The evaluation of slamming loads on structures is a challenging problem in both civil and industrial engineering. For instance, waves on coastal works and ship structures may occasionally induce high pressure peaks, causing significant stresses, despite their short duration. Similar problems arise in debris flow impact on structures. The work here presented is aimed at clarifying some aspects of a corrected version of the Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) technique which need to be re-examined in order to improve the performance of the method in highly critical fluid-solid impact problems. The paper is particularly focused on the first stages – here specified as short term – of the phenomenon, where compressibility plays a key role. A test case is considered whereby a rectangular mass of fluid particles, 3.95 m long and 0.5 m high impacts against a vertical obstacle, with velocities ranging from 2 m/s to 10 m/s. The employed WCSPH code is applied with various computational options for the state equations and for the boundary conditions. Results show that the pressure distribution on the wall quickly increases as the impact process takes place, from the bottom to the free surface, with value which may reach up to tens of atmospheres according to fluid compressibility. The effects on the results of Tait equation parameters are investigated, as well as the effects of additional repulsive central forces – the so called "Lennard Jones" – on the boundary. Results should provide useful suggestions concerning the SPH practise for slamming loads.