NOVELTY - The fastener has mounting rails (1) arranged on substructure of a roof such that a narrow side runs parallel to modules (2). A clamp (4) is formed such that the clamp encloses the rails on a longitudinal side and the narrow side. U-shaped top hat rails (3) are arranged between the modules, where a slot (5) is attached in a lower bar of the top hat rails. Longitudinally running elevations (10) are provided in inner sides of the rails (3), where a side of the rails (1) that is turned to the modules is provided with a longitudinally running point or line shaped running knurled or roughened area.    USE - Fastener for fastening solar elements of solar modules of solar systems on a structure i.e. roof (claimed). Can also be used on a wall.    ADVANTAGE - The longitudinally running elevations are provided in inner sides of the top hat rails, where the side of the mounting rails that is turned to the modules is provided with a longitudinally running point or line shaped running knurled or roughened area, so that the fastener can be simply and economically manufactured and easily mounted to use the top hat rails for the modules as a locking end fastener at the edge of the entire solar system and to provide a good electrical contact between the mounting rails and the modules, thus mechanically improving the locking of the modules, and hence simplifying the installation of the fastener and reducing the cost for manufacturing the fastening elements and assembling times, while reducing the number of different elements both in the form and number. The clamp can be economically manufactured in the extruding process with high dimensional accuracy and mounted only on the mounting rail without requiring an additional connecting element e.g. screw, bolt and nut. The clamp can be attached in any area on the mounting rail during assembling.    DESCRIPTION OF DRAWING(S) - The drawing shows a partial cross sectional view of a clamp that runs transverse to a roof in a mounted condition.    Mounting rails (1)    Modules (2)    Top hat rails (3)    Clamp (4)    Slot (5)    Longitudinally running elevations (10)