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'''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.<ref name="White-4-260">White (2002), p. 4-260.</ref><ref name="Kodur-Harmathy-1-164">Kodur and Harmathy (2002), pp. 1-164–1-165.</ref>
'''Timber in fire''' describes the heating, decomposition, charring and structural response of wood members and assemblies during fire exposure. Timber can supply fuel to a fire while an interior portion continues to carry load. Its endurance depends on protective layers, the extent of charring and the capacity of the remaining uncharred section.<ref name="KH_1-157">Kodur and Harmathy (2002), p. 1-157.</ref><ref name="W_4-257">White (2002), p. 4-257.</ref>


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.<ref name="White-4-260" /><ref name="Kodur-Harmathy-1-164" />
The charred surface and the intact-looking interior perform different roles. Char has very little load-bearing capacity, while the remaining wood can also lose performance as it heats. Charring rates depend on exposure, grain direction and material properties. A single rate therefore describes a particular approximation rather than every timber fire.<ref name="W_4-260_4-261">White (2002), pp. 4-260–4-261.</ref>


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.<ref name="VR-4.4">Van Rossum (2027), §4.4.</ref><ref name="White-4-261">White (2002), pp. 4-260–4-261.</ref>
Assessment connects these material changes to the whole construction. Membranes and their attachments affect heating, while member geometry and loading determine the significance of section loss. Standard furnace results provide evidence for specified assemblies. Applying them to another arrangement requires attention to the conditions and limits of the test or calculation method.<ref name="W_4-257_4-259">White (2002), pp. 4-257–4-259.</ref>


== Pyrolysis and degradation zones ==
== Heating, decomposition and combustion ==


[[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.<ref name="VR-3.1">Van Rossum (2027), §3.1.</ref><ref name="Beyler-Hirschler-1-111">Beyler and Hirschler (2002), p. 1-111.</ref>
Heating initially raises the temperature of the exposed timber surface and then affects material farther inside. Moisture evaporation absorbs some of the incoming energy. At higher temperatures, [[Pyrolysis]] breaks down the wood and releases gases while leaving a carbon-rich char. These processes change both the material properties and the geometry of the remaining solid.<ref name="VR_5.2">Van Rossum (2027), §5.2.</ref><ref name="W_4-260">White (2002), p. 4-260.</ref>


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.<ref name="White-4-260" />
The released gases can burn in flames outside the timber surface. The char can also react with oxygen through glowing combustion. White explains that oxidation and mechanical disintegration can progressively erode the outer char layer. The surface is therefore not a permanent, unchanged insulating shell throughout every exposure.<ref name="W_4-260" />
[[File:fse-timber-in-fire-v1.svg|thumb|upright=1.5|alt=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.<ref name="Kodur-Harmathy-1-164" /><ref name="Kanury-2-231">Kanury (2002), pp. 2-231–2-232.</ref>
[[Smouldering]] differs from pyrolysis alone. Ohlemiller defines it as flameless combustion sustained by heat from oxygen reacting with the solid fuel. A material can release smoke while externally heated without sustaining that oxidation process. The appearance of smoke therefore does not, by itself, identify the combustion mechanism taking place.<ref name="O_2-200">Ohlemiller (2002), p. 2-200.</ref>


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.<ref name="Beyler-Hirschler-1-112">Beyler and Hirschler (2002), p. 1-112.</ref><ref name="White-4-260" />
White distinguishes an outer char layer, a zone undergoing decomposition and the inner wood. The base of the char layer is commonly associated with a temperature around 300 °C in the methods he describes. This boundary is useful for discussing section loss. It is not a statement that every point inside the remaining wood retains its original properties.<ref name="W_4-260" />


== Charring rate ==
[[File:fse-timber-in-fire-v1.svg|thumb|upright=1.5|alt=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.]]


