The present invention relates a set comprising a supporting structure 2, such as a joist, a first and panel 31, a second panel 32 and a fastening device 1for securing the first and the second panel 31, 32 to the supporting structure 2. The fastening device comprises a top portion 3, having top and bottom surfaces, first and second edge portions, and first and second legs protruding downwardly from the bottom surface. The first and second panels respectively comprise opposite first and second edge grooves into which the first and second edge portions are configured to be respectively inserted. The first and second legs are flexible and bendable towards each other, and respectively comprise first and second locking elements. An upper surface 18 of the supporting structure 2 comprises an insertion groove 12 comprising a first undercut groove 13and an opposite second undercut groove 14. The first and second locking elements are respectively configured to be snapped into the first and second undercut grooves to a locked position.