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Contents
  1. 1Wildfire behaviour
  2. 2How fire reaches buildings
  3. 3Building protection
  4. 4Spread between buildings and conflagration
  5. 5Modelling
  6. 6Limitations
  7. 7See also
  8. 8Notes
  9. 9References

The wildland-urban interface (WUI) is the term used for areas where buildings are exposed to fires in natural vegetation. In such areas a wildfire can spread from the vegetation to buildings and from one building to the next.[1][2] Fire engineering guidance notes that flying brands and embers may need to be considered to limit fire spread to adjoining property, and that this can have design implications in areas prone to bushfires and wildfires.[2]

Fire can reach a building in three ways: through burning embers and other debris, through radiation and through direct flame contact.[3] Wind can carry embers, also called firebrands, over great distances and start spot fires at other locations.[4] The United States Forest Service supported the development of sub-models in the Fire Dynamics Simulator for fire spread in the wildland-urban interface.[1] Once buildings are alight, the methods for fire spread between buildings apply, and these rest on the same processes of radiant ignition and brand transport.[5][6]

Wildfire behaviour

Flame spread in a forest depends on the type and arrangement of the fuel, the terrain, the wind and the humidity. It is driven by radiant heat transfer and by convective heating caused by wind or by the slope of the terrain.[4] In most fires the porous brush on the forest floor burns. In severe fires the crowns of the trees also burn, or only the crowns.[4] Forest fires accelerate up slopes because the flames lean, an effect that was also seen in the escalator trench in the fire at King's Cross underground station.[7]

Thomas proposed an approximate relation for the rate of spread of wildland fires, wood cribs and urban conflagrations. In it, the spread rate times the bulk density of the fuel is proportional to one plus the wind speed, with a constant of 0.07 for wildland fires.[8] The relation shows two trends: the spread rate rises with the wind speed, and it falls as the fuel is packed more densely. Quintiere also compares spread rates for cribs, gorse and heather and urban fires in Japan at wind speeds of up to 30 m/s.[4] He lists the spread rate of forest and urban fires as of the order of 1 to 100 cm/s.[9] A glossary in NUREG-1805 gives 1 to 30 cm/s for wind-driven spread through forest debris.[10]

The United States Department of Agriculture Forest Service has developed practical methods for estimating flame spread in forests under a range of field conditions. Rothermel reviewed the theory and models for fire danger rating and fire behaviour prediction, including the computer model BEHAVE.[4] Breaks in the fuel can stop the spread, but the width needed depends on the size of the fire and its heat transfer.[4] When a forest fire jumps across a firebreak, this is generally regarded as ignition by radiation.[5]

The advancing front of a forest fire is an example of a line fire, like a row of burning townhouses, and like other natural fires it burns as a diffusion flame.[11][12] Behind the front, the passage of a forest fire often ignites the duff layer, the porous layer of decaying organic material on the forest floor, to smouldering. This smouldering is very irregular in extent and can reignite a flaming fire.[13]

How fire reaches buildings

The International Fire Engineering Guidelines name three routes by which fire spreads to an adjacent enclosure, building or property boundary: burning embers and other debris, radiation, and direct flame contact.[3] Wind speed and direction influence how far flames project from the windows of a burning building, and openings on both the windward and the leeward side make the effect of wind more significant.[2] A firebrand is a flaming or smouldering airborne object that emerges from a fire and can sometimes ignite remote combustibles. An ember is a particle of solid material that emits radiant energy because of its temperature or because of combustion at its surface.[14][15] Burning brands that are lifted by the convective flow of a fire and fall onto combustible materials are a common mechanism of fire spread, observed in wildland fires and in warehouse fires.[16] How far a burning brand rising in the plume may be carried by the wind is often difficult to establish with any certainty.[17]

Radiation from the flames heats building surfaces until they ignite. For fire growth over separate items, such as buildings in a city, radiant heating to ignition is the main process.[5] Kanury distinguishes piloted ignition, caused by a small flame, a heated wire or a spark, from spontaneous ignition without such a pilot.[18] Wood heated for less than 30 min needs a minimum heat flux of about 12 kW/m² for piloted ignition and 20 kW/m² for autoignition. For exposures lasting hours or days, a value of 4.3 kW/m² should be used.[19] Tewarson's critical heat flux for ignition of wood is 10 kW/m²; below this level, ignition and flame spread are not possible.[20]

The combustibility of the materials on and around a building therefore matters. Embers that land on combustible surfaces may ignite them, and a radiant flux of 10 to 20 kW/m² may cause objects to ignite.[16][21] The heat flux that a surface receives from a fire depends on the flame radiation and on the geometry between flame and surface, which engineers express with a view factor.[17][22]