[[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.<ref name="Kodur-Harmathy-1-164" /><ref name="White-4-260" />
== Charring rate and grain direction ==


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.<ref name="Kodur-Harmathy-1-164" /><ref name="White-4-261" />
[[Charring rate]] describes how rapidly the boundary between char and uncharred wood advances. White explains that the rate can be relatively high initially and then become approximately steady during a standard exposure. A constant-rate description is therefore an empirical approximation. It represents a selected exposure and material response rather than a universal chemical constant.<ref name="W_4-260" />
[[File:fse-timber-in-fire-v2.svg|thumb|upright=1.5|alt=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.<ref name="Kodur-Harmathy-1-164" /><ref name="White-4-261" />
For transverse-to-grain charring under the standard ASTM E119 exposure, White describes a commonly used approximation of 0.6 mm/min. He also reports that charring parallel to the grain is approximately twice as fast. These are historical modelling conventions in the 2002 handbook. They are not complete contemporary design rules for every timber product.<ref name="W_4-260" />


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.<ref name="White-4-261" />
[[File:fse-timber-in-fire-v2.svg|thumb|upright=1.5|alt=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. 4-260.]]


== Mass timber and residual section ==
Density, moisture, species, chemical composition and permeability affect charring. Greater moisture also changes the energy needed before the wood decomposes. Kodur and Harmathy report study values spanning approximately 0.137 to 0.85 mm/min under differing conditions. That range demonstrates variation across evidence; it is not an uncertainty band to apply around every design value.<ref name="KH_1-164_1-165">Kodur and Harmathy (2002), pp. 1-164–1-165.</ref>


[[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.<ref name="White-4-260" /><ref name="White-4-261" />
[[File:fse-timber-in-fire-v3.svg|thumb|upright=1.5|alt=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.]]
 
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.<ref name="White-4-260" /><ref name="White-4-261" />
 
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.<ref name="Fleischmann-Buchanan-4-242">Fleischmann and Buchanan (2002), p. 4-242.</ref><ref name="White-4-260" />
 
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.<ref name="Rockett-Milke-1-29">Rockett and Milke (2002), p. 1-29.</ref><ref name="White-4-260" />


== Smouldering and char oxidation ==
The direction and number of exposed faces change the shape of the remaining section. Corners exposed from adjacent faces become rounded as charring proceeds. White describes an approximate corner radius related to the char depth along the sides. Simply subtracting a rectangular layer can therefore miss geometry relevant to the residual section.<ref name="W_4-260" />


[[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.<ref name="Ohlemiller-2-200">Ohlemiller (2002), p. 2-200.</ref><ref name="Ohlemiller-2-208">Ohlemiller (2002), p. 2-208.</ref>
== A worked charring example ==


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.<ref name="Beyler-Hirschler-1-110">Beyler and Hirschler (2002), p. 1-110.</ref><ref name="Ohlemiller-2-200" />
For a deliberately simplified constant-rate calculation, char depth is the rate multiplied by exposure time. The relationship describes advance from one exposed face in the selected direction. It does not calculate the load capacity of the remaining section. Its applicability depends on whether the assumed rate continues to represent the exposure and member.<ref name="W_4-260_4-261" />


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.<ref name="Ohlemiller-2-204">Ohlemiller (2002), p. 2-204.</ref>
<math display="block">d_{\mathrm{char}}=\beta t</math>


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.<ref name="Ohlemiller-2-206">Ohlemiller (2002), p. 2-206.</ref><ref name="Gray-2-212">Gray (2002), p. 2-212.</ref>
{| class="wikitable"
! Symbol !! Description !! Unit
|-
| <math>d_{\mathrm{char}}</math> || Depth of char measured from the original exposed face || mm
|-
| <math>\beta</math> || Assumed constant charring rate || mm/min
|-
| <math>t</math> || Time under the assumed exposure || min
|}


== Fire resistance and protection ==
Using White's transverse-to-grain approximation of 0.6 mm/min for an illustrative 60 min exposure gives a char depth of 36 mm. This is the arithmetic consequence of the adopted constant rate. It is not a measured result for a particular beam, nor does it establish a 60 min structural rating. The remaining section and its temperature-dependent capacity still require assessment.<ref name="W_4-257" /><ref name="W_4-259_4-261">White (2002), pp. 4-259–4-261.</ref>


