Ventilation stack leak

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New Member
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Chesterfield, Mass
We have heavily used shower in an upstairs bathroom (with exhaust fan) that drains into a pipe that runs up/down an outside wall into the attic where it then turns and runs horizontally in toward the center of the building where it joins a larger diameter stack pipe. Its very cold in our attic right now (10 degrees outside) I've felt around the this joint after taking a short shower to see if I can feel moisture and it seems to be dry. But in the adjoining room the ceiling below this joint has a dark circular stain that has grown worse over the years.

I see the remains of a wasps nest around the stack just as it exits the roof which might mean they have a nearby entrance around the pipe.

Seems to be two possible sources of this leak: either condensation at this joint from excessive showers or incoming rain/snow through the stack.

Can you suggest ways to verify which one it is and what to do to remediate?


In the Trades
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Seattle. WA
Run hot water in the shower. Then go up into attic and observe joint. The vent will quickly be filled with steam, which
will condense on inner walls of cold pipe, and start seeping out joint if it has a leak.

Go up on roof and closely examing flashing around vent pipe. Or wait for a rainy day and check for leakage from roof
around the pipe.

Leaky pipe joint will have to be redone. Leaky roof flashing will have to be replaced.

Master Plumber Mark

Sensitivity trainer and plumber of mens souls
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indianapolis indiana - land of the free, home of
you might have just a hair line crack in the stack or a spot growing wiskers.......
or it could just be the flashing on the roof needs a good load of tar pitch thrown on it to seal the pipe

if you had a real serious leak you probably would already know it..

if you do find a minor pin hole leak in the cast iron I suggest you wait for a hot dry day this spring or summer
and just get a tube of clear silicone and take a large handful of it and smear it over the spot
that appears to be compromised and this repair will hold for probably another 60 years more or less

This is a very inexpensive way to patch and repair an issue in cast pipe... and I have done this many
times to get people and myself out of huge jams........... as long as you clean the area first of dust and dirt
it will last forever
Hey, wait a minute.

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