Furnace Vent Pipe Flashing

A furnace vent pipe is the pipe that protrudes out the top of the roof. It is the pipe that vents the exhaust coming from your furnace. It is double wall constructed and made from galvanized steel. It can be painted but is oftentimes unpainted leaving it gray in color.

It will be topped with a cap at the top. The flange around the base of the pipe that partially extends underneath the shingles is the actual flashing. It is normally re-used when the roof gets replaced unless it has sustained damage. We sometimes see a furnace vent pipe flashing that has many nail holes around the base that where placed too close to the inside of the flashing. This can be the source of a leak so it's important to replace the flashing if this is the case.

The other reason these will leak is because the shingles weren't installed around the base properly. Either too much of the base that lies on the deck was exposed or the shingles were not trimmed back from the vertical part of the flashing. This kind of defect may not reveal itself for quite a long period of time. It is important to allow a space of 1/2 of an inch between the shingles and the vertical portion of the flashing.

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