[[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.<ref name="White-4-260" /><ref name="CVM2-64">Ministry of Business, Innovation and Employment (2025), p. 64.</ref>
The same calculation cannot be extended indefinitely into a small member. White explains that heating of the centre can increase the charring rate as the residual section becomes thin. The approximation also omits corner rounding and any change in the exposed surface. Those effects can become influential before a simple geometric subtraction predicts complete consumption.<ref name="W_4-260" /><ref name="W_4-264">White (2002), p. 4-264.</ref>


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.<ref name="White-4-260" />
Protection introduces another limit. If a membrane delays the onset of charring, total furnace time and actual charring time differ. The membrane can also affect the subsequent heating rate after decomposition begins. Subtracting one assumed delay from every protected assembly does not reproduce all the protective behaviour described in the handbook.<ref name="W_4-257" /><ref name="W_4-259_4-260">White (2002), pp. 4-259–4-260.</ref>
[[File:fse-timber-in-fire-v3.svg|thumb|upright=1.5|alt=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.<ref name="White-4-259">White (2002), pp. 4-259–4-260.</ref>
== The remaining section and structural capacity ==


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.<ref name="White-4-260" /><ref name="Beyler-Hirschler-1-110">Beyler and Hirschler (2002), p. 1-110.</ref>
Char has very little load-bearing capacity in the structural assessment described by White. Resistance therefore depends primarily on the remaining uncharred wood and its condition. The [[Reduced cross-section method]] expresses the related idea of evaluating a section after fire-related losses. A geometric char-depth estimate supplies only part of that assessment.<ref name="W_4-257" /><ref name="W_4-260" />


== Fire exposure and modelling ==
The residual section can be warm even where it has not converted to char. Under natural exposure, White notes that heating during the cooling period can affect more than a narrow outer layer. Residual dimensions and temperature consequently describe different aspects of the member. Treating all uncharred wood as unchanged would omit that thermal history.<ref name="W_4-264" />


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.<ref name="Kodur-Harmathy-1-164" /><ref name="Lie-4-201">Lie (2002), pp. 4-201–4-203.</ref>
Member size influences the relationship between those effects. Large solid and glued timber members can retain a substantial interior while their exposed surfaces char. These members connect the subject to [[Mass timber]], although a calculation for one solid or glued member does not establish every assembly's behaviour. White's example methods specify material, dimensional and exposure limits.<ref name="W_4-261_4-264">White (2002), pp. 4-261–4-264.</ref>


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.<ref name="White-4-260" /><ref name="Rockett-Milke-1-29" />
Loading remains part of the structural problem. White identifies the capacity of the remaining wood as a separate contribution from the membrane and the char depth. A member can lose structural function before all its wood is consumed. Conversely, visible charring does not by itself specify the time at which the applied load can no longer be carried.<ref name="W_4-257" /><ref name="W_4-260" />


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.<ref name="White-4-260" /><ref name="Cadorin-Franssen-430">Cadorin et al. (2003), p. 430.</ref>
== Heat penetration is not char depth ==


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.<ref name="VR-1.7">Van Rossum (2027), §1.7.</ref><ref name="White-4-261" />
Van Rossum uses thermal diffusivity to illustrate how a temperature disturbance penetrates a thick wall. Diffusivity combines conductivity, density and specific heat. The characteristic depth grows with the square root of time when those properties are held constant. This thermal estimate differs from the constant-rate char-depth model because it describes another physical quantity.<ref name="VR_5.2" />


== Limits and common misunderstandings ==
The source's timber example uses conductivity of 0.13 W/(m·K), density of 500 kg/m³ and specific heat of 1,600 J/(kg·K). Their combination gives a diffusivity of 1.63 × 10⁻⁷ m²/s. After 3,600 s, the characteristic thermal penetration depth is about 24 mm. The calculation is a simplified comparison using fixed properties, not a prediction of the char boundary.<ref name="VR_5.2" />


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.<ref name="White-4-260" /><ref name="VR-3.1" />
That 24 mm result and the earlier 36 mm char estimate should not be combined as if they described one tested member. They come from different simplifying models with different purposes. The penetration model does not include the changing geometry and properties caused by charring. A model intended to follow both processes requires additional treatment of decomposition and heat transfer.<ref name="VR_5.2" /><ref name="W_4-264" />


