The flame propagation regimes for the stoichiometric hydrogen-air mixtures in an obstructed semi- confined flat layer have been numerically investigated in this paper. Conditions defining fast or sonic propagation regime were established as a function of the main dimensions characterizing the system and the layout of the obstacles. It was found that the major dependencies were the following: the thickness of the layer of H2-air mixture, the blockage ratio, and the distance between obstacles and the obstacle size. A parametric study was performed to determine the combination of the above variables prone to produce strong combustions. Finally, a criterion that separates experiments resulting in slow subsonic from fast sonic propagations regimes was proposed.