The protection of the future lunar base from radiation, thermal stresses and impacts of meteoroids can be achieved by several options, generally consisting in a shielding system, made of either regolith or combined materials. In the present paper, the incidence of two sources of uncertainty on the FEM calculation of stress propagation through the covering regolith layer has been assessed. First, the investigation has pointed out, for a given impact and a given constitutive model, the uncertainty in the stress prediction related to the strain interval adopted for the soil stiffness measurement. Thereafter, calculation has been performed, for a given stiffness value, changing the assumed collision duration of one order of magnitude for equal impact momentum, that is, changing the maximum impact force too, correspondingly. The simulation has been performed based on physical and mechanical parameters of DNA-1A lunar simulant. The results provide indication of the relative importance of the calculation assumptions, which could address the design of a regolith shield.