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.<ref name="White-4-260" /><ref name="Kodur-Harmathy-1-164" />
The physical distinction also explains why a cool interior can coexist with a damaged surface. Timber does not heat uniformly through a thick section at the beginning of an exposure. However, the depth and magnitude of that temperature gradient change with time. The available interior cannot be characterised solely by its original dimensions or by a surface-temperature observation.<ref name="VR_5.2" /><ref name="W_4-264" />


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.<ref name="White-4-261" /><ref name="Kodur-Harmathy-1-164" />
== Membranes and protected assemblies ==


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.<ref name="White-4-260" /><ref name="CVM2-52">Ministry of Business, Innovation and Employment (2025), p. 52.</ref>
A protective membrane is the first thermal barrier in many timber assemblies. White describes gypsum wallboard and plywood panelling as examples within historical test and calculation methods. Their type and thickness influence how long they remain effective. The performance of their fasteners and joints is part of the assembled construction.<ref name="W_4-257_4-258">White (2002), pp. 4-257–4-258.</ref>


== Evidence and application ==
White's component additive method assigns contributions to specified membranes, framing and other permitted features. The assignments come from correlations with ASTM E119 assembly tests. They are not independently measured endurance times that can be added for arbitrary products. The method includes construction details and limits on the assemblies to which it applies.<ref name="W_4-258_4-259">White (2002), pp. 4-258–4-259.</ref>


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.<ref name="White-4-260" /><ref name="Kodur-Harmathy-1-156">Kodur and Harmathy (2002), p. 1-156.</ref>
Adding a layer or filling a cavity can change heat transfer and attachment behaviour. White notes that insulation may improve some wall assemblies while producing different effects in floors or roofs. Changes in member spacing, internal finish and membrane arrangement can also fall outside the tested configuration. More material does not therefore create an automatic increase in the established rating.<ref name="W_4-257_4-259" />


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.<ref name="White-4-261" /><ref name="Kodur-Harmathy-1-164" />
Protective effects can continue after the onset of charring. White reports experiments in which gypsum board both delayed charring and reduced its subsequent extent. This distinction matters when representing a membrane by a simple time delay. A delay-only model can omit part of the interaction between the board, the timber and the developing fire exposure.<ref name="W_4-260" />


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.<ref name="White-4-261" /><ref name="Fleischmann-Buchanan-4-242" />
== Reaction to fire and fire resistance ==


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.<ref name="White-4-260" />
Timber's contribution as fuel and its performance as a structural member are separate questions. [[Fire resistance]] concerns maintaining specified assembly functions during a defined exposure. Surface burning behaviour concerns the contribution to developing fire conditions. A material can therefore require assessment for both its combustion behaviour and its remaining structural capacity.<ref name="KH_1-157" /><ref name="W_4-257" />


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.<ref name="Ohlemiller-2-206" /><ref name="Ohlemiller-2-208" />
A fire-retardant surface treatment does not automatically provide structural protection. White distinguishes treatments aimed at surface flame spread from protection that reduces the heating of a member. Some treatments can change decomposition behaviour without producing the expected structural benefit. The relevant evidence depends on which outcome is being claimed.<ref name="W_4-259">White (2002), p. 4-259.</ref><ref name="W_4-261">White (2002), p. 4-261.</ref>


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.<ref name="White-4-260" /><ref name="White-4-259" />
Regulatory treatment also depends on the use of the timber. New Zealand's C/VM2, second edition of 2025, lists specific exceptions within its internal surface-lining scenario. Its distinction between certain structural timber elements and exposed timber panels is part of that defined scope. It does not remove the separate need to evaluate the structural fire performance of an assembly.<ref name="CVM_50">Ministry of Business, Innovation and Employment (2025), p. 50.</ref>


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.<ref name="Lie-4-201" /><ref name="VR-1.7" />
The same New Zealand document uses ordinary timber products as examples associated with lining Group 3. Many fire-retardant-treated timbers are examples associated with Group 2. Those explanatory groups concern surface-lining performance within that method. They are not charring rates or structural endurance ratings for every product within either description.<ref name="CVM_52">Ministry of Business, Innovation and Employment (2025), p. 52.</ref>


