The present work presents a numerical study of the neutral gas dynamics in the JET sub-divertor. A complex model of the sub-divertor geometry is implemented and successful comparisons between corresponding numerical and experimental data have been performed. The experimental data represent the neutral gas pressure obtained by a sub-divertor pressure gauge. The recently developed Divertor Gas Simulator (DIVGAS) which is based on the Direct Simulation Monte Carlo (DSMC) method is applied. DIVGAS is able to predict the behaviour of the flow including macroscopic quantities of practical interest as for instance the pressure, temperature and bulk velocity. The non-linear feedback of the sub-divertor gas flow on the divertor plasma vicinity is not taken into account. For all presented plasma cases, the deduced flow pattern is non-isothermal and covers the free molecular up to the transition flow regime. Furthermore, for low intermediate and high divertor density simulations, recirculation effects occur through gaps between the vertical target tiles, which seem to be two order of magnitude less compared with the recycling ion flux onto the divertor walls.