The calculation of old existing masonry structures needs a homogeneous mechanical behaviour law under tensile and compressive load until collapse. The samples extracted from existing buildings are necessarily limited in dimension and number and it is not possible to perform tests directly on a representative volume, so the numerical modelling of masonry mechanical behaviour has to be predicted from the behaviour of its constitutive elements. The aim of the study presented here was to develop a method able to define the homogeneous representative law for brick masonry from those of its components, bricks and mortar, in tension and in compression, including linear and non-linear domains. The method is numeric, fitted on experimental results. Tests were carried out on mortar samples, brick samples, and multilayer samples. Their experimental behaviour is described and a damage model is used to describe the behaviour of the masonry. An explanation is given of how the homogeneous characteristics of the masonry were defined and the influence of some parameters is presented.