Schematic of a plumbing stack vent (C) Carson Dunlop AssociatesPlumbing Vent Defects
Drain Noises, Gas Smells & Septic Tank Odors

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Troubleshooting plumbing vents:

This article describes how to examine the building's plumbing vent system piping and connections in order to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or plumbing drain and fixture noises that may appear indoors or outside

. Here we list common causes of plumbing problems related to improper plumbing vents.

We describe how to track down and correct these "gas odors" in buildings with a focus on homes with a private onsite septic tank but including tips for owners whose home is connected to a sewer system as well. What makes the smell in sewer gas? Sewer gases are more than an obnoxious odor.

How to prevent or cure sewer gas odors from septic systems, building plumbing, & other causes. Sewer gas smell Septic gas smell diagnosis - plumbing checklist.

What causes that gurgling noise in a sink, tub or shower drain, and why do some fixtures "gurgle" when a toilet is flushed nearby? Here are the answers.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Plumbing Vent Piping Defects May Cause Drain Noises or May Release Dangerous Sewer Gas Odors Indoors

Short plumbing vent (C) Daniel FriedmanOur page top sketch of a plumbing stack vent and other sketches included below are provided courtesy of Carson Dunlop Associates. At left is a plumbing vent installed on an angle, making us question the workmanship of this job and raising an alert for other plumbing vent snafus.

[Click to enlarge any image]

Plumbing drain or sewer gas odors: If you smell sewer gases in your building conditions could be dangerous (risking a methane gas explosion) or unsanitary.


Watch out: Because sewer gas contains methane gas (CH4) there is a risk of an explosion hazard or even fatal asphyxiation. Sewer gases also probably contain hydrogen sulfide gas (H2S) In addition some writers opine that there are possible health hazards from sewer gas exposure, such as a bacterial infection of the sinuses (which can occur due to any sinus irritation).

Depending on the sewer gas source and other factors such as humidity and building and weather conditions, mold spores may also be present in sewer gases.

Plumbing drain or fixture noises: Other advice about controlling plumbing noises in buildings is at PLUMBING DRAIN NOISE DIAGNOSIS and also at SOUND CONTROL for PLUMBING.

Photograph of a loose, improperly installed, slanted plumbing vent above the building roof Photograph of a plumbing vent terminating by a window Plumbing vent with collapsed top screen became blocked (C) Daniel Friedman Plumbing vent with screen reopened (C) Daniel Friedman Plumbing Vent leaks (C) Daniel Friedman Cheryl Lidawer Plumbing Vent leaks (C) Daniel Friedman Cheryl Lidawer Plumbing vent into attic (C) D Friedman C Lidawer

Troubles and mistaken sewer gas odor-sources along the way to finding this plumbing vent sewer gas leak problem included

Transite vent pipe clogged (C) Daniel Friedman

Plumbing vent line has been cut, releasing sewer gases in an attic (C) Daniel Friedman

Sewer vent by porch chair (C) Daniel Friedman

Original installation of plumbing vent stack flashing on a metal roof

Below are details from the original plumbing vent installation of a 3-inch ABS plumbing vent through a standing seam metal roof.

We had installed a conventional plumbing vent stack and boot (below left) along with a roof membrane, while waiting for the metal roof to arrive at the jobsite.

To properly seal the plumbing vent to the metal roof we replaced the shingle-roof type vent flashing with the white rubber flashing shown at below right. That flashing is sealed to the metal roof surface by both adhesive sealant and a ring of metal screws.

Temporary roof vent flashing while waiting for metal roof installation (C) Daniel Friedman Eric Galow Permanent roof vent flashing for metal roof installation (C) Daniel Friedman Eric Galow

Photos courtesy of Eric Galow, Galow Homes in Poughkeepsie, NY.

At below left you'll see that even with the plumbing vent stack broken and bent over nearly touching the roof surface, the rubber boot installed in the photos above managed to avoid a flood of melting snow into the roof cavity. This broken plumbing vent was caused by the property-owner's reluctance to finish the job of installing snow guards on the roof.

Details about the snow guard system and its proper installation (done after this damage occurred) are


and step by step details of how we repaired this snapped-off plumbing vent without having to cut open the roof from above nor the cathedral ceiling from below are


Rubber boot keeps most water out of the roof cavity even with a broken roof vent stack pipe (C) Daniel Friedman

We discuss how to diagnose and cure drain sounds (gurgling, bubbling, smells, poor drainage)

at PLUMBING DRAIN NOISE DIAGNOSIS (gurgling drains means bad vent piping or clogged drains)


We discuss drainage for plumbing fixtures, such as loose toilets, as sources of sewer gas odors separately


Plumbing Codes, Citations & Plumbing Code References

Watch out: while the 2015 IPC is the most-recent version of the International Plumbing Code, while the 2018 IPC is under development. To understand which generation of the model building code or plumbing code applies to your specific project you will need to check with your local building official. For example many jurisdictions are on the 2012 IPC or still older versions.

Question: Roof leaks at plumbing vent flashing

29 October 2014 Beverly Gaydos said:

We had High Definition Roofing Timberline installed over 3 tab asphalt roofing.The flashing &vent piping were not redone & still leak. WE purchased new flashing for vent pipes & chimney & swamp cooler. What would be the best way to install this new flashing over these 2 layers of roofing ?

I was thinking of removing the few appropriate courses of HIGH Definition Roofing install the new flashing then cutting out the new Timberline asphalt shingles appropriately & replacing it with the lower portion of flashing exposed & sandwiching it between old 3 tab & new high definition roofing using adhesive nails etc.

As recommended on a new one layer roof. Can't seem to find info. about this on roof overlayment.

What is your recommendation ? Everyone asked seems to have different opinions.

The so called roofers aren't worth calling back. So please give your PRO ADVICE. THANK YOU Roof is 3to 12 pitch in windy snowy



As the old vent piping flashing installation leaks and there are now three layers of roofing, the best approach is probably to remove enough of the new roofing to give access to cut and remove the old flashing down flush with the original 2 layers of roofing that remain,

then install a new vent flashing boot and shingle over it with proper flashing positioning so that any water entering around the new flashing and boot is delivered atop the shingles down roof.

For added protection you can include sealant both beneath and atop the top and sides of the new flashing boot.

Question: removed plumbing vent, cut holes in roof for new vent - structural damage?

(July 22, 2014) Anonymous said:

We had our roof re shingled and had an electric vent removed and 2 new holes were cut to install 2 of the larger max vents.

What is the correct procedure for repairing the hole in the roof where the old electric vent was removed.

Also is there any chance of structural damage to the roof if approximately one inch x the length of the vent was cut off of the roof truss where the new vents were installed?


Anon I can't say I've got a clear idea of the situation, but generally if we are removing something that leaves a hole in roof sheathing, before re-roofing we patch the hole, typically by a filler inside the sheathing opening (plywood?), supported by additional plywood screwed to the sheathing underside from below.

Roof Trusses: manufacturer's instructions, many texts, and in this InspectApedia article

you're warned not to chop, hack, shorten or modify trusses as the truss strength is compromised. From what I *think* you are describing, if repair is needed it's probably minor.



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