In this paper, an improved version of a simplified method to assess lateral shear forces attracted by shear walls of regular, low-rise masonry structures is presented. This simplified method for seismic analysis (SMSA) is allowed by Mexican building codes since the 1970s. The impact of shear deformations in the three-dimensional distribution of the forces absorbed by these walls is assessed for different wall aspect ratios �H/L�. Based on extensive parametric studies, effective shear area factors �FAE� originally proposed in the SMSA are modified to improve the estimates of shear forces using this method. New FAE are proposed for three different performance levels for the structure: �1� elastic response; �2� completely nonlinear �cracked� response of all walls along the building height; and �3� partially nonlinear �cracked� response along the height.