Brick- and tile manufacturing is one of the most rapidly developing industries. Professionals are constantly seeking modern technological solutions with which much better and longer lifetime of ceramic roof tiles can be reached. The aim of this R&D work was to optimize the mixing ratio of two different clay minerals of the industrial partner. The Authors have tried to develop a mixture by increasing the mixing ratio of the clay „Jamina”, which reaches or exceeds the mechanical strength and frost-resistance of the ceramic roof tiles which were made according to the present recipe used by the industrial partner. After XRD tests in virtue of preliminary mixture design, the Authors have made clay mineral mixtures that were milled and homogenized in an edge mill for an appropriate grain size. After mixing and homogenizing, samples with different length were made for further investigations with the use of a laboratory vacuum extrusion system. Shrinkage after drying and firing, apparent porosity and water absorption were measured on these samples. Mechanical test were also executed, as well as bending strength tests and compressive strength tests. On the basis of the results of the executed tests, it can be seen that the Authors were successfully developed clay mixtures, with which ceramic roof tiles can be produced with properties that are more favourable than the present mixture used by the industrial partner. The increase of the ratio of ‘Csaba I.’ clay increases the bending and compressive strength of the fired ceramic roof tiles. So the reduction of the amount of clay „Jamina” affects the mechanical properties of the ceramic roof tiles favourably.