Wood will burn when exposed to high enough temperatures and in the presence of oxygen. Thermal degradation of wood occurs in stages. The degradation process and the exact products of thermal degradation depend upon the rate of heating as well as temperatures. For wood in a fire:
- As the surface temperature of wood increases due to fire exposure, flammable vapors are produced and a char layer (burnt wood) is formed on the external surfaces.
- In the presence of fire, these flammable vapors ignite and contribute to the fire.
- As the char layer gets thicker, it insulates the remaining unburned wood and slows the rate of vapor production, thereby slowing the charring process.
Lumber, plywood, and other wood-based materials, including the components of I-joists, and trusses, exhibit a relatively narrow range of flame spread. Flame spread rates for engineered wood products, such as, laminated veneer lumber (LVL), parallel strand lumber (PSL) and laminated strand lumber (LSL) are within the same range as solid wood materials. Differences result from factors such as density, thickness, surface characteristics, and coatings or other chemicals applied, if any. Typically, at thicknesses greater than 1/4″, flame spread is almost independent of material thickness.
Wood exposed to fire develops an insulating layer of char that further slows wood degradation. The char layer contributes no strength to the remaining cross-section, but acts to insulate the underlying wood from further charring, thus retarding the char rate. The structural capacity of a wood member exposed to fire depends upon its unburned-wood cross-section. Accordingly, char rate is a major factor in the determination of the fire endurance of wood products.
The major chemical elements found in natural wood products are carbon, hydrogen, and oxygen. When thermally decomposed, these elements primarily produce carbon monoxide, carbon dioxide, and water. Where nitrogen or halogen containing compounds, such as adhesives and laminates, are added to make composite wood products, the potential for production of hydrogen cyanide and hydrogen halide exists during the burning process. However, the toxic potency of the smoke from these composite wood products is no higher than the smoke from natural wood, as shown by smoke toxicity test data supplied to New York State on a large number of wood species and products. A review of combustion toxicity by John Hall, NFPA, revealed that toxicants other than carbon monoxide are not a major problem in fire toxicity or overall fire safety.