Clogged or blocked plumbing vents as a cause of sewer or septic system odors:

Diagnose odors traced to clogged, frozen or snow-covered plumbing vents. This article describes the causes of blocked plumbing vents & resulting smells & odors in and around buildings; why do some plumbing vents get blocked in freezing weather? Effects of snow cover on plumbing system vents & odors. Plumbing drain trap siphonage, odors, noises due to clogged plumbing vents.

Plumbing Vent Blockage Problems as a Source of Building Smells, Septic or Sewage Odors

What's different in cold weather that could cause indoor sewer gas odors only during freezing weather?

[Click to enlarge any image]

• An under-sized plumbing vent
  
   (less than 2" diameter or too short above the roof line) can become blocked by frost in the vent line above the roof, blocked by snow cover, or have its gases blown down to a lower level by cold or varying air movement. If it's a plumbing vent diameter problem in a freezing climate, you'll see the vent basically blocked by frost above the roof line.
• Too-short plumbing vents:
  
  Look also for short plumbing vent stacks above the roof line - they can be covered and blocked by snow.

If in cold weather a plumbing vent pipe is frosting up, sewer gases may vent backwards out through building drains. In that case, even re-priming building drain traps won't prevent sewer gas entry as flushing a toilet or running a nearby drain can siphon water out of a nearby trap.

[Illustration provided courtesy Carson Dunlop Associates.]

A thoughtful reader, Jill Elwert, suggested insulating the vent piping to try to delay vent pipe blockage by freezing moisture. We are afraid that even if you insulate the vent line in a cold attic, it'll freeze above the roof surface outdoors. You could try it but if that proves the problem, ultimately you'll want to have a plumber or handyman install a 2" or larger diameter vent from the attic floor up through the roof.

See REMEDIES for SEWER ODORS, PLUMBING.

Diagnose frozen or snow-covered plumbing vent lines:

• Noisy drains only in freezing weather:
  
  Look for a clue of blocked vents (from freezing moisture) by listening for a glub glub sound at nearby drains, say when flushing a toilet or emptying a bathtub.
• Poor drainage only in freezing weather:
  
  Another blocked vent line symptom might be poor drainage that occurs only in freezing weather.

• Rooftop inspection for frozen or snow-covered plumbing vents:
  
  Watch out: don't try going on an icy, wet, high, steep, or snow-covered roof. But if the roof is safely accessible, or you can get visual access, perhaps from a higher elevation, or some safe location with binoculars, see if you see frost build-up in the plumbing vent.
• Priming the plumbing trap
  
   (where sewage odors are most observed) by pouring water into it may stop the sewer gas odor, but if the plumbing vent is blocked, the trap is likely to lose its prime and the odors will return.
• Inspect for small diameter plumbing vents 
  
  above the roof line or in the attic - just a simple visual inspection can tell you where to look first for frozen, blocked plumbing vents. Start with the smallest diameter vents, especially if the plumbing vent is one that vents a place where lots of hot (steamy) water is run such as a vent for a clothes washer, kitchen sink, or shower.

See PLUMBING VENT DEFECTS & NOISESfor diagnostic details.

PlumbingTrap Siphonage, Dry Plumbing Traps, Sewer Gas Leaks & Freezing Weather-Related Plumbing Vent Problems

Reader Question: 16 Feb 2015 rose said: do I need to dig out the septic vents that are snow-covered?

I hear about the need to clear vents in the house in this snow, but can't find anything about when the septic vent pipes are buried in snow -- do those need to be dug out?

Reply: frost blocked or snow-covered vents can cause dangerous sewer gas backups

Rose,

My OPINION is that the answer is ... it depends.

On what your sewer piping connects to and where you live.

For example: North Dakota State University has advised that in deep-snow-winter weather, blocked sewer vents can cause a [dangerous in my opinion] sewer gas backup in homes that can "make people sick" (sewer gas is also explosive). - see "Clogged vents can lead to sewer gas backup, which can make you or your family ill.", Roxanne Johnson, Water Quality Associate, (2011), - ag.ndsu.edu

Johnson also notes that deep snow on a roof can block the roof vents of the plumbing system too; indeed I've published photos of that condition here at InspectApedia.

Recently, in New York, dealing with deep snow and ice dam leaks on a flat roof, we had what seems to me a clever solution: having to make an emergency "repair' by adding a heat tape to a leaky roof we just stuck an extra length of the heating tape cable into the rooftop plumbing vent stack. That small amount of heat won't damage the plastic piping of the vent, and the heat can avoid a frost-blocked plumbing vent that can occur in freezing weather whether there is deep snow cover or not.

Freezing weather can cause a complete blockage of a plumbing vent stack by frost that forms inside the portion of the vent that extends outdoors above the roof, or by deep snow cover that completely covers a short plumbing vent.

For example steam from long showers moves up the vent pipe where it freezes in the pipe section extending above the building roof.

The absence of venting (missing or blocked) causes plumbing trap siphonage and loss of water in building traps. (Water flowing down a drain line without a nearby supply of makeup air to follow the water creates a vacuum that pulls water out of nearby plumbing traps. Flushing a toilet can siphon out a nearby sink or tub trap.)

What happens when the water seal is lost from a plumbing trap?

