A zone model, which incorporates a recently developed horizontal ceiling/floor vent flow algorithm, was written to simulate a compartment fire ventilated only through an opening in the ceiling. Development of this algorithm makes it possible to characterize vertical flow through a horizontal vent in those scenarios where the standard vent flow model is invalid, i.e. where the difference in pressure across the horizontal vent is at or near zero and the density of the gas above the vent is greater than the gas below the vent. Model predictions are shown to be comparable to reduced scale experimental results.