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Timber in fire

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Contents
  1. 1Pyrolysis and degradation zones
  2. 2Charring rate
  3. 3Mass timber and residual section
  4. 4Smouldering and char oxidation
  5. 5Fire resistance and protection
  6. 6Fire exposure and modelling
  7. 7Limits and common misunderstandings
  8. 8Evidence and application
  9. 9See also
  10. 10Notes
  11. 11References

Timber in fire describes the decomposition, charring and loss of structural capacity of wood exposed to heat. Timber is a combustible, char-forming material whose pyrolysis gases can feed flames while the remaining section loses density and strength. Fire resistance depends on species, moisture, section geometry, protection and the actual fire exposure.[1][2]

Heating produces a char layer and volatile gases rather than a uniformly weakened version of the original timber. The char has virtually no load-bearing strength, so the uncharred core becomes the relevant structural section. Charring rate and residual cross-section are therefore central to assessment.[1][2]

Timber can also contribute to fire growth through its energy content and combustion products. Its behaviour in a small test and in a developed compartment fire can differ, making the actual exposure and installation essential to interpretation.[3][4]

Pyrolysis and degradation zones

Pyrolysis is thermal decomposition that breaks large wood molecules into smaller products, including combustible gases and char. It does not require oxygen at the decomposition site, although those gases can later burn after mixing with air. The original solid and the flame are therefore separate but coupled regions.[5][6]

White describes normal wood, a pyrolysis zone and an outer char layer. As heating continues, the pyrolysis region advances into the member and the char thickens. Pyrolysis gases leave the surface and can undergo flaming combustion, while glowing combustion and mechanical disintegration gradually erode the outer char.[1]

Timber section with uncharred core, pyrolysis zone and char, with combustible gases leaving.
Timber develops an uncharred core, pyrolysis zone and char. Based on White (2002), p. 4-260, Figure 4-11.2, redraw, do not copy.

Kodur and Harmathy place the charring temperature for wood in the range 280 to 300 °C in their review. This is a material-specific range rather than a universal ignition threshold for every timber product and exposure. Pilot arrangement, heat flux and heat loss still influence actual ignition.[2][7]

Moisture desorption occurs before and during decomposition and changes the energy required for heating. Wood density and moisture also affect charring, so a dry, constant-property model can be inadequate. Product form and permeability further influence gas movement and oxidation.[8][1]

Charring rate

Charring rate is the rate at which timber is converted to char, commonly expressed as depth per unit time. It determines how quickly the uncharred structural section shrinks. A constant rate can be a reasonable approximation for thick members under standard exposure, but actual fires can change it.[2][1]

The standard average transverse-to-grain rate is approximately 0.6 mm/min in the sources. The rate parallel to grain is approximately twice that transverse rate. These conventions are widely used for standard fire exposure but do not represent every species, moisture condition and fire curve.[2][4]

Transverse charring 0.6 mm/min and parallel charring about 1.2 mm/min.
Standard charring-rate convention by grain direction. Based on Kodur and Harmathy (2002), p. 1-164 and White (2002), p. 4-261.

Reported study rates range from 0.137 to 0.85 mm/min, showing variation across specimens and test arrangements. Density and moisture content influence the rate, with the cited Douglas fir relationship showing decreases as density and moisture increase. The values used in a project should match the timber and exposure.[2][4]

Charring also rounds beam and column corners. White describes a common approximation in which the corner radius equals the char depth on the sides. Residual-section calculations should account for this shape loss rather than assuming the original sharp rectangle.[4]

Mass timber and residual section

Mass timber includes large engineered wood sections whose fire performance depends on the uncharred core and connections. Char has virtually no strength, so the original full section cannot be used in structural calculations once charring has developed. Residual geometry and material properties at temperature become decisive.[1][4]

The Reduced cross-section method evaluates structural performance using the section remaining after subtracting charred material. Charring depth, corner rounding and the required load-bearing function determine that remainder. Protection that delays char onset or slows its advance increases the residual section at a given time.[1][4]

Connections and adjacent elements can govern system performance even when the core retains capacity. Heat and deformation can weaken fasteners and change load paths, while different parts char at different rates. A structural assessment should include those details rather than treating the member as isolated material.[9][1]

The remaining core can also be affected by temperature before charring reaches it. Mechanical properties at elevated temperature and internal moisture movement can change response. Zero-strength char and changing uncharred properties are separate considerations.[10][1]

Smouldering and char oxidation

Smouldering is flameless surface oxidation that can persist in porous char and timber after flaming declines. It produces heat and toxic products and can provide a later path back to flaming. Apparent extinction of open flame therefore does not prove that timber has stopped reacting.[11][12]

Char is carbon-rich and porous, making it susceptible to oxygen attack. Its oxidation supplies heat for continued smoulder propagation and can erode the protective layer. Char can insulate underlying timber and simultaneously sustain this surface combustion.[13][11]

