Knowledge of the first principles defining fire behaviour in large enclosures remains limited despite their common use in modern tall buildings. The evolution of a fire in large enclosures can be defined by the relationship between the flame front and burnout velocities (VS/VBO). This relationship can be classified into three distinct fire spread modes being Mode 1 (VS/VBO → ∞), Mode 2 (VS/VBO > 1), and Mode 3 (VS/VBO ≈ 1). The mechanisms governing flame spread and burnout are investigated using four full-scale enclosure fire experiments with high porosity wood cribs with similar enclosure geometries. Flame and burnout front positions and velocities are estimated using video data. Velocities are affected by the heat feedback from the enclosure and smoke layer to the fuel. The spread velocity shows two regimes, a minimum external heat flux above which there is surface spread (q''s,min) and a heat flux that defines the onset of very rapid flame spread ((q''rs,crit)). A phenomenological model is developed to help identify the underlying mechanisms controlling the transition between the different spread modes. Both the model and data show that for wood cribs, the dependence of the burnout front velocity to the external radiation is weak, whereas the dependence of the flame spread velocity to the external and flame heat flux is strong. A transition from Mode 3 (VS/VBO ≈ 1) to Mode 2 (VS/VBO > 1) occurs with increasing external heat fluxes above q''s,min. The transition to Mode 1 (VS/VBO → ∞) is further defined once (q''rs,crit) is attained due to a sudden increase in the flame heat flux by changing the ventilation condition, or by significant increases in the external heat flux from the enclosure
