Fire spread

Fire spread is the extension of combustion from its initial location to additional fuel, spaces or structures. It can occur as continuous flame movement, successive ignition of separate objects, heat transfer through a barrier, burning debris carried to a new location or a combination of these routes. The process links ignition with fire growth beyond the first burning item.

Spread changes both the size and the location of the fire. Additional fuel can increase heat release rate, while new openings and spaces can alter ventilation and smoke movement. A developing fire therefore cannot be assessed only from its initial source and fuel package.

The relevant scale ranges from millimetres across a surface to flames and brands travelling between buildings. Each scale has different controlling heat-transfer and ignition conditions. Spread rate and direction are consequently scenario properties rather than fixed material constants.

Successive ignition and heat transfer

Kanury describes fire spread over combustibles as continuous ignition of successive upstream material. The existing fire heats fuel ahead of the burning region until that fuel reaches its own ignition condition. Fuel production, mixing and heat loss at the new location all influence the time required.

Direct flame contact supplies intense local heating and a pilot. Radiation can act across a gap without flame or smoke touching the target, while convection carries hot gases to remote surfaces. Conduction transmits energy through solids, including walls and structural boundaries.

Four fire-spread pathways to new fuel. Based on Australian Building Codes Board et al. (2005), p. 1.6-7.

The target's arrangement affects each route. Thin or light items can respond quickly, while thick items can absorb energy internally. Orientation changes convective flow and the area exposed to flame radiation, and nearby surfaces can provide shielding or additional reflected and emitted energy.

Heat received by a target is not equal to total fire heat release. Only part reaches the object, and that part is balanced against losses and storage. Spread analysis therefore requires source exposure, target ignition response and time, not just the size of the original fire.

Flame spread over continuous fuel

Flame spread over a solid or liquid involves a moving gas-phase flame close to the condensed fuel that supplies its vapour. Quintiere distinguishes the visible flame front from the evaporation or pyrolysis front in the fuel. The movement of the latter defines surface flame-spread velocity.

The flame must heat material ahead of itself for spread to continue. If that heating is too weak or heat losses are too large, the front can slow or stop. If preheating is strong, spread can accelerate as more fuel becomes involved.

Direction matters because fire-induced flow changes with orientation. Upward spread can preheat a large region above the flame, whereas lateral or downward spread relies on different forward heat-transfer paths. Opposed and wind-aided flow can therefore produce very different rates on the same material.

Spread over a continuous surface differs from flame propagation through a premixed gas cloud. In the latter, fuel and oxidiser are already distributed in the gas phase. Surface spread instead requires continuing evaporation or decomposition at the condensed-fuel boundary.

Spread between separate objects

Separate objects introduce gaps across which heat must travel. Flame contact, radiation and burning debris can bridge these gaps, while convective flow can expose objects farther away. The time of each new ignition determines the spatial development of the overall fire.

The 2005 International Fire Engineering Guidelines describe a simplified pre-flashover method for radiant ignition of a second item. It neglects radiation from the upper layer and room surfaces and uses flame radiation from the first item. This assumption belongs to that stage and method, and is unsuitable for a developed hot compartment.

Fire spread between buildings can involve radiation through closed or fixed windows, direct flame contact and burning debris. The guidelines note that radiation can pass through glass without glass breaking. If breakage occurs, it can create both a new spread route and a ventilation change.

Firebrands and embers add discontinuous spread. Burning material can rise in a thermal plume and be carried by wind before igniting a remote target. The 2005 guidelines caution that travel distance is difficult to establish with certainty, so precise brand trajectories should not be invented.

Barriers and compartmentation

Fire compartmentation divides a building into enclosures intended to limit fire and smoke spread. Its effectiveness depends on barriers, openings, firestopping, structural stability and the combustibles on both sides. An assessment therefore considers failure modes as well as nominal fire resistance.

Conductive failure can occur when enough heat passes through a barrier to meet a failure criterion, even without loss of integrity or stability. Combustibles in the next enclosure can then be exposed on the unexposed side. The barrier can remain physically intact while this thermal pathway develops.

Barrier failure can be thermal, integrity-related or structural. Based on Australian Building Codes Board et al. (2005), p. 1.6-7.

