This research aims to determine the actual deflection of a concrete joist and to correlate the result with the theoretical deflection. This is based on the theories of elasticity as the double integration method, considering the homogeneous, isotropic and linearly elastic material from an ideal theoretical model. The construction of a concrete joist does not make a 100% homogeneous, isotropic and linearly elastic element, since its manufacture depends on many conditions such as the choice of aggregates, water, the manufacture of cement, tests carried out for the elaboration of the mix design, the operator who is going to perform the mix and the construction of the joist. The variation of the real deflection with respect to the theoretical one has been investigated. For this, 30 simple concrete joists and 30 reinforced concrete joists were manufactured. The dimensions of these joists were 15cm x15cmx53.5cm (b x h x L). The reinforcement of the last 30 joists was 4 Փ 1/4 "as longitudinal reinforcement and Փ 1/4" @ 0.10 m of transverse reinforcement. The joists were tested for flexion by measuring the maximum deflection and compared with the theoretical one, calculated by the double integration method, having much greater experimental results than those calculated with the theory of the double integration method, not being within the ranges expected in literature