according to the double arrow f 2 and a second connection of the pivot type according to the double arrow f 3 these two connections and the constituent elements thereof are arranged so that when the roof element i is raised above the ground cf fig7 and 8 the castor support 320 is released through its own weight while remaining vertical to the vertical guidance elements 330 forming said runner connection and is free to rotate according to the double arrow f 3 in order to pivot through its pivot connection for the purpose of retraction in the thickness of the roof element i during stacking cf fig9 when the roof element i is raised and returns to its initial height the castor support 320 can pivot double arrow f 3 and return to the vertical position through its pivot connection and engage by vertical sliding double arrow f 2 in said guidance means 330 as illustrated the castor support 320 is preformed with an oblong hole 321 enabling both translation and pivot connection with respect to a shaft 340 fixed to the end crossmember this oblong hole 321 is preformed so that its elongation is oriented vertically and so as to define a translational travel where when the roof element i rests on its castors the support 320 is engaged in the elements 330 making up the runner connection and where when the roof element i is raised the support 320 driven by its weight disengages from said elements 330 constituting the runner until the top end of the oblong hole 321 comes into abutment against.