Timber, being an organic material, is inherently combustible. However, when evaluating its application in building services and maintenance, particularly in the context of safety and performance under fire conditions, the fire resistance of timber becomes a critical parameter. Fire resistance refers to the ability of a building material or structural element to withstand fire or give protection from it, for a specified period under standard conditions of heating and load. Despite being flammable, timber displays predictable behavior when exposed to fire, which can be leveraged in designing fire-resistant structures.

Fire resistance of timber is influenced by several factors such as the species of wood, moisture content, density, size of the section, type of treatment applied, and the presence of protective coverings. One of the most significant characteristics of timber under fire conditions is that it chars at a predictable and uniform rate. This charring process forms a layer of charcoal on the surface that acts as an insulator, slowing down the further combustion of the inner core. The typical charring rate for softwood is around 0.6 mm per minute, while hardwoods may char at a slightly slower rate. This predictable behavior allows engineers to calculate the residual load-bearing capacity of timber structures during fire.

The larger the cross-sectional area of the timber, the better its fire resistance. Massive timber elements such as glued laminated timber (glulam) and cross-laminated timber (CLT) have a substantial fire resistance due to their thickness. In contrast to steel, which weakens and loses strength rapidly when heated, thick timber sections retain significant structural integrity even when exposed to high temperatures. This property has led to increased interest in using engineered timber in modern construction, including in high-rise buildings.

Moisture content plays a significant role in determining the fire resistance of timber. Green wood, with high moisture content, resists ignition longer than dry wood because the heat energy is initially used to evaporate water from the wood. However, once dry, the timber becomes more susceptible to ignition and faster charring. In building construction, seasoned wood with a moisture content of 12% to 19% is commonly used, and such timber should be treated appropriately to enhance fire resistance.

One of the key strategies for improving the fire resistance of timber is through fire retardant treatments. These include impregnation with fire retardant chemicals under pressure, surface coatings with intumescent paints, and the use of protective cladding materials such as gypsum board or mineral wool. Intumescent paints swell up when exposed to heat, forming a charred layer that protects the timber