[Click to enlarge any image]

When the water seal is lost from a plumbing trap sewer gases can back up out of that fixture and not just smell bad. Sewer gases contain methane which is an explosive gas - possibly quite dangerous. In addition to occasional methane gas explosions inside buildings, I've had a report of an outdoor septic tank explosion too when an owner built a brush fire atop the tank.

Sketch above courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

In sum, dry plumbing traps are caused by evaporation at an unused fixture, leaks at the trap, or siphonage due to improper plumbing vent line installation. Dry traps can leak smelly or dangerous sewer gases into a building.

Check for dry plumbing traps, particularly at un-used basement or lower floor fixtures and at floor drains which might be connected to the sewer line. "Dry trap" means that there is no water in the trap bend or weir.

See PLUMBING VENT DEFECTS & NOISESfor diagnostic details of venting problems

and see PLUMBING FIXTURE TRAPS for fixture trap inspection and defect diagnosis.

Also see METHANE GAS SOURCES and

See PLUMBING DRAIN NOISES where we explain the basics of proper plumbing vent piping and how errors cause trap siphonage, odors, and noises

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-11-28 - by (mod) -

Martha

If you think that the owner is coming out of dry Plumbing traps in a seldom-used bathroom you might try just pouring enough mineral oil into the drain to fill the Trap.

That won't evaporate and so it will reduce the odor problem. Let me know if that works for you.

On 2020-11-28 by Martha Schultz

We have a second bathroom that we don't use too much. When the weather turns colder septic/sewer smells start coming up from the bathtub drain,shower head and sink drains. Should we start using this bathroom more on a regular basis so the drains will not dry out and cause odor?

On 2019-08-10 - by (mod) -

I guess damage is possible, particularly at that check valve.

But your traps ought all to be water-filled so once everything's cleaned-up there ought to be no further odor.

On 2019-08-10 by mark

we had a sewage blockage and had to snake the drains with a commercial electric snake.

We cleared it but directly following tht we had sewage smells in our bathroom and near the drainpipes its a 1029 house on town water and sewage.

could we have damaged something with the snake? is there a vent flap where our drain meets the main town drain?

On 2018-01-05 - by (mod) -

Don

Check out also these COLD WEATHER SEWER ODORS diagnostic descriptions https://inspectapedia.com/odor_diagnosis/Sewer_Odors_Cold_Weather_Sources.php

On 2018-01-05 by don

Smelling sewer gas when temp dropped to 0. Only thing done recently is "Had a vent boot changed to a lead soil stack covering.
thinking blocked soil stack by ice

On 2017-06-23 by Monika

Hi ...actually really worried can't get my answer ...the thing is water is draining everywhere

but still I can feel some smell..earlier it was like septic smell then poured vinegar baking soda and some other things now foul smell is still there ...

in the night when I sit in my master bedroom I can smell the same duck gel which I used a long back ago to freshen the toilet ...don't know what to do my head is spinning ...we have 3 air vent 2air vents are open but one is covered with concrete slab ...is it because of that ?

On 2017-02-12 - by (mod) -

Most-often the problem is a clogged main drain line.

On 2017-02-12 by Cathy

Sewer is backing up in my downstairs toilet....the water is seeping slowly...what could the problem be

On 2015-09-29 by josh

Strong odor coming from vents on the roof vent pipes but only when it gets cool outside and have checked vents and they are not clogged

On 2015-08-03 - by (mod) -

If the toilet is not flushing properly in just some circumstances I'd check to see that the tank and bowl are filling to the proper flush level. Inadequate water volume can cause poor toilet flushing.

Once that's resolved we can discuss leaky toilet seals or vents.

On 2015-08-03 by Richard

Sewer gasses rush up the toilet waste pipe (in my bathroom) when flushing stool and not when flushing urine.

Please advise how to perform problem determination and fix the problem. It seems that a bladder infection has resulted in me. My wife's toilet is NOT affected nor is the downstairs toilet.

Question: raw sewage odors in very cold weather

I read your article and I would like to explain my problem to see if I have come up with the problem. I live in Cheyenne Wyoming and when it is very cold (below 25 degrees Far.) we get a very strong raw sewage odor from our basement bathroom. I have narrowed it down to the bathtub.

We did not finish the basement, it was finished when we moved in. The odor was not disclosed to us when we moved in 3 years ago and this winter it is worse but it is colder.

We have smelled it every winter since we moved in. Now tonight I decided to try an experiment

. I closed the drain on the tub and put water in the tub and this seemed to fix the problem

. So my opinion is that they did not put a "P" trap on the tub. So what do you think?
I do not smell the odor from the sink, the toilet, or the floor drains.
Thank you - John 12/6/11

Reply:

John,

There may be a corroded leaky P-trap that thus doesn't maintain its water seal. Try pouring water into the trap and inspect with a good light to see if it remains in the drain; Unfortunately some demo and digging may be in order to fix this odor source.

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Continue reading at TRACK DOWN COLD WEATHER SEWER ODORS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• PLUMBING FIXTURE TRAPS for fixture trap inspection and defect diagnosis.
• PLUMBING VENT DEFECTS & NOISES for diagnostic details of venting problems

Suggested citation for this web page

PLUMBING VENT BLOCKAGE ODORS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to DRAIN SEPTIC SEWER PIPES

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