Solid wood has lower permeability than loose fibres, so its smouldering configuration differs from a porous insulation bed. Ohlemiller reports that a single wood layer may need about 10 kW/m² of additional heat input to sustain smouldering in the cited configuration. This is not a universal threshold for all timber products.[14]

Oxygen readmission and airflow can change the process after apparent suppression. Deep-seated char can reignite if it remains hot, and cooling such material can be difficult. Assessment after a fire should include the condition of the remaining char and core.[15][16]

Fire resistance and protection

Fire resistance for timber members and assemblies depends on delaying char formation, limiting heat transfer and preserving the residual section and connections. Protective membranes can delay char onset and reduce subsequent charring rate. The complete assembly, not just the timber species, determines performance.[1][17]

White reports that 13 mm gypsum board on wood beams reduced char depth by approximately 40% in two-hour tests, with only part of that benefit credited to the initial delay in char formation. This demonstrates protection through both delayed onset and reduced subsequent rate. The result belongs to the tested assembly.[1]

Study range 0.137 to 0.85 mm/min with the standard average 0.6 marked.
Reported charring rates vary around a standard average. Based on Kodur and Harmathy (2002), p. 1-164.

Component-additive methods assign fire-resistance contributions to membranes and framing from test evidence. Fastener type and spacing can be critical, and codes limit how the method is applied. Adding insulation can improve a wall rating, while the position and attachment of insulation in floor or roof assemblies can change the result.[18]

Coatings and intumescent treatments can provide additional protection, but adhesion and deformation matter. White reports coated beams that improved less in full-scale testing than in earlier small-scale tests because bending caused adhesion problems. Product-scale evaluation is therefore necessary.[1][13]

Fire exposure and modelling

A standard fire curve provides a controlled exposure for comparing assemblies, but real compartment fires vary in temperature and duration. Charring rate can change with that severity, so a standard 0.6 mm/min rate should not be used as an unqualified natural-fire constant. The exposure must be stated.[2][19]

Heat-transfer models can evaluate protective membranes and residual timber temperature, but they must also represent charring and panel deterioration. White notes that these physical changes complicate numerical methods. A conduction-only model may not capture the whole fire-resistance mechanism.[1][10]

Natural and parametric exposures can be used in performance-based analysis when supported by the model and data. Zone or compartment models supply the gas environment, after which heat transfer and structural analysis evaluate the member. Each stage needs appropriate validation.[1][20]

Sensitivity to species, density, moisture, section shape and protection should be made visible. Rounding char depth with excessive precision would hide those uncertainties. The structural conclusion should identify which assumptions control it.[21][4]

Limits and common misunderstandings

Timber does not simply melt away at one temperature. It decomposes into gases and char, loses section and can continue to smoulder. The residual structure and the combustion process must be assessed separately.[1][5]

Char is not a reliable structural material. Although it can insulate the core, it has virtually no load-bearing strength and can erode or oxidise. Structural calculations must use the residual uncharred section rather than crediting the full original member.[1][2]

The standard charring rate is not universal. Species, density, moisture, grain direction, protection and fire severity all affect it. Applying 0.6 mm/min without that context can misrepresent the residual section.[4][2]

Finally, a flame-retardant treatment does not necessarily provide fire resistance. It may delay ignition or reduce surface spread while offering limited improvement under deformation. The claim must be supported by the complete assembly and exposure.[1][22]

Evidence and application

A timber assessment should begin with species, product form, density, moisture and section geometry. These determine the thermal response and how the residual section develops. Engineered products and connections can add further materials and failure paths that a solid-wood model would miss.[1][23]

Charring data should be tied to the exposure and grain direction. The familiar 0.6 mm/min transverse average and the approximate doubling parallel to grain are conventions for the cited standard exposure. Real fire severity, protective layers and product variation can produce different rates and should be addressed explicitly.[4][2]

Residual-section calculations should use the same char-depth convention as the structural analysis. Corner rounding, connections and any nonuniform charring can change the effective shape and load path. Subtracting an idealised rectangle without those details can overestimate the remaining capacity.[4][9]

Protection should be evaluated through both onset and rate. Gypsum and other membranes can delay char formation, but White's example also shows a reduced subsequent charring rate. Reporting only the initial delay would miss part of the protection's effect on the residual section at a given time.[1]

Fire and smouldering history should be included after peak exposure. Hot char can continue to oxidise and can later support flaming, changing both the remaining section and the exposure of nearby objects. An analysis that stops when open flames disappear can therefore miss continuing degradation.[15][12]

Test evidence should match the assembly and its deformation. Fasteners, panel attachment and bending can change membrane performance and expose timber earlier than expected. Small-scale material results do not remove the need to evaluate those full-scale conditions.[1][18]

