The study investigates the determination of selected fire properties of spruce wood, specifically the charring rate, using a modified testing method described and registered at the Industrial Property Office of the Slovak Republic PUV 50121-2020, utility model no. 9373. The samples were exposed to a square ceramic infrared heater, FTE-750W, with a power output of 750 W, using which we determined the heat flux as a function of voltage (V). Spruce wood specimens with dimensions of 75 mm × 75 mm × 50 mm (l × w × h) were subjected to thermal exposure under heat fluxes of 10, 15, 20, and 25 kW∙m⁻². The charring rate was evaluated using two distinct approaches: the first method measured the thickness of the char layer formed after a duration of 1800 s, while the second method was based on reaching a temperature threshold of 300 °C. The findings demonstrated a positive correlation between the thermal load and the charring rate. The charring rates obtained using the first method ranged from 0.2397 to 0.6933 mm∙min⁻¹, whereas those derived from the second method varied from 0 to 1.0344 mm∙min⁻¹. This suggests that the 300 °C temperature criterion may not be a reliable parameter for calculating the charring rate. The precision of the results was corroborated through numerical simulations.