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.<ref name="VR-4.4" /><ref name="White-4-260" />
Natural-fire calculations also require attention to the period after the highest gas temperature. White discusses changes in the residual timber section during cooling, when the interior temperature distribution continues to evolve. The mechanical condition cannot therefore be inferred solely from the time of the external temperature peak. A model following the complete exposure addresses a different question from one stopping at that peak.<ref name="W_4-264" />


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.<ref name="White-4-260" /><ref name="Fleischmann-Buchanan-4-242" />
This distinction separates extinguishing flames from completing a structural assessment. The source's nonstandard-fire discussion considers the subsequent temperature history within the remaining section. Its implications depend on the member, exposure and adopted material model. A calculation of maximum char depth alone does not describe every temperature-dependent change in the uncharred wood.<ref name="W_4-264" />


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.<ref name="White-4-260" /><ref name="Ohlemiller-2-206" />
== Exposure limits and model evidence ==


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.<ref name="VR-6.2">Van Rossum (2027), §6.2.</ref><ref name="Purser-2-85">Purser (2002), pp. 2-85–2-86.</ref>
The standard furnace exposure provides a repeatable heating history. A natural compartment fire instead depends on combustible contents, openings and enclosure behaviour. Van Rossum explains that a standard temperature curve does not reproduce every real fire's growth and decay. A charring model calibrated to one exposure consequently needs justification before being transferred to another.<ref name="VR_8.6">Van Rossum (2027), §8.6.</ref><ref name="W_4-261_4-262">White (2002), pp. 4-261–4-262.</ref>


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.<ref name="White-4-261" /><ref name="VR-1.7" />
White describes empirical models for nonstandard exposure with explicit limits on duration, member dimensions and fire conditions. Such limits are part of the equations, not optional additions to them. His worked example checks whether the calculated depth remains within the model's dimensional range. Once the residual section heats more widely, the earlier approximation can cease to apply.<ref name="W_4-262_4-264">White (2002), pp. 4-262–4-264.</ref>


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.<ref name="Lie-4-201" /><ref name="White-4-260" />
More detailed models follow energy storage, conduction and the energy effects of decomposition. Some also represent the movement of decomposition gases or moisture. Their flexibility allows a wider range of exposure histories, but it increases the demand for suitable physical data. White identifies both modelling the interacting processes and obtaining those data as difficulties in predicting timber behaviour.<ref name="W_4-264" />


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.<ref name="White-4-261" /><ref name="CVM2-64">Ministry of Business, Innovation and Employment (2025), p. 64.</ref>
Assembly evidence remains specific to the tested construction. A membrane, adhesive system or arrangement of timber pieces can change the sequence of heating and section loss. White's descriptions of solid and glued members therefore support the conditions he specifies. They do not establish a universal response for every later timber product or composite assembly.<ref name="W_4-257" /><ref name="W_4-261_4-264" />
 
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.<ref name="White-4-261" /><ref name="CVM2-64">Ministry of Business, Innovation and Employment (2025), p. 64.</ref>


== See also ==
== See also ==


* [[Structural fire engineering]]
* [[Strength reduction at elevated temperatures]]
* [[Strength reduction at elevated temperatures]]
* [[Structural fire engineering]]
* [[Fire testing]]
* [[Mass timber]]


== Notes ==
== Notes ==
Line 122: Line 119:
== References ==
== 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.
* 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., pp. 1-155–1-181). 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.
* Ministry of Business, Innovation and Employment. (2025). ''C protection from fire: Verification method C/VM2'' (2nd ed.).
* 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.
* Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-200–2-210). 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.
* 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.
* 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., pp. 4-257–4-273). National Fire Protection Association.


