Abstract | We investigate the influence of the uncertain value of dead loads, live loads and the vertical seismic excitation on the seismic performance of a reinforced concrete structure. Seismic guidelines typically enforce the use of the Gκ + 0.3Qκ combination, where Gκ are the dead loads and Qκ the live loads. However, their true values and their influence to the seismic response are uncertain. Using a typical existing five-story building we employ Incremental Dynamic Analysis to evaluate the seismic performance under probabilistic load distributions in combination with the use or not of the vertical component of ground motion. Multiple accelerograms are scaled to several levels of intensity both in the horizontal and in the vertical sense to evaluate the building capacity in limit-states ranging from serviceability to global dynamic instability and collapse of the structure. In summary, the influence of the loads and the use of the vertical component on the estimated demand and capacity of the structure are quantified, allowing for the extraction of useful conclusions on the assumptions of the code and their impact.