A system and method are provided for simple yet reliable replacement and sealing of existing vent pipe penetrations through an external surface of a structure such as a building's roof. The system and method replace a portion of an existing vent pipe with a sealed replacement pipe extension. The replacement pipe extension and remaining portion of the existing vent pipe are preferably combined with a pipe flashing structure to provide a sealed replacement pipe solution capable of withstanding torsional and side loads encountered in rooftop applications. The system and method enable replacement and sealing tasks quickly and at low cost.