A series of large-scale experiments has been conducted on under-ventilated mass timber room compartments with kerosene pool fires. To characterise the role of exposed timber walls on the fully-developed phase of the fire, the exposed surface area of mass timber was varied between experiments. Experiments with all timber surfaces protected were compared against compartments with the ceiling and one or two walls exposed. The results show that increasing the surface area of exposed timber alters the flow fields both inside the compartment and at the doorway. The momentum-driven flow fields control the spatial distribution of temperatures, surface heat fluxes, and the neutral plane. Temperatures, heat fluxes and velocities within the compartment are shown to be spatially heterogeneous in both horizontal and vertical planes, highlighting the complexity of the flow fields. This represents a significant divergence from the conventional assumptions for under-ventilated compartments of minimal velocities and homogeneous thermal conditions governed by the opening factor. The results demonstrate that the momentum-driven flow fields created by the burning walls control the compartment fire dynamics. Therefore, typical simplifications of the momentum equation in zone models are not valid for mass timber compartments, and these solutions should only be used in a conservative manner.