the cover means 158 has an integral inwardly and downwardly inclined tongue 172 which extends into the groove 154 in the manner shown most clearly in fig2 thus the extrusion 158 will be extruded in one piece so as to have the configuration shown in fig4 and will be cut into suitable lengths which can be placed one next to the other along the periphery of the pool of course where the pool is curved the extrusions 158 will be suitably curved by being placed against suitable forms while they are still of sufficient pliability upon issuing from the extruder as is shown most clearly in fig2 the liner sheet 22 has an upper edge region 174 in the form of a bead which is thicker than the remainder of the liner sheet 22 when the components of the pool are assembled the sheet 22 is placed against the backing means 34 and in a known manner a suitable vacuum pipe can be applied between the sheet 22 and the backing means to extract any residual air which may remain so that the liner sheet will snugly rest against the inner surface of the backing means the upper thicker edge 174 of the liner sheet 172 is introduced into the groove 154 all around the periphery of the pool and then the cover means 158 is assembled with the remaining structure as illustrated in fig2 the thickness.