacham, B. J., & Custer, R. L. P. (2002). Design of detection systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-1–4-43). National Fire Protection Association; Society of Fire Protection Engineers.
• Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-82–3-161). National Fire Protection Association; Society of Fire Protection Engineers.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.