[[Category:Structural Fire Engineering]]
[[Category:Structural Fire Engineering]]

Revision as of 18:54, 2 November 2026

Timber in fire describes the heating, decomposition, charring and structural response of wood members and assemblies during fire exposure. Timber can supply fuel to a fire while an interior portion continues to carry load. Its endurance depends on protective layers, the extent of charring and the capacity of the remaining uncharred section.[1][2]

The charred surface and the intact-looking interior perform different roles. Char has very little load-bearing capacity, while the remaining wood can also lose performance as it heats. Charring rates depend on exposure, grain direction and material properties. A single rate therefore describes a particular approximation rather than every timber fire.[3]

Assessment connects these material changes to the whole construction. Membranes and their attachments affect heating, while member geometry and loading determine the significance of section loss. Standard furnace results provide evidence for specified assemblies. Applying them to another arrangement requires attention to the conditions and limits of the test or calculation method.[4]

Heating, decomposition and combustion

Heating initially raises the temperature of the exposed timber surface and then affects material farther inside. Moisture evaporation absorbs some of the incoming energy. At higher temperatures, Pyrolysis breaks down the wood and releases gases while leaving a carbon-rich char. These processes change both the material properties and the geometry of the remaining solid.[5][6]

The released gases can burn in flames outside the timber surface. The char can also react with oxygen through glowing combustion. White explains that oxidation and mechanical disintegration can progressively erode the outer char layer. The surface is therefore not a permanent, unchanged insulating shell throughout every exposure.[6]

Smouldering differs from pyrolysis alone. Ohlemiller defines it as flameless combustion sustained by heat from oxygen reacting with the solid fuel. A material can release smoke while externally heated without sustaining that oxidation process. The appearance of smoke therefore does not, by itself, identify the combustion mechanism taking place.[7]

White distinguishes an outer char layer, a zone undergoing decomposition and the inner wood. The base of the char layer is commonly associated with a temperature around 300 °C in the methods he describes. This boundary is useful for discussing section loss. It is not a statement that every point inside the remaining wood retains its original properties.[6]

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.

Charring rate and grain direction

Charring rate describes how rapidly the boundary between char and uncharred wood advances. White explains that the rate can be relatively high initially and then become approximately steady during a standard exposure. A constant-rate description is therefore an empirical approximation. It represents a selected exposure and material response rather than a universal chemical constant.[6]

For transverse-to-grain charring under the standard ASTM E119 exposure, White describes a commonly used approximation of 0.6 mm/min. He also reports that charring parallel to the grain is approximately twice as fast. These are historical modelling conventions in the 2002 handbook. They are not complete contemporary design rules for every timber product.[6]

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. 4-260.

Density, moisture, species, chemical composition and permeability affect charring. Greater moisture also changes the energy needed before the wood decomposes. Kodur and Harmathy report study values spanning approximately 0.137 to 0.85 mm/min under differing conditions. That range demonstrates variation across evidence; it is not an uncertainty band to apply around every design value.[8]

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.

The direction and number of exposed faces change the shape of the remaining section. Corners exposed from adjacent faces become rounded as charring proceeds. White describes an approximate corner radius related to the char depth along the sides. Simply subtracting a rectangular layer can therefore miss geometry relevant to the residual section.[6]

A worked charring example

For a deliberately simplified constant-rate calculation, char depth is the rate multiplied by exposure time. The relationship describes advance from one exposed face in the selected direction. It does not calculate the load capacity of the remaining section. Its applicability depends on whether the assumed rate continues to represent the exposure and member.[3]

dchar=βt

Symbol Description Unit
dchar Depth of char measured from the original exposed face mm
β Assumed constant charring rate mm/min
t Time under the assumed exposure min

Using White's transverse-to-grain approximation of 0.6 mm/min for an illustrative 60 min exposure gives a char depth of 36 mm. This is the arithmetic consequence of the adopted constant rate. It is not a measured result for a particular beam, nor does it establish a 60 min structural rating. The remaining section and its temperature-dependent capacity still require assessment.[2][9]

The same calculation cannot be extended indefinitely into a small member. White explains that heating of the centre can increase the charring rate as the residual section becomes thin. The approximation also omits corner rounding and any change in the exposed surface. Those effects can become influential before a simple geometric subtraction predicts complete consumption.[6][10]

