A series of full-scale room burn experiments has been conducted on fully-developed fires to study their destructive potentials in terms of the thermal response of room boundaries. It is shown that normalized heat load is a convenient measure of the destructive potential (severity) of fire, irrespective of the nature of the boundary elements. A mathematical model for calculating normalized heat load yields satisfactory predictions, given the amount of combustible material in the compartment, its geometry, the thermal properties of the boundaries, and the size of ventilation openings.