Abstract: The impulse delivered to the surface of a building by a blast wave travelling perpendicular to the building surface is not usually the fully reflected impulse produced on an infinite surface. Instead, it is lessened by 'clearing; as expansion waves propagate inwards from the regions of lower pressure at the building's edges. The actual time-varying load delivered to any point on the building surface can be approximated by an instantaneous rise to the reflected (or oblique reflected) pressure followed by a positive phase foreshortened by the arrival of the expansion wave. The assumption in this clearing model is that building surface dimensions are similar, ensuring that reflected pressures are achieved across the whole surface parallel to the blast wavefront. For a tall building whose height is considerably greater than its width, expansion waves originating from the sides of the building will propagate inwards and reduce the (oblique) reflected pressure as the blast wave progresses up the building. This paper illustrates the phenomenon of clearing on tall buildings, identifying the key factors that govern the process. Results of numerical simulations are presented, supported by small-scale experiments in which a broad range of important parameters, such as stand-off and building width, were investigated.