which are provided to help distribute load transmitted to the riser pan 10 from a next higher riser 12 stacked thereon in addition to the ribs 32 several screw bosses 33 each having sidewalls 34 36 an inner wall defined by an outer portion of the upper cylindrical wall 16 and an outer wall 37 are provided at periodic locations about the upper cylindrical wall 16 which also extend above the annular ledge 30 these screw bosses 33 may take the form of a pair of closely spaced ribs which are adapted to securely receive a threaded screw therebetween but it is preferred that the screw bosses 33 be enclosed on the bottom and sides thereof so as to prevent dirt or more importantly concrete when the riser pan 10 is cast into a concrete septic tank lid from entering the screw bosses 33 and obstructing the screw receiving opening therein preferably the screw bosses 33 may be hollow cylindrical or in the embodiment shown substantially rectangular hollow polygonal members the purpose of such screw bosses 33 is to enable securement of an injection molded polyethelene riser cover 38 directly to the top of the riser pan 10 or riser 12 14 if it is desired to place a riser pan 10 at or near grade level i e at.