The semi-empirical assessment of seismic vulnerability of ancient church buildings is possible only if sufficient knowledge of the expected seismic behavior is available for a wide variety of typologies. For this reason, the information inferred from seismic damage observation may need to be complemented by numerical analysis. A simplified material model is proposed here for predicting the damage from out-of-plane behavior of large walls in old masonry churches subjected to seismic loading. For a specific substructure, the church façade, the effects of geometry, strength and post-elastic behavior of the material, as well as excitation characteristics are then analyzed with reference to the formation of a collapse mechanism. Comparison with observed damage thoroughly confirms the crack patterns developed numerically. Thence, the material model proposed may be considered satisfactory and suitable for use in seismic vulnerability studies.