What is a roof vent pipe, and what are the different pipes in my roof?
Short answer. A roof vent pipe is any pipe that passes up through the roof to release air or gases outside. The most common is the plumbing vent stack, usually white PVC or black cast iron, but roofs also carry metal flues for gas furnaces and water heaters, plastic vents for high efficiency appliances, and sometimes radon mitigation pipes.
How to tell them apart
- Plumbing vent: an open topped PVC or cast iron pipe, 1.5 to 4 inches, with a rubber, silicone or lead boot at the base and no cap.
- Gas appliance flue: double wall metal pipe with a round cap and a storm collar where it meets the roof flashing. It gets hot and serves a furnace or water heater.
- High efficiency appliance vents: PVC pipes, often in pairs or as one concentric fitting, serving a high efficiency furnace or water heater.
- Radon mitigation stack: a PVC pipe, often labeled, running from the foundation with a fan in the attic or outside.
- Bath and kitchen exhaust: usually a low hooded cap rather than a bare pipe.
What roofers need to know
Each one needs the correct flashing. Plumbing vents use boots. Metal flues need flashing suited to heat, a storm collar and the required clearance from combustible materials. Plastic appliance vents have termination heights set by the appliance maker. None should be shortened, capped or moved during reroofing without the right trade involved.
Common problems
- Cracked boots on plumbing vents, the most frequent leak source among roof penetrations.
- Rusted storm collars on metal flues.
- Pipes left too short after a roof was built up with new layers.
See pipe boot leaks and roof flashing explained.