The role of horizontal diaphragms in the seismic response of buildings is significant both in terms of stiffening and transfer of horizontal loads to vertical elements. Their behaviour in 3D modelling is frequently assumed (with a rough approximation) as rigid: this hypothesis may be not completely acceptable in case of existing buildings (with reference to historical masonry structures), where various archaic constructive technologies can be found (e.g. structural brick or stone vaults). In the framework of equivalent-frame models, the adequate definition of the equivalent elastic stiffness of various ancient floor technologies is noteworthy. In this contribution, the attention is focused on the structural brick or stone vaults, frequently built in historical heritage. Detailed FEM elastic analyses are performed on different typologies of vaults (barrel vault, cross vault, cloister vault), in order to correlate their axial and shear stiffness to that of an equivalent membrane. Various geometrical parameters and boundary conditions are analysed. On the other hand, stiffness degradation, due to the non-linear behaviour of masonry, and failure mechanisms have to be studied. In fact, damage in the vaults may substantially alter their stiffening effect and the distribution of horizontal loads to vertical elements. Also in this case, detailed FEM analyses, using a non-linear constitutive model are performed, focusing the attention on the effect of masonry pattern
