07:46AM | 02/24/07
Member Since: 02/23/07
5 lifetime posts
Sorry in advance for this long scenario for a hopefully simple question. The main one is: can there be an almost-steady leak from a PVC vent pipe in a bathroom that's not operational?

We are building a new bathroom on our first floor partially below the footprint of a second-floor bathroom. We had assembled the exhaust/vent pipes using PVC pipe that goes up through the ceiling and on into the attic, where it connects via a Y-pipe to the main vent pipe going out to the roof.

After we had finished installing the ceiling and started work on the finishes, I noticed discoloration in one area. I broke away the ceiling and found extensive dampness at a point just underneath one of the PVC pipe joints. (There are five elbow joints or connections in this area - it's where the exhaust from the shower and sink meet up and then bend upwards to go into the attic). The dampness deeply saturated two pieces of wood - a joist through which we had put a hole that the PVC pipe runs through, and a 2x4 acting as a cat brace between two joists (and which ran directly beneath the PVC pipe, both parallel to the ceiling). I assume some white stuff now on the wood is mold.

Here's the problem: It was really wet - at one point I placed a plate on the floor below because it was collecting a steady drip. But it was intermittent and there was no clear pattern.

Here's a twist: I got up there and wrapped some plumbing pipe tape around the joint just above the cat brace - not cleanly. The cat brace was so close to the pipe that the wrap wasn't tight at all. But since then, I haven't had the leak come back.

Here's the mystery - the bathroom was not yet operational. No fixtures had been attached. I can't figure out how the vent pipe for a bathroom that isn't being used can cause that much condensation or excessive wetness. (This was in December/January). Is that possible?

I wondered if it was possible that there was a leak from the upstairs bathroom but there is no sign of it from the floorboards above nor any pattern I could re-create (leaving the faucets upstairs running or dripping - none of it led to the leak). We even had big rainstorms before and after (rain coming down the vent pipe opening?) - but no pattern.

I think the easy solution is to remove the cat brace, really tightly wrap the pipe, replace the brace, and then move on. But I want to be sure before I close up the ceiling that the leak was coming from the vent pipe, because I don't want it to return after we have all the fixtures in, etc.

So the questions are – (again) can there be an almost-steady leak from a PVC vent pipe in a bathroom that's not operational? Will I be safe if I just wrap up those PVC pipe connections/joints or do I need to re-glue them? Should I keep looking for a leak from the upstairs bathroom?

Thanks for any help you can offer.


10:47AM | 02/24/07
Member Since: 01/24/06
1548 lifetime posts
Yes as you know it is very possible for a vent pipe to have lots and lots of moisture as the vapor from the vent One day if you live in a cold area look up at a vent terminal (vent through roof) and you will see the vapor (called hoar frost when it freezes )

Just make sure the joints are properly primed and glued and PITCHED AWAY from the fixture being serviced.

Drainage leads from the fixture vent pitches back to the fixture to drain the condensate.

A lot of folks wrongfully think because it is only vent they do not have to prime and glue the joints.

Also plastic sags leaving lots of pockets for water to accumulate as plastic installers are known to use band iron or tie wire rather then clevis hangers properly spaced as one clevis hanger cost more then a roll of band iron.

Check the flashing around the vent going through the roof and make sure the installer did NOT use PVC exposed to direct sun light


11:02AM | 02/24/07
Member Since: 04/25/05
1915 lifetime posts
It is not clear to me where the condensation is.

Normal condensation in a plumbing vent pipe should stay within the pipe.

The other possibility is that a large amount of air is leaking around the holes in the DW and depending on the house contruction from the basement/crawlspace through the plumbing chase into the attic and condensing on the OUTSIDE of the vent pipe.

If that is what is happening you need to seal those air leaks.


12:23PM | 02/25/07
Member Since: 02/23/07
5 lifetime posts
Thanks for your reply - it's a relief to know it is probably not a leak from elsewhere (and now makes more sense).

I will bother you for two small questions based on your reply.

First - when you say pitched away, I want to make sure you are talking up/down angle and not proximity to the fixtures. The pipe joint that is causing me trouble is basically in the center of the room between the shower and sink, where we connected the vent pipes before the stem heads up toward the attic - in parallel/horizontal terms it's not going away from the fixtures. (If by pitched you mean angled upward, I think there is a slight upward slant already that hopefully covers us).

Secondly, I am pretty sure that the plumber who helped us install the vent replaced our cast iron pipe that went through the roof with a PVC pipe, and I am sure that it is exposed to direct sunlight. Is there anything I should do to replace it or cover it?

Thanks again for your help. This problem was driving me crazy.


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