What is the gas vent on my roof, and what should I know about gas vents on a roof?
How to recognize it
Gas vents are typically metal pipes a few inches in diameter, often rising higher above the roof than plumbing vents, with a round or conical cap. Some high efficiency furnaces use white PVC pipes that exit through a sidewall or the roof instead. Plumbing vents, by contrast, are usually short plastic or cast iron pipes with a rubber or lead boot.
Requirements that matter
- Height above the roof. Gas codes and the vent manufacturer set a minimum height that increases with roof pitch, plus clearances from nearby walls.
- Clearance to combustibles. B-vent needs a specified air gap to framing and decking as it passes through the attic and roof.
- Cap. A listed cap keeps out rain and birds and helps the vent draft.
- Flashing and storm collar. A metal flashing base woven into the shingles, plus a storm collar sealed around the pipe above it, keeps water from following the pipe down.
Common problems
- Leaks where the storm collar has pulled loose or its sealant has cracked in the sun.
- Missing or damaged caps after storms, which let in rain and nesting birds.
- Rust on older single wall pipes.
- Pipes cut too short during a previous reroof.
Safety notes
Never cover, seal, or block a gas vent, even temporarily during roof work. If the cap is missing, the pipe is disconnected in the attic, or you smell exhaust, have a licensed HVAC or gas professional check it, and make sure your carbon monoxide alarms work. During a reroof, the flashing around the vent should be replaced along with the shingles. See roof flashing explained and common roof leak causes.