In the article, the authors present the results of a study of a flat reinforced concrete beamless floor of an existing building. The experiment consisted of loading a slab in span with water tanks. There were 15 loading stages in total, on average 4 kN. During the experiment, data was obtained on the relative deformations of concrete and reinforcement in the span and on the support. Also, observations of the growth of vertical deformations and the growth of cracks in the span were introduced.