Building protection

Roofs are exposed to both embers and radiation. British Standard BS 476: Part 3 describes a test of the ignitability and fire spread of external roof surfaces under radiation from a fire in an adjacent building and piloted ignition from flying brands.[17] In the United States, roof coverings are rated in the test of ASTM E108, UL 790 and NFPA 256, which measures the spread of flame, intermittent flame, burning brands and flying brands. Class A roof coverings must limit flame spread to less than 1.82 m, Class B to less than 2.44 m and Class C to less than 3.96 m.[23][24] For all classes, no flying brands or particles may continue to flame or glow after reaching the floor.[23]

Separation distance limits the radiation that reaches a building. The International Fire Engineering Guidelines prefer a first-principles approach for each case to methods derived from codes. They refer to an SFPE guide for the time to piloted ignition under flame radiation and for safe separation distances, which often require view factor calculations.[25][17]

These methods were developed for fires in buildings. In New Zealand, for example, the Building Code limits the radiation from a building fire to 30 kW/m² at the relevant boundary and 16 kW/m² at 1 m beyond it. External walls closer than 1 m to the boundary must be non-combustible, or must not ignite within 15 or 30 min under a radiant flux of 30 kW/m². The required time depends on the importance level of the building.[26]

Firefighting foams can protect exposed buildings. Class A foams have been used extensively to suppress wildland fires. In tests, Class A foam lengthened the time to ignition of a combustible surface compared with the same rate and duration of water.[27] In portable equipment, foam is mainly used against wildland fires in Class A fuels, mixed with water at 0.1 to 1.0 %.[28]

Spread between buildings and conflagration

Once buildings burn, the fire can spread through a built-up area as it does through a forest. Ramachandran describes models in which buildings are linked by bonds whose chance of transmitting fire depends on the building and its contents, the wind and the distance between buildings. He notes that the same problem arises when predicting fire spread in a forest or from building to building in an urban area.[29]

Data from fires in Tokyo were used to express the probability of fire spread between two buildings as an exponential function that decreases with the distance between them. The analysis showed that the construction of the buildings was the main factor in fire spread.[30] In a forest model with randomly placed combustible and non-combustible blocks, fire stopped spreading without wind when about 39 % of the blocks were non-combustible. With wind, up to 65 % non-combustible blocks were needed to stop it.[31]

Albini and Rand modelled urban fire spread as depending on whether a burning building is closer to its unburnt neighbours than a safe distance for radiation or brand transfer.[6] Thomas's spread relation was also applied to the Great Fire of London, with a constant of 0.046 at a wind speed of 4.5 m/s, close to the value of 0.07 for wildland fires.[8] Wildland fires are also listed among the natural catastrophes, such as lightning, earthquakes and wind storms, that make a fire risk of zero unachievable.[32]

Modelling

The Fire Dynamics Simulator offers two ways to model fire spread through vegetation. In the particle model, vegetation such as grass, leaves or airborne burning brands is represented by Lagrangian particles that exchange heat and mass with the gas.[33][34] For radiation, vegetation is treated as a collection of large, opaque particles that do not scatter.[35]

The level set model replaces the details of heat transfer and decomposition by an empirical spread rate, which greatly reduces the complexity of the model.[34] By default each point of the fire front spreads as an ellipse, whose length-to-breadth ratio depends on the combined effect of wind and slope.[34][36] The spread rate of the head of the fire follows Rothermel's model: a rate without wind and slope, increased by factors for wind and slope. The model treats the spread rate as the ratio of the energy received by unburnt fuel to the energy needed for its ignition.[37] The wind is taken from a reference height of 6.1 m, equivalent to a standard 20 ft weather station.[38]

Limitations

Estimates of flame spread require complex analysis and specific data on the fuel, its arrangement and the ambient conditions, and the available formulas are limited.[39] The distance over which brands are carried is difficult to establish.[17] The ignition fluxes and radiation limits above come from studies of materials and building fires, not from wildfires, and Thomas's relation is an approximation.[19][26][8] A common misconception is that a wildfire only threatens buildings that the flame front touches. Embers carried by the wind can start fires at a distance, and smouldering duff can reignite the fire after the front has passed.[4][13]