Loss of integrity involves cracks, fissures or failure of firestopping. Hot gases, flames and radiation can then reach adjacent fuel through paths that were previously limited. The timing of such failure can also change smoke movement and ventilation.

Loss of stability involves collapse of the barrier, whether or not it is a structural element. This can create a large opening and expose new areas to flames and radiation. Structural failure and integrity failure are therefore distinct events in spread analysis.

A barrier can also depend on other structural elements for support. Failure of a supporting member may cause the barrier to fail before its own thermal resistance is exhausted. The IFEG consequently requires the structural sequence and its consequences for dependent barriers to be considered together.

Doors and windows introduce time-dependent opening states. The guidelines suggest obtaining door-opening times from evacuation analysis and evaluating glass breakage from fire severity. These changes connect occupant movement and component response with the fire's ventilation and spread path.

Compartment fire and flashover

Within a compartment, flames, hot gases and heated boundaries preheat additional fuel. As more objects ignite, heat release rate and gas temperature can increase, producing stronger feedback. Spread is therefore coupled with the compartment energy balance rather than being a simple sequence of independent objects.

Flashover represents a transition towards widespread involvement of combustible surfaces in the compartment. It can follow successive ignition and radiative preheating, but its timing depends on the complete enclosure and ventilation scenario. Spread before flashover requires assumptions appropriate to that stage.

After widespread involvement, unburnt volatiles can leave the compartment and burn outside after mixing with air. This external flaming can expose upper facades and neighbouring surfaces. The location of heat release consequently extends beyond the original fuel and room.

Facade fire introduces exterior surfaces, cavities and cladding arrangements into the spread problem. The 2005 guidelines include concealed spaces behind exterior cladding and flames projecting through windows among possible paths. These arrangements require their own geometry and material assessment rather than a room-only model.

Large spaces and travelling involvement

Large open-plan enclosures make simultaneous burning of the whole floor unrealistic in many scenarios. Travel time between fuel packages becomes significant relative to the duration of a localised fire. Assuming the entire area burns at once can produce unrealistically high temperatures.

The 2005 guidelines describe a grid approximation for large enclosures, using time offsets as fire moves between grid areas. Where object locations and ignition criteria are known, spatial development can instead be based on individual items. Both methods require defensible fuel arrangement and spread timing.

Grid time offsets represent spread across a large enclosure. Based on Australian Building Codes Board et al. (2005), p. 2.6-12.

The suggested grid areas are 10–50 m² in the 2005 method. Each region contributes its heat release after the assigned spread delay. The grid therefore represents a sequence of involvement; it does not calculate the delay from ignition physics. Using individual objects can improve the representation when their type and location remain predictable throughout the building's life.

Vertical shafts and concealed spaces extend the relevant enclosure beyond the room of origin. Flame and hot gas can reach other floors through these routes. Analysis limited to horizontal spread across one floor would miss them.

Fire spread can also alter suppression and evacuation conditions. New ignition locations change detection, available escape time and the area requiring intervention. The guidelines use spread and barrier-failure times as inputs to these wider assessments.

Rates, acceleration and extinction

Spread accelerates when newly involved fuel increases heat release and preheating faster than losses and fuel depletion limit them. Quintiere compares spread time with burn time: rapid spread relative to burning can accelerate growth, while slow spread can allow the original fuel to burn out before new ignition occurs.

The process is therefore unstable rather than inherently steady. A small change in heat flux, initial temperature or fuel arrangement can shift the balance towards acceleration or extinction. Initial temperatures closer to the ignition condition reduce additional heating required ahead of the front.

Representative rates illustrate scale rather than universal values. Quintiere gives lateral or downward spread on thick solids as roughly 0.1 cm/s in order of magnitude, compared with 1 to 100 cm/s for upward spread and several other phenomena. These ranges from the 2002 handbook should not be assigned to an untested material without matching conditions.

Spread can stop when heat transfer ahead of the flame becomes insufficient, fuel becomes discontinuous or oxygen and combustion conditions no longer support propagation. Fire growth is therefore not inevitable even after a substantial initial fire.

Assessment and radiation calculations

A spread assessment should identify the first item, potential targets, heat-transfer routes and barrier states. It then needs timing for each new ignition or failure and the corresponding changes in heat release and ventilation. This structure makes the assumed path explicit.

