to one embodiment of the invention the embodiment shown comprises a flange 30 with two ridges 38 on its upper surface 32 and two ridges 38 on its lower surface 34 the ridges of opposite surfaces are shown offset with respect to one another although the spacing of the ridges may vary as a matter of design additionally any number of ridges may be used the cross sectional shape of the ridges 38 is a matter of design choice the ridges 38 depicted in fig4 have generally rectangular cross sections which are relatively easy to mold in the embodiment shown the collar 18 is molded across the upper 32 inner 36 and lower 34 surfaces of the flange 30 this maximizes the bonding area between the collar 18 and flange 30 as well as increasing the effective width of the seam 24 in a typical base element made in accordance with the invention the flange 30 may be on the order of 1 4 to 1 2 inches or more in axial width since water would have to pass completely over the upper 32 inner 36 and lower 34 surfaces to penetrate the assembly the distance involved to penetrate the seam i e the minimum seam passage distance would be on the order of 1 2 to 1 inches or more thus in order for water to penetrate the seam the seam would have to fail across its entire effective width which is equivalent to the minimum seam passage distance and then water would have to travel the minimum seam passage distance fig5 shows in fragmentary cross section another embodiment of the invention where the flange 30 has.