In smoke management design, the characteristics of a horizontally flowing hot gas layer below the spill edge of a thermal spill plume are key input parameters for empirical and analytical entrainment calculation methods. All of these methods have been derived for a flow emerging from a free spill edge, where there is no wall vertically extending above the edge. This work presents experimental results using physical scale modeling to characterize the horizontal flow of gases below a flat spill edge of a spill plume, both with and without a vertical wall projecting above the edge. The presence of a vertical wall above the spill edge can affect the behavior of the plume beyond the edge which in turn affects the characteristics of the flow below the edge. This article presents an assessment of the performance of current calculation methods to predict the mass flow rate of gases below a spill edge for the range of geometries examined and proposes modifications where necessary. (C) 2010 Elsevier Ltd. All rights reserved.