Technical Overview: Fire Performance Properties of Timber

/ by Jon Smith / in Blogs

Technical Overview: Fire Performance Properties of Timber

Wood is a unique structural material because it possesses inherent, predictable fire-resistance properties. Unlike steel, which can lose structural integrity rapidly at high temperatures, wood protects itself through a natural charring process.

1. Fundamental Mechanisms of Resistance

The fire performance of timber is governed by its physical structure and chemical response to heat.

  • The Charring Effect: When exposed to flame, the outer layer of wood undergoes thermal decomposition to form char. This carbonaceous layer acts as an efficient insulator, significantly slowing heat penetration to the “cold” inner core, which retains its structural load-bearing capacity.

  • Pyrolysis and Ignition: Wood generally reaches ignition between 250°C and 300°C. During this phase, it releases flammable gases (flaming combustion) and solid char (smoldering combustion).

  • Thermal Mass & Density: High-density species offer superior fire resistance because they contain more solid fiber and less air, requiring more energy to sustain combustion.

2. Key Influencing Factors

Three primary variables dictate how a specific timber element will behave in a fire:

Factor Impact on Fire Resistance
Moisture Content Higher moisture levels delay ignition, as thermal energy is first consumed by the evaporation of water.
Density Denser hardwoods generally burn slower and more predictably than lighter softwoods.
Chemical Composition The presence of resins and lignin varies by species, affecting the rate of flame spread and heat release.

3. Engineering and Enhancements

Modern construction utilizes both chemical and structural engineering to maximize safety.

  • Mass Timber (CLT/Glulam): Large-scale engineered wood products take advantage of the charring effect. Because of their massive cross-sections, they char predictably, maintaining structural stability long enough to allow for safe evacuation.

  • Fire Retardant Treatments (FRT): Chemical applications can be used to improve fire ratings (reaching Class II or III). These treatments work by:

    • Reducing the production of volatile flammable gases.

    • Promoting the formation of protective char at lower temperatures.

Summary: While untreated wood is a combustible material, its predictable charring rate and high insulating value make it a reliable structural component when factored into fire-safe engineering and design.