being present in said mixture in a proportion comprised within the range of 20 to 30 by weight by way of example for this first layer there can be fused an unsaturated polyester resin diluted with styrene and methacrylate this resin is accelerated by means of an accelerator such as a solution of cobalt octoate at 6 by weight then hardened by means of a catalyst such as a peroxide either of methyl ethyl ketone or of acetyl acetone 2 4 pentanedione such a resin is available commercially and sold by the reichold company under the mark dion fr 7722 00 other unsaturated pre accelerated polyester resins such as the resin viapal vup 4754 bemt produced by vianova resins can be used the second layer of this element is a continuous layer of a protective coating a so called gel coat constituted by a mixture of at least one synthetic resin preferably thermosetting such as polyester resin a constituent coloring said resin throughout its mass and a catalyst the thickness of this second layer is generally comprised within the range of 0 3 to 0 5 mm moreover the constituents of the first and second layers that color said layers are selected such that the color of the first layer is identical to that of the second layer as a coloring constituent throughout the mass of said resin can be cited titania rutile treated with chlorine more generally the coloring materials of the core and of the second layer are obtained from white blue or black bases for a covering element imitating slate and from yellow red or orange for a covering element imitating tile in other words the final colors of the core and of the second layer can be for a covering element imitating slate of the.