Fire behaviour: charring, ignition and reaction-to-fire classes

Wood in a fire

Wood does not burn as a solid. Heat breaks it down (pyrolysis) into combustible gases that ignite, leaving a carbon-rich char. Main pyrolysis begins above about 225 degrees C. Sources are mainly European and North American reviews; the portal article marks which is which.

Ignition is better described by heat flux than by one ignition temperature, because reported auto-ignition temperatures vary by more than 150 degrees C with test conditions. A review gives about 10-13 kW/m2 for piloted ignition and 25-33 kW/m2 for unpiloted ignition. Moisture delays ignition. So never quote one ignition temperature to a customer.

Why large timber sections are predictable

The charred layer has about 20 % of the density of the original wood and insulates: about 6 mm below the char line the temperature is only around 180 degrees C, so the wood behind it is largely intact and keeps most of its strength. Charring is a surface process advancing at a nearly constant rate:

• about 0.8 mm/min for light dry softwoods down to 0.4-0.5 mm/min for dense, moist hardwoods, averaging about 0.6 mm/min;
• Eurocode 5 (EN 1995-1-2) uses a design charring rate of 0.65 mm/min for solid softwood of at least 290 kg/m3 (second-hand from conference and secondary papers; the standard was not opened).

Small sections lose a much larger fraction of their cross-section, and a protective layer falling away accelerates charring. Real design uses the standard and the national annex, not this lesson.

Worked example: char depth

Charring rate 0.65 mm/min, standard fire, no protection.

• After 30 minutes: 0.65 x 30 = 19.5 mm per exposed face.
• After 60 minutes: 0.65 x 60 = 39 mm per exposed face.
• A softwood beam 120 mm wide by 240 mm deep, exposed on three sides (both sides and the bottom) for 60 minutes, has charred 39 mm from each exposed face. Remaining width: 120 - 2 x 39 = 42 mm. Remaining depth: 240 - 39 = 201 mm.

This ignores corner rounding and the fixed zero-strength layer that current practice adds to the char depth, so it is an illustration, not a design calculation. It shows why a 120 mm member keeps a usable core while a thin one would not.

Reaction to fire (Europe)

Reaction to fire is how a material contributes to fire growth. Europe classifies it in EN 13501-1 into A1, A2, B, C, D, E and F, with smoke classes s1-s3 and burning droplet classes d0-d2.

• Untreated wood and wood-based panels typically fall in class D.
• Fire retardant treatment (impregnation or coating) can lift wood into class B or C, but cannot make it non-combustible, and treated products must be tested and classified separately.
• Cross-laminated timber is classified D-s2,d0 for walls and ceilings under an EU delegated regulation when it meets thickness and density minimums (panel at least 54 mm, each layer at least 18 mm, mean density at least 350 kg/m3). Check the current legal text before quoting anything.
• "Fire-rated" depends on thickness, density and mounting, not on species alone.

Related thermal facts for the mill

• Wood insulates because it is porous: conductivity is lowest across the grain and in light species, and rises with moisture and density.
• Time for heat or moisture to move a distance L scales with L squared. Doubling thickness roughly quadruples the time; relative to 1 inch stock a report gives drying-time factors of about 4 for 2 inch and 9 for 3 inch.

Checklist before answering a fire question

• Is this a structural question (refer to EN 1995-1-2 and the national annex) or a reaction-to-fire claim (quote certified product documents only)?
• Which product, thickness, density and mounting?
• Is the wood treated, and does the certificate cover the end use (interior or exterior)?
• Is the information current? Euroclass regulations are updated.

Key points

• Wood chars at a nearly constant rate, about 0.6-0.65 mm/min, and the char insulates the core.
• Ignition depends on heat flux, moisture and test method; do not quote one ignition temperature.
• Untreated wood is typically Euroclass D; retardants can reach B or C but need certification.
• Doubling thickness roughly quadruples heat or drying time.
• Refer structural calculations to the standard and quote only certified classes.