Abstract: A blow off cap is provided for use in conjunction with a standard-type vent stack or blow off pipe. This blow off cap structure includes a cap structure fabricated from a plastic material having a top planar portion and a side wall having an upper and lower section, the upper section being a solid portion which extends in underlying relationship to the top surface, while the lower section has a shaped side wall adapted to engage the outer surface of a vent stack. A retention structure fabricated from an elastomeric material connects the cap structure to a clamping structure, which in turn fixedly secures the blow off cap to the pipe.