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Wet Weather or Cold Weather Sewage or Septic Odors: Diagnosis and Repair Guide SepticAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• How to diagnose sewer odors in wet or cold weather
• Causes and cures for sewer gas odors related to wet or cold weather
• How to find and cure bad smells in buildings

This article discusses the diagnosis and correction of sewer gas or septic odors (and other building smells and odors with focus on diagnosing odor sources and causes in cold weather. Some of the diagnostic steps pertain to all seasons. The photograph provides a lot of septic odor diagnostic information if you look closely: there is a home made septic tank nearly touching the building wall (by those steps), and in the foreground is a pony pump that the owner was using to try to move septic effluent uphill to his drainfield. This system would not work reliably: the tank is too small for normal use and the pump is exposed to freezing. Odors from the septic tank were strongest at the front entry door to the home, perhaps in part because the system was in failure and backing up. This article on diagnosing sewer gas or septic odors is a special edition of our more general advice on finding and curing sewage odor problems. Here we focus on sewage or septic odor problems that occur during cold weather or wet weather. Also see our broad-scope article on diagnosis and cure of sewer gas and septic odors: Sewer Gas Odors diagnosing, finding, and curing septic tank and sewer line smells. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical reviewers are welcome and are listed at "References." This is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

The Short Answer to Diagnosing Bad Smells: Septic Odors in Cold Weather

Locate the outdoor septic components - septic tank and fields, or their probable location. Melted areas of snow can be helpful as they often mark the location of even properly-working septic system components - the soil is warmer at the septic tank and at the drainfield lines. As in the two photographs of snow over septic fields (left) and over a septic tank (right), if you see snowmelt, the system is probably not frozen. Warm effluent is entering from the septic tank and bacterial action is progressing in the soils. If you do not see septic effluent coming to the surface of the yard and if drains are not backing up inside the outdoor parts of the septic system appear to be working. But how do we know if the septic system is frozen and not accepting waste from the building? What if we think the septic system is blocked by freezing? Before doing anything costly it's important to do your best to properly diagnose the problem. Do NOT hire a company to "rejuvenate" your septic field nor to install or flush or use any other magic product. It would be a better investment to take the cash you'd have paid them and rip it into small pieces and flush it down the drain. Frozen septic fields: A septic system can become frozen in prolonged very cold weather if the system is also not in regular use. If the septic field froze and stopped accepting effluent, or if it were totally blocked and in failure (see below), your entire septic system would back up as you used the building drains, and you'd see drains backing up into the lowest areas of the home like a basement toilet or sink. I don't mean odors, I mean sewage backup. Partial septic blockage: If a building drain or septic system or system drainfield is partially blocked, it's possible for that condition to generate odors in or out of the building. In this case odors often correspond to surges in system use, and odors should not correspond particularly to cold weather, except insofar as cold still air may permit gases to fall (from a plumbing vent stack, for example) , or different seasonal wind patterns may cause gases to move to areas where they're not noticed in warmer weather. If you're using the system and it's not backing up it's not so likely that the problem is a frozen septic field. There could be a drain vent line problem such as a frozen vent line (frost from moisture moving up the vent line above the roof from use of that drain for laundry or showering). Be sure the traps in the lower bath are not dry as a dry trap will often send sewer gases into a building.. Check out the above items first.

Diagnosing and Eliminating Sewer Gas Odors

Smelling sewage odors inside only, or mostly inside

Here are some places to look and perhaps to fix if you're smelling sewage or septic like odors inside of a building:

• Blocked or Inadequate Plumbing Vents can cause odor backdrafting into a building
• Dry Plumbing Traps can serve as a Sewer Gas Odor Source indoors - check all of your fixtures, especially ones which have not been used in some time or ones which are lacking proper venting. A plumbing trap serving an unused sink or shower or tub can dry out and permit sewer gases to enter the building up through the fixture.
• All drains and traps may smell and produce a little methane:: But beware, a little gas odor is detectable at many fixture drains and traps since there is likely to be decaying organic matter there. Check for dry plumbing traps.
• Fixtures which are not properly vented may lose their water trap: If the fixture is not properly vented, even if it's in-use, the water that is supposed to be in the fixture trap may be siphoned away, permitting sewer gases to enter the building.
• Plumbing vents as an odor source: a leaky plumbing vent could be an odor source indoors. The odor may not be coming from plumbing drains: If none of the building drains are clogged or slow and if the septic system is otherwise working one's first guess would be a problem with the plumbing the vent system. This condition can occur year round.

You can usually spot inadequate or blocked plumbing venting by noticing, for example, that tub, shower, or sink drain makes a gurgling or "glub glub" sound when a nearby toilet is flushed or when a full fixture is being drained. Inadequate or blocked plumbing venting can also mean that a plumbing fixture does not drain well (even though the drain line is not blocked). Inadequate venting produces a glub glub sound as water passes down a drain and cannot draw air in after itself through a plumbing vent stack. Instead the drain has to draw air in to itself through various plumbing traps which, as they normally contain water, produces the glub glub sound as air passes through the trap. This defect can siphon water out of traps and let sewer gases back into a home - smelly and potentially dangerous (methane gas).

Also see Drain Noises: may indicate defective or clogged plumbing: how to diagnose and cure drain sounds

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

What's different in cold weather? An under-sized 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.

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.

Plumbing Vent Problems and Trap Siphonage, Dry Plumbing Traps and Sewer Gases

Freezing weather can cause a complete blockage of a plumbing vent stack by frost. 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 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?

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.

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.

Blocked or slow building drains as a sewage gas odor source

A second indoor sewer gas odor explanation is that there may be a partial blockage in a waste line such as between house and septic tank or less likely between septic and fields. The "glub glub" sound discussed above can also be caused by a partially-blocked drain which is simply slow.

Drain noises due to bad venting or due to a partial blockage will be invariant day or night, depending on drain usage, but in the more quiet still and cool night air, drain noises are more easily heard.

As we said earlier, partially blocked building drain, septic tank, or drainfield can generate odors indoors or outside. Odors in this case will usually correspond to surges in system use, and odors should not correspond particularly to cold weather, except insofar as cold still air may permit gases to fall (from a plumbing vent stack, for example) , or different seasonal wind patterns may cause gases to move to areas where they're not noticed in warmer weather.

Diagnosing fixture versus plumbing system blockage problems for correcting septic or sewer gas odors

How to tell a local problem at one fixture from a system wide plumbing problem:: If one fixture or one group of fixtures is not draining well, suspect a local problem with blockage or venting; if all building drains are slow or blocked, suspect a system waste line blockage or blocked septic system.

Odors caused by slow drains or septic drainfields: