soften somewhat at this temperature asphalt paving material is usually applied to roads at about 300 degrees f the softening of the plastic extender ring at the asphalt application temperature will actually improve the seating of the heavy manhole cover 12 as the plastic ring will conform to its shape as well as the shape of the support shoulder 14 the color of the plastic ring 15 is black or dark red to prevent deterioration caused by ultraviolet radiation cold flow in the ring 15 is not more than ten per cent and the plastic material is slightly pliable above 250 degrees f all of these stated physical characteristics of the plastic extender ring 15 contribute toward its improved performance over the customary prior art welded metal extender rings or much more costly adjustable screw threaded extender means fig1 illustrates the invention where only a single ring 15 is required to position the manhole cover 12 at the required elevation while fig3 illustrates a condition where three of the rings 15 in nested relationship are employed ordinarily three rings is the maximum number of rings which would be used to elevate the manhole cover as shown in fig1 and 3 the ring or rings 15 are seated on caulking composition 19 or other suitable adhesive as shown in fig2 this material 19 may be applied as a narrow bead initially and the weight of the manhold cover 12 will spread the caulking or adhesive and actually assist in seating the ring evenly as well as permanently bonding it to the base ring 11.