Source and target data must share compatible conditions. Flame-spread tests, ignition tests and barrier tests answer different questions, and their results cannot be substituted without a model. Geometry and scale should remain visible throughout.

Firebrand travel and large-space spread carry particular uncertainty. Precise distances and simultaneous involvement should not be asserted without supporting evidence. Sensitivity to target location and ignition timing can show whether the overall conclusion changes.

A simplified radiation calculation illustrates one link in the spread sequence. The IFEG represents a luminous flame as a radiating panel with a uniform effective temperature and emissivity. A configuration factor describes how the target's position and orientation relate to that panel. Multiplying these terms gives the radiation incident at the target.

$$q_r=\varphi \sigma \epsilon T_f^4$$

Symbol	Description	Unit
$q_r$	Incident radiant heat flux at the target	W/m²
$\varphi$	Configuration factor for the radiating panel and receiving point	Dimensionless
$\sigma$	Stefan–Boltzmann constant, 5.68 × 10⁻⁸ in the cited IFEG equation	W/(m²·K⁴)
$\epsilon$	Effective emissivity of the source	Dimensionless
$T_f$	Effective source temperature	K

The temperature must be entered in kelvin because the equation contains its fourth power. The configuration factor is geometric; it accounts for the visible panel size, separation and orientation. An obstruction or a different receiving position can change the exposure. This is why fire temperature alone cannot establish the heat flux reaching another object.

For an illustrative calculation, choose a source temperature of 1,000 K and a configuration factor of 0.10. The IFEG permits an emissivity of 1.0 as a conservative assumption for thick luminous flames. The chosen temperature and factor are example inputs, not measurements of a particular fire. Using the constant printed in the 2005 guidelines gives approximately 5.7 kW/m².

$$q_r=0.10\times (5.68\times 10^{-8})\times 1.0\times 1000^4=5680\mathrm{W}/{\mathrm{m}}^{\mathrm{2}}\approx 5.7\mathrm{k}\mathrm{W}/{\mathrm{m}}^{\mathrm{2}}$$

If geometry reduces the configuration factor to 0.05, with the other assumptions unchanged, the calculated exposure falls to about 2.8 kW/m². This demonstrates the effect of geometric separation and orientation through the factor. It does not determine a specific separation distance, which would require the actual panel dimensions and target position.

The result is incident radiation from the represented source. It is not the target's net heat gain or proof that ignition occurs. Target absorption, heat losses, internal heating and exposure duration still determine its thermal response. Kanury's ignition analysis also requires sufficient combustible gas production, mixing with oxygen and a suitable ignition condition.

The uniform panel is a simplification of the flame's changing shape and temperature. The guidelines also discuss point-source and cylindrical-flame methods, each with its own assumptions. A point-source screening model uses an inverse-square relationship, whereas a panel model explicitly represents the emitting geometry. Their results cannot be combined without retaining those different representations.

Radiation across intact glazing requires further care. The possibility of transmission does not mean the glass transmits all incident radiation. Glass breakage can subsequently create a different exposure and alter the compartment's ventilation. The IFEG therefore treats the initial radiation path and the later opening event separately in its fire-spread sequence.

Likewise, a barrier reaching an assumed failure criterion does not itself describe the receiving fuel's response. The guidelines allow barrier failure to stand for spread in simplified analyses, but distinguish that assumption from actual ignition. The fuel's ignitability and arrangement on the receiving side remain relevant. This distinction makes the scope of a calculated spread time explicit.

See also

• Car park fire
• Cavity barrier
• Heat transfer
• Wildfire

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-90–1-98). National Fire Protection Association.
• Kanury, A. M. (2002). Flaming ignition of solid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-229–2-245). National Fire Protection Association.
• Quintiere, J. G. (2002). Surface flame spread. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-246–2-257). National Fire Protection Association.
• Rockett, J. A., & Milke, J. A. (2002). Conduction of heat in solids. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-27–1-43). National Fire Protection Association.
• Tien, C. L., Lee, K. Y., & Stretton, A. J. (2002). Radiation heat transfer. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-73–1-89). National Fire Protection Association.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.