The legislator, in the technical regulations of the last 50 years, has always imposed very stringent and invariable conditions for a reinforced concrete (r.c.) masonry deck to be assumed to be infinitely rigid in its own plane; among them, the required presence of a reinforced slab of thickness ≥  4 cm is very onerous. The article identifies the conditions under which a deck, equipped with such a slab even without reinforcement, may be assumed to be infinitely rigid in its own plane, illustrating the calculations carried out to achieve this result using a simplified numerical model and on the basis of the geometric characteristics of the construction and the stresses induced by the seismic action. It is hoped that the new technical regulations will make use of these results, thus simplifying the construction process in order to better align it with market requirements and historical and current construction practice.