Protection introduces another limit. If a membrane delays the onset of charring, total furnace time and actual charring time differ. The membrane can also affect the subsequent heating rate after decomposition begins. Subtracting one assumed delay from every protected assembly does not reproduce all the protective behaviour described in the handbook.[2][11]

The remaining section and structural capacity

Char has very little load-bearing capacity in the structural assessment described by White. Resistance therefore depends primarily on the remaining uncharred wood and its condition. The Reduced cross-section method expresses the related idea of evaluating a section after fire-related losses. A geometric char-depth estimate supplies only part of that assessment.[2][6]

The residual section can be warm even where it has not converted to char. Under natural exposure, White notes that heating during the cooling period can affect more than a narrow outer layer. Residual dimensions and temperature consequently describe different aspects of the member. Treating all uncharred wood as unchanged would omit that thermal history.[10]

Member size influences the relationship between those effects. Large solid and glued timber members can retain a substantial interior while their exposed surfaces char. These members connect the subject to Mass timber, although a calculation for one solid or glued member does not establish every assembly's behaviour. White's example methods specify material, dimensional and exposure limits.[12]

Loading remains part of the structural problem. White identifies the capacity of the remaining wood as a separate contribution from the membrane and the char depth. A member can lose structural function before all its wood is consumed. Conversely, visible charring does not by itself specify the time at which the applied load can no longer be carried.[2][6]

Heat penetration is not char depth

Van Rossum uses thermal diffusivity to illustrate how a temperature disturbance penetrates a thick wall. Diffusivity combines conductivity, density and specific heat. The characteristic depth grows with the square root of time when those properties are held constant. This thermal estimate differs from the constant-rate char-depth model because it describes another physical quantity.[5]

The source's timber example uses conductivity of 0.13 W/(m·K), density of 500 kg/m³ and specific heat of 1,600 J/(kg·K). Their combination gives a diffusivity of 1.63 × 10⁻⁷ m²/s. After 3,600 s, the characteristic thermal penetration depth is about 24 mm. The calculation is a simplified comparison using fixed properties, not a prediction of the char boundary.[5]

That 24 mm result and the earlier 36 mm char estimate should not be combined as if they described one tested member. They come from different simplifying models with different purposes. The penetration model does not include the changing geometry and properties caused by charring. A model intended to follow both processes requires additional treatment of decomposition and heat transfer.[5][10]

The physical distinction also explains why a cool interior can coexist with a damaged surface. Timber does not heat uniformly through a thick section at the beginning of an exposure. However, the depth and magnitude of that temperature gradient change with time. The available interior cannot be characterised solely by its original dimensions or by a surface-temperature observation.[5][10]

Membranes and protected assemblies

A protective membrane is the first thermal barrier in many timber assemblies. White describes gypsum wallboard and plywood panelling as examples within historical test and calculation methods. Their type and thickness influence how long they remain effective. The performance of their fasteners and joints is part of the assembled construction.[13]

White's component additive method assigns contributions to specified membranes, framing and other permitted features. The assignments come from correlations with ASTM E119 assembly tests. They are not independently measured endurance times that can be added for arbitrary products. The method includes construction details and limits on the assemblies to which it applies.[14]

Adding a layer or filling a cavity can change heat transfer and attachment behaviour. White notes that insulation may improve some wall assemblies while producing different effects in floors or roofs. Changes in member spacing, internal finish and membrane arrangement can also fall outside the tested configuration. More material does not therefore create an automatic increase in the established rating.[4]

Protective effects can continue after the onset of charring. White reports experiments in which gypsum board both delayed charring and reduced its subsequent extent. This distinction matters when representing a membrane by a simple time delay. A delay-only model can omit part of the interaction between the board, the timber and the developing fire exposure.[6]

Reaction to fire and fire resistance

Timber's contribution as fuel and its performance as a structural member are separate questions. Fire resistance concerns maintaining specified assembly functions during a defined exposure. Surface burning behaviour concerns the contribution to developing fire conditions. A material can therefore require assessment for both its combustion behaviour and its remaining structural capacity.[1][2]

