The present invention relates to a fastening element with a tolerance-compensation function for fastening add-on elements, in particular roof rails. To this end, a threaded bolt is provided which has a threaded region, a fastening region and a projection. A positioning and assistance element is supported on the protection, and an adjustment element is arranged in the threaded region. An add-on element is arranged on the fastening region. A space between a first and a second component can be compensated for with the aid of the positioning and assistance element in cooperation with the adjustment element. Furthermore, a fastening means is arranged on the threaded region, with the result that the second component is arranged between the adjustment element and the fastening means. The add-on element is fixedly connected to the second component by means of the fastening element in this way.