Disclosed is a method for detecting a fault condition in the measurement of the level of a medium in a tank having a floating roof. The method uses a radar level gauge to measure the level of the floating roof and a level measuring device to measure the level of the medium. The method includes recording the level of the floating roof during a defined time interval that comprises some filling/emptying cycles of the medium, recording the level of the medium during said time interval, determining the deviations between the level of the medium and the corresponding level of the floating roof, centering the deviations between the level of the medium and the level of the floating roof reduced by the thickness of the floating roof with the corresponding level data, and generating an alert message when the centered deviations exceed a tolerance threshold.