See also

Notes

  1. ↑ 1.0 1.1 McGrattan et al. (2026), p. ix.
  2. ↑ 2.0 2.1 2.2 National Research Council of Canada et al. (2005), p. 1.6-5.
  3. ↑ 3.0 3.1 National Research Council of Canada et al. (2005), p. 1.6-7.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Quintiere (2002b), p. 2-255.
  5. ↑ 5.0 5.1 5.2 Kanury (2002a), p. 2-229.
  6. ↑ 6.0 6.1 Albini and Rand, as cited in Ramachandran (2002c), p. 3-396.
  7. ↑ Cox and Kumar (2002), p. 3-207.
  8. ↑ 8.0 8.1 8.2 Thomas (1971), as cited in Quintiere (2002b), p. 2-255.
  9. ↑ Quintiere (2002b), p. 2-247.
  10. ↑ Nolan (2000), as cited in Iqbal and Salley (2004), p. F-31.
  11. ↑ Iqbal and Salley (2004), p. 9-2.
  12. ↑ Iqbal and Salley (2004), p. 3-12.
  13. ↑ 13.0 13.1 Ohlemiller (2002), p. 2-206.
  14. ↑ Iqbal and Salley (2004), p. F-24.
  15. ↑ Iqbal and Salley (2004), p. F-22.
  16. ↑ 16.0 16.1 Hamins, Maranghides, McGrattan, Ohlemiller, and Anleitner (2005), p. 30.
  17. ↑ 17.0 17.1 17.2 17.3 17.4 National Research Council of Canada et al. (2005), p. 2.6-12.
  18. ↑ Kanury (2002a), p. 2-230.
  19. ↑ 19.0 19.1 Babrauskas (2003), as cited in National Research Council of Canada et al. (2005), p. 3.3-4.
  20. ↑ Tewarson (1995), as cited in Iqbal and Salley (2004), p. 6-4.
  21. ↑ Nolan (2000), as cited in Iqbal and Salley (2004), p. F-35.
  22. ↑ Van Rossum (2027), §5.4.
  23. ↑ 23.0 23.1 Iqbal and Salley (2004), p. C-11.
  24. ↑ Iqbal and Salley (2004), p. C-12.
  25. ↑ National Research Council of Canada et al. (2005), p. 2.6-11.
  26. ↑ 26.0 26.1 Ministry of Business, Innovation and Employment (2025b), p. 43.
  27. ↑ Scheffey and Hickey (2002), p. 4-144.
  28. ↑ Iqbal and Salley (2004), p. B-60.
  29. ↑ Ramachandran (2002c), p. 3-395.
  30. ↑ Sasaki and Jin, as cited in Ramachandran (2002c), p. 3-396.
  31. ↑ Nahmias et al., as cited in Ramachandran (2002c), p. 3-396.
  32. ↑ Watts and Hall (2002), p. 5-4.
  33. ↑ McGrattan et al. (2026), p. 96.
  34. ↑ 34.0 34.1 34.2 McGrattan et al. (2026), p. 135.
  35. ↑ McGrattan et al. (2026), p. 76.
  36. ↑ McGrattan et al. (2026), p. 136.
  37. ↑ McGrattan et al. (2026), p. 137.
  38. ↑ McGrattan et al. (2026), p. 138.
  39. ↑ Quintiere (2002b), p. 2-256.

References

  • Cox, G., & Kumar, S. (2002). Modeling enclosure fires using CFD. 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. 3-194–3-218). National Fire Protection Association; Society of Fire Protection Engineers.
  • Hamins, A., Maranghides, A., McGrattan, K. B., Ohlemiller, T. J., & Anleitner, R. L. (2005). Experiments and modeling of multiple workstations burning in a compartment (NIST NCSTAR 1-5E). National Institute of Standards and Technology.
  • 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.
  • Kanury, A. M. (2002a). Flaming ignition of solid fuels. 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. 2-229–2-245). National Fire Protection Association; Society of Fire Protection Engineers.
  • McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
  • 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.
  • Ohlemiller, T. J. (2002). Smoldering combustion. 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. 2-200–2-210). National Fire Protection Association; Society of Fire Protection Engineers.
  • Quintiere, J. G. (2002b). Surface flame spread. 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. 2-246–2-257). National Fire Protection Association; Society of Fire Protection Engineers.
  • Ramachandran, G. (2002c). Stochastic models of fire growth. 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. 3-381–3-401). National Fire Protection Association; Society of Fire Protection Engineers.
  • Scheffey, J. L., & Hickey, H. E. (2002). Foam 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. 4-123–4-148). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • Watts, J. M., Jr., & Hall, J. R., Jr. (2002). Introduction to fire risk analysis. 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. 5-1–5-7). National Fire Protection Association; Society of Fire Protection Engineers.
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