Structural timber represents an efficient solution combining the environmental sustainability of the constructions and their structural safety against natural events, contemporarily. In this context, timber-based systems used for multi-storey vertical additions in urbanized areas have the further advantage into facing the issues related to soil consumption due to urban densification.

From structural standpoint, the analysis of the seismic behavior of existing constructions with timber Cross-Laminated Timber (CLT) vertical extensions still represents a challenging task, in fact no specific seismic design rules are available in existing Standards. This paper deals with the assessment of seismic demand represented by the distribution of shear forces in existing masonry buildings with multi-storey timber vertical additions based on the results of modal dynamic analysis. A simplified model of the building is used to perform a parametric analysis by varying the number of storey (i.e. mass ratios) in a way that the role of single vibration mode is pointed out in the shear force distribution. The results offer an insight into the contributions of different parameters affecting the seismic demand and to deliver considerations about the definition of seismic design rules of this kind of structures both according to linear and non-linear design procedures.