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Leak from roof flashing for plumbing vent pipe

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  • Mar 14th, 2022 1:08 am
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May 8, 2007
1199 posts

Leak from roof flashing for plumbing vent pipe

Recently after a windy day I noticed on my backyard deck there was a circular rubber ring part about 4" diameter. I wondered if it maybe blew there from the neighbouring tall condo building, or maybe it was dropped by a crow (they pick up weird stuff). Or maybe it was from MY ROOF (shudder).

I looked at my roof vent pipe with binoculars and it seemed OK. But a few days later the weather forecast said a period of heavy rain was coming so I decided to go up on the roof and check.

I hate going on the roof, but I reviewed the ladder safety procedures and went up there. Yes indeed the rubber flashing for the plumbing vent pipe had started to deteriorate and fall apart. You can see the area around the vent pipe is no longer sealed and rain will get in (the missing rubber is the ring part found on my deck). Also there is a crack in the flashing base and rain will get in there too.

I didn't have time to fix the problem before the rain came. But I went in the attic when the rain started and I could see water coming in on the surface of the vent pipe. As a temporary fix, I wrapped some rags around the pipe to wick the water off the pipe surface. Also I put a large plastic lid from a wine making barrel under the pipe to catch any water dripping off the rags or the pipe. Fortunately the vent pipe has a 90 degree bend just above the roof insulation so that's a good place to put the lid to catch the water.

A few days later when the rain stopped I went on the roof to measure the vent pipe diameter (2" I.D.). Also I installed another temporary fix. I took a scrap 3' long 2x2" board and screwed a lid from a 4L ice cream bucket on one end. Then I inserted the 2x2 in the vent pipe. This acts as a mini roof over the vent pipe to block most of the rain from hitting the vent pipe and flashing.

I have ordered a new flashing from homedepot.ca. It's the "Permaboot" version for new roofs (another version is for fast repairs). These flashings do not have any rubber parts or exposed caulking so they should last many years. The standard pipe flashings all use rubber and most need caulk so they eventually fall apart or need repair. Mine was only 5 years old, installed when my roof was redone 5 years ago.
https://www.homedepot.ca/product/perma- ... 1000750356

My new Permaboot flashing has a large rectangular base so it will involve removing shingles to install. However PermaBoot also has baseless versions that are just caulked at the bottom, these are much easier to install. The base of my old flashing is starting to rot so I think the conventional base version is better for my situation.
https://www.homedepot.ca/product/perma- ... 1000745590

An alternative is to just cut off the vent pipe in the attic, just below the roof surface, and then just seal up the roof at the former vent location. I think this would work, but would not be approved under construction regulations, due to possible sewage smells being released into the attic. Some places allow a vent pipe in the attic, if a special air valve is installed on the vent pipe end, the valve allows air to enter the pipe but does allow air (and smells) to exit. Smaller versions of this valve are used in island sink installations. It's called an AAV - Air Admittance Valve.

If you have any unexplained stains on the ceiling, check your roof vent pipe flashing, it might be shot. Fortunately I caught this problem early and have no ceiling damage.
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1 reply
Sr. Member
Oct 22, 2016
866 posts
Comox Valley
You deserve credit, for the work you have put into it.
As you state “ Mine was only 5 years old, installed when my roof was redone 5 years ago.”
Do you think that maybe they reused the old flashing for this roof pipe, when they redid your roof?
It should not have failed after 5 years.


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