Drift is a relevant issue in concentrated solar tower facilities since this time-dependent pointing error severely affects their performance. This work deals with the optical analysis of drift for tilt-roll heliostats in terms of the geometrical parameters associated to the heliostat mechanical structure and the local time. For each drift source or error, flux maps have been obtained by means of Monte Carlo ray-tracing calculations as a function of the source magnitude and daily time in winter and summer solstices. Then the corresponding pointing errors and the drift curves are determined from the displacement of the center of gravity of the flux maps. As a result, drift related to each source is identified and characterized. Additionally, drift sources are compared in terms of the standard deviation and the mean of their daily pointing errors. The analysis evidences that main drift sources are the misalignments of the pedestal like the one of an inclination with respect to the zenith.