A fire-retardant surface treatment does not automatically provide structural protection. White distinguishes treatments aimed at surface flame spread from protection that reduces the heating of a member. Some treatments can change decomposition behaviour without producing the expected structural benefit. The relevant evidence depends on which outcome is being claimed.[15][16]

Regulatory treatment also depends on the use of the timber. New Zealand's C/VM2, second edition of 2025, lists specific exceptions within its internal surface-lining scenario. Its distinction between certain structural timber elements and exposed timber panels is part of that defined scope. It does not remove the separate need to evaluate the structural fire performance of an assembly.[17]

The same New Zealand document uses ordinary timber products as examples associated with lining Group 3. Many fire-retardant-treated timbers are examples associated with Group 2. Those explanatory groups concern surface-lining performance within that method. They are not charring rates or structural endurance ratings for every product within either description.[18]

Natural-fire calculations also require attention to the period after the highest gas temperature. White discusses changes in the residual timber section during cooling, when the interior temperature distribution continues to evolve. The mechanical condition cannot therefore be inferred solely from the time of the external temperature peak. A model following the complete exposure addresses a different question from one stopping at that peak.[10]

This distinction separates extinguishing flames from completing a structural assessment. The source's nonstandard-fire discussion considers the subsequent temperature history within the remaining section. Its implications depend on the member, exposure and adopted material model. A calculation of maximum char depth alone does not describe every temperature-dependent change in the uncharred wood.[10]

Exposure limits and model evidence

The standard furnace exposure provides a repeatable heating history. A natural compartment fire instead depends on combustible contents, openings and enclosure behaviour. Van Rossum explains that a standard temperature curve does not reproduce every real fire's growth and decay. A charring model calibrated to one exposure consequently needs justification before being transferred to another.[19][20]

White describes empirical models for nonstandard exposure with explicit limits on duration, member dimensions and fire conditions. Such limits are part of the equations, not optional additions to them. His worked example checks whether the calculated depth remains within the model's dimensional range. Once the residual section heats more widely, the earlier approximation can cease to apply.[21]

More detailed models follow energy storage, conduction and the energy effects of decomposition. Some also represent the movement of decomposition gases or moisture. Their flexibility allows a wider range of exposure histories, but it increases the demand for suitable physical data. White identifies both modelling the interacting processes and obtaining those data as difficulties in predicting timber behaviour.[10]

Assembly evidence remains specific to the tested construction. A membrane, adhesive system or arrangement of timber pieces can change the sequence of heating and section loss. White's descriptions of solid and glued members therefore support the conditions he specifies. They do not establish a universal response for every later timber product or composite assembly.[2][12]

See also

Notes

  1. ↑ 1.0 1.1 Kodur and Harmathy (2002), p. 1-157.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 White (2002), p. 4-257.
  3. ↑ 3.0 3.1 White (2002), pp. 4-260–4-261.
  4. ↑ 4.0 4.1 White (2002), pp. 4-257–4-259.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Van Rossum (2027), §5.2.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 White (2002), p. 4-260.
  7. ↑ Ohlemiller (2002), p. 2-200.
  8. ↑ Kodur and Harmathy (2002), pp. 1-164–1-165.
  9. ↑ White (2002), pp. 4-259–4-261.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 White (2002), p. 4-264.
  11. ↑ White (2002), pp. 4-259–4-260.
  12. ↑ 12.0 12.1 White (2002), pp. 4-261–4-264.
  13. ↑ White (2002), pp. 4-257–4-258.
  14. ↑ White (2002), pp. 4-258–4-259.
  15. ↑ White (2002), p. 4-259.
  16. ↑ White (2002), p. 4-261.
  17. ↑ Ministry of Business, Innovation and Employment (2025), p. 50.
  18. ↑ Ministry of Business, Innovation and Employment (2025), p. 52.
  19. ↑ Van Rossum (2027), §8.6.
  20. ↑ White (2002), pp. 4-261–4-262.
  21. ↑ White (2002), pp. 4-262–4-264.

References

  • 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., pp. 1-155–1-181). National Fire Protection Association.
  • Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification method C/VM2 (2nd ed.).
  • Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-200–2-210). 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., pp. 4-257–4-273). National Fire Protection Association.
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