Building codes require all building systems to perform to the same level of safety, regardless of material used. Heavy timbers have a particular advantage in a fire because they char on the outside while retaining strength, slowing combustion and allowing time to evacuate the building.
Mass timber has an inherent resistance to fire. The char, created on the wood surface as it burns, helps protect and insulate unburnt wood below the charred layer. The unburnt portion of a thick member retains 85 to 90 percent of its strength. Hence, a wood member with a large cross-section can burn for a significant amount of time before its size is reduced to the point where it can no longer carry its assigned loads.