The paper contains the results of experimental research on the elaboration of the composition and optimization of the properties of the self-levelling screed based on cement and sand, intended for the fulfilment of the intermediate layer of floors, including industrial enterprises with the aggressive environment; sedentary parts of tanks, basins; bunkers; different containers, etc. The reinforced self-levelling screed has an absolutely horizontal surface. It allows the application of finished layers of composite polymer materials with a uniform thickness, and the finished layers of floor materials in rolls or shunt pieces to be absolutely smooth. To carry out the experimental testing the design of experiments method was used, which is the most advanced instrument of planning, realization, analysis and optimization existing in the scientific field that requires experimental testing, which is capable of correctly choosing the experimental design to provide the user with mathematical properties of the matrix used to define it. The experiments was carried out according to the D-Optimal design with 5 factors, which allowed the optimization of the composition of the complex addition, which includes the addition of plasticizer, clogged up, biocide and reinforcing accelerator, which in particular relation to the water content allows to ensure the self-levelling of the screed during its application, increased density, short grip time and increased chemical and biological resistance. With this method, we can also obtain the influence of each parameter studied on the properties of the self-levelling screed in order to eliminate the least influential from the regression equation.