Abstract: A plastic sewer vent line extending through the roof of a building is provided with a defroster for preventing blockage of the vent line by freezing and the consequent back-up sewage gas into the building. The defroster is located above the roof and includes a metal pipe, e.g., of copper, having its bottom end coupled to the plastic sewer vent line and provided with an electric heating coil in contact with a portion of the exterior thereof. An outer shell is spaced outwardly from and surrounds the exterior of the pipe and has a top sealed to the pipe top and a bottom sealed to the roof to provide an annular providing an insulating effect and enhancing the effectiveness of the heating element. A heat limiting switch is provided to prevent overheating.