The aim of bachelor thesis was to design the RC slab of apartment building. It was necessary to design the slab in more alternatives. I decided to work out the cast in situ slab reinforced in two directions placed on masonry walls. Shape drawing and calculation of internal forces was elaborated from the dimensions of the choosen alternative. It was followed by design of reinforcement on beam model in RSTAB Dlubal program, that i drew in a reinforcement scheme. Then was simulated a 2D model in Scia Engineer, which was followed by reinforcement drawing for the lower and for the upper surface of the slab. Whole structure was verified for the serviceability limit state manually and in Scia Engineer program. In the serviceability limit state the deflection and crack width were calculated. The calculation proved that RC slab is stable, rigid and in accordance with the standards STN EN.