Recently I removed an old water heater stovepipe coming through my roof, because we moved our water heater. Right next to it was my plumbing vent pipe that I thought was associated with venting for the water heater, so I cut it off below the roof and patched my roof. So now my plumbing vent pipe terminates in my attic, in a little corner to be exact. How bad is this? I did it a week and a half ago and I never noticed any bad smells.

3 Answers 3


Bad. Not only are you introducing stink and moisture to your attic, but toxic and potentially explosive gases as well.

Decomposing waste materials in public and private sewer and septic systems create sewer gases. Methane is the largest single constituent of sewer gas, which includes an assortment of toxic and non-toxic gases, such as hydrogen sulfide, carbon dioxide, ammonia, nitrogen oxides, and sulfur dioxide. Improperly disposed gasoline and mineral spirits may also contribute to sewer gases.


Fix it now.

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    This would have been a much better answer with a 'how to fix it now' in the last paragraph.
    – Tim
    Commented Feb 17, 2017 at 16:30
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    I'm not so sure. For one thing, it's fairly obvious what needs to happen. For another, I'm not really in the business of rewriting what already exists in countless places on the web. I'd rather spend my time helping with unique or more difficult problems.
    – isherwood
    Commented Feb 17, 2017 at 16:47
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    Fair comment, for me the obvious would be an air admittance valve, in the attic. No need to go through the roof. I'd have thought that all the u-bends and water traps would stop smells and gas coming through that pipe, anyway. It should only be there to stop hydraulicing when a loo is flushed, i believe.
    – Tim
    Commented Feb 17, 2017 at 17:11

You should fix this (see isherwood's answer), but you don't necessarily need to extend it back out of the roof again, assuming that you have at least one vent to the outside somewhere else in the system.

You could instead cap it with an internal drainage vent (also known as an air admittance valve). This lets air in (necessary for good drainage) without letting sewer gases out.

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    Oh, too late. Already ripped the roof up again and stuck a pipe out of it. And made a big mess with Henry Wet Patch, cuz the existing shingles are old and stiff and all out of adhesive (and one ripped).
    – Ian
    Commented Feb 17, 2017 at 14:52
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    AIUI you want to have at least one open vent in a drainage system. Otherwise rotting waste can cause pressure to build up in the system until it forces it's way past a trap. Commented Feb 17, 2017 at 15:40
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    While the manufacturers of AAVs say they can be used to terminate your vent stack inside of your attic it seems pretty hacky if you have easy access through the roof. What happens when the main vent stack has a positive pressure situation from sewer gasses? Where do the gasses go? What happens in 20 years when the AAV sticks in the open or closed position? Do it right, right now. It's cheaper in the long run.
    – self.name
    Commented Feb 17, 2017 at 21:25
  • @self.name Or, provided you're roof is a little on the older side, use the AAV until it's time for a new roof and then remove the AAV and extend it through the roof at the same time as you're getting your new roof installed :-) Probably less than 20 years of use for a lot of people. Commented Feb 17, 2017 at 21:48
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    @AndyT You're right that it depends on the roofing material. Most rooves in my area are done with asphalt shingles and last for 15-30 years depending on shingle quality and workmanship. That said, needing to replace a roof in an older home is not uncommon: When was the tiled roof installed on your house, 95 years ago? :-) Commented Feb 6, 2018 at 17:46

It's probably a vent not a drain. Yes, it allows moisture and odors into you attic and it needs to be extended at least 12" above the adjacent roof. If you cap it, the fixture it comes from will not drain properly. If it's a drain, then add a vent through the roof too.

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