As factory-manufactured, ultra-light and highly optimised products for load-bearing frame structures, wooden I-joists provide an extended range of applications for light timber frame assemblies. The fire resistance of such wooden structural products may be the deciding factor limiting their wider use. Moreover, the current European design standard for timber structures in fire does not include guidance for I-joists. The development of charring coefficients is based on six model-scale furnace fire tests and an extensive series of thermal simulations. The results of fire tests and finite element simulations were compared. Thermal simulations were on the safe side. The charred cross-section areas obtained from thermal simulations were analysed. Charring coefficients were developed in accordance with the current draft of FprEN 1995-1-2. The charring coefficients were developed so that their combination would capture the simulation results as accurately as possible, therefore creating a conservative result compared to furnace tests. The charring coefficients proposed within this paper were validated against the model-scale fire tests. The new set of coefficients appropriately captures the available test data.