There is concern over the use of enhanced surface roughness values in the modelling of dense gas dispersion on process sites. We review the effect of roughness on dispersion and present the results of a set of dense gas dispersion wind tunnel experiments in which the roughness elements are large compared to the cloud depth. Measured concentrations and cloud widths are compared with the predictions of a dense gas model, HEGADAS, which was developed to predict dispersion over flat terrain. Model predictions are found to be in good agreement with measurement using appropriate values for the aerodynamic roughness length of up to about 0.5 m at full scale. Interactions between roughness elements and the flow field need to be taken into account when comparing model predictions for boundary layers with a greater roughness length.