Model uncertainty should include the fire curve as well as material data. Standard exposure supports comparison, but natural fire severity can alter charring and protection. Sensitivity to exposure, density, moisture and residual geometry should be reported rather than hidden in a single char-depth value.[19][21]

A final review should separate the fire's fuel contribution from the member's structural performance. Timber can feed flames and smoke while the remaining core continues to support load, and those states change over time. Evaluating both is necessary for a complete account of timber in fire.[3][1]

The assessment should also distinguish material loss from loss of structural function. Charred material has virtually no strength, but the remaining core and connections can continue to support load for a period. Structural performance therefore depends on the residual system, not simply on whether charring is visible.[1][9]

For protected assemblies, the timing of each transition should be retained. Delayed char onset, a changed charring rate and later char oxidation affect the residual section in different ways. Combining them into one nominal protection factor would hide how the protection actually works.[1][15]

For occupants and adjacent property, timber's combustion products must be assessed as well as its structure. Heat release, soot, carbon monoxide and smouldering can continue after the core has weakened, and external flaming can move the exposure elsewhere. The member's structural condition does not by itself describe the fire hazard.[24][25]

A complete report therefore identifies the timber product, exposure, char-depth convention, residual geometry and structural criteria. It also states which protective and decay processes were included. This keeps the analysis tied to the actual assembly instead of treating the standard charring rate as a complete model.[4][21]

The fire exposure should also include its duration and spatial variation. Prolonged heating can advance pyrolysis and char even when the peak temperature is limited, and local hot regions can produce uneven residual geometry. A single average fire condition may not capture those effects on a large timber member.[19][1]

A final assessment should connect the residual section with the required structural function and exposure period. The remaining core, connections and load paths must satisfy the selected criteria at that time. This makes reduced cross-section and fire resistance practical evaluation tools rather than descriptions of appearance alone.[4][17]

For final acceptance, the complete load and thermal history should be compared with the member's required function. This makes reduced cross-section and fire resistance practical evaluation tools rather than descriptions of appearance alone.[4][17]

See also

Notes

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 White (2002), p. 4-260.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Kodur and Harmathy (2002), pp. 1-164–1-165.
  3. ↑ 3.0 3.1 Van Rossum (2027), §4.4.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 White (2002), pp. 4-260–4-261.
  5. ↑ 5.0 5.1 Van Rossum (2027), §3.1.
  6. ↑ Beyler and Hirschler (2002), p. 1-111.
  7. ↑ Kanury (2002), pp. 2-231–2-232.
  8. ↑ Beyler and Hirschler (2002), p. 1-112.
  9. ↑ 9.0 9.1 9.2 Fleischmann and Buchanan (2002), p. 4-242.
  10. ↑ 10.0 10.1 Rockett and Milke (2002), p. 1-29.
  11. ↑ 11.0 11.1 Ohlemiller (2002), p. 2-200.
  12. ↑ 12.0 12.1 Ohlemiller (2002), p. 2-208.
  13. ↑ 13.0 13.1 Beyler and Hirschler (2002), p. 1-110.
  14. ↑ Ohlemiller (2002), p. 2-204.
  15. ↑ 15.0 15.1 15.2 Ohlemiller (2002), p. 2-206.
  16. ↑ Gray (2002), p. 2-212.
  17. ↑ 17.0 17.1 17.2 Ministry of Business, Innovation and Employment (2025), p. 64.
  18. ↑ 18.0 18.1 White (2002), pp. 4-259–4-260.
  19. ↑ 19.0 19.1 19.2 Lie (2002), pp. 4-201–4-203.
  20. ↑ Cadorin et al. (2003), p. 430.
  21. ↑ 21.0 21.1 21.2 Van Rossum (2027), §1.7.
  22. ↑ Ministry of Business, Innovation and Employment (2025), p. 52.
  23. ↑ Kodur and Harmathy (2002), p. 1-156.
  24. ↑ Van Rossum (2027), §6.2.
  25. ↑ Purser (2002), pp. 2-85–2-86.

References

  • Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Cadorin, J.-F., Pintea, D., Dotreppe, J.-C., & Franssen, J.-M. (2003). A tool to design steel elements submitted to compartment fires: OZone V2. Part 2: Methodology and application. Fire Safety Journal, 38, 429–451.
  • Fleischmann, C., & Buchanan, A. (2002). Analytical methods for determining fire resistance of concrete members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Gray, B. (2002). Spontaneous combustion and self-heating. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). 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.). National Fire Protection Association.
  • Kodur, V. K. R., & Harmathy, T. Z. (2002). Properties of building materials. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Lie, T. T. (2002). Fire temperature-time relations. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification method C/VM2 (2nd ed.). New Zealand Government.
  • Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). 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.). National Fire Protection Association.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • White, R. H. (2002). Analytical methods for determining fire resistance of timber members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
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