While timber is indeed a combustible material, in construction it has significant insulating properties and burns in a slow, predictable and measurable way. These factors see timber perform strongly against fire and give designers the ability to confidently create strong, durable, fire resistant timber constructions.
When exposed to the heat of a fire, timber goes through a process of thermal breakdown into combustible gases. During this process, a layer of charcoal forms on the burning surface of the timber and it is this charred layer that is the key contributing factor in timber's fire resistance. The layer acts as an insulator protecting the inner core of the timber, making it resist heat penetration and thus burn more slowly; while the temperature of the inner, uncharred core remains low, enabling it to continue to carry its load. Initially the rate of charring is fast but as the char depth increases it provides a stronger protective layer to the timber, slowing the overall combustion rate.
The self protecting nature of the charring layer increases the likelihood of a timber structure surviving fire as the uncharred inner core remains unaffected, maintaining its strength and with it the structure's stability.