Timber in fire

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.

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.

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.

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.

The released gases can burn in flames outside the timber surface, and 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.

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.

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.

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.

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, and these are historical modelling conventions in the 2002 handbook. They are not complete contemporary design rules for every timber product.

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.

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.

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, but 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.

$$d_{\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}}=\beta t$$

Symbol	Description	Unit
$d_{\mathrm{c}\mathrm{h}\mathrm{a}\mathrm{r}}$	Depth of char measured from the original exposed face	mm
$\beta$	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.

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.

Protection introduces another limit. If a membrane delays the onset of charring, total furnace time and actual charring time differ, and 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.

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, and 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.

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.

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.

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.

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, and 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.

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.

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.

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.

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, and their type and thickness influence how long they remain effective. The performance of their fasteners and joints is part of the assembled construction.

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.

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.

Protective effects can continue after the onset of charring. White reports experiments in which gypsum board both delayed charring and reduced its subsequent extent, and 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.

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.

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.

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.

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.

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.

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, and 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.

Exposure limits and model evidence

The standard furnace exposure provides a repeatable heating history, while 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.

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.

More detailed models follow energy storage, conduction and the energy effects of decomposition. Some also represent the movement of decomposition gases or moisture, and 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.

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.

See also

• Structural fire engineering
• Strength reduction at elevated temperatures
• Fire testing

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.