In this paper, a combination of the Lagrangian meshfree method of SPH and Polyhedral DEM is presented to simulate the interaction between the free surface of water and solid objects possessing sharp edges and flat surfaces, such as armour units of breakwaters. Both SPH and DEM schemes are validated successfully against experimental data. The numerical scheme is utilized to inspect the stability of concrete cubic armours in rubble-mound breakwaters through systematic analyses with various geometrical parameters and environmental conditions. The numerical results regarding the required dimensions of the cubic blocks for providing stability of the armour units under the wave attack are compared with the existing empirical formulae. It is shown that armour units located at the still water level are more exposed to damage due to wave attack. The proposed numerical technique proved to be accurate and therefore it can be used for designing the armour blocks of the sloping breakwaters and also for inspecting the possible level of damage to the system of armour units.