A new foundation factor is proposed to account for the soil amplification effects in the calculation of seismic base shear, as specified in the 1985 edition of the National Building Code of Canada. Three types of soil conditions are considered, viz., stiff soil, deep cohesionless soil, and soft clay deposits. In order to capture the resonance effect at the site–structure period, the foundation factor is expressed as a function of the ratio between fixed base period of the building and site period. The proposed foundation factor takes into account both the site–structure period coincidence effect and soil–structure interaction. The application of the proposed foundation factor is illustrated for three particular sites that have been subjected to an actual recorded earthquake. Key words: foundation factor, site effect, soil–structure interaction, buildings, seismic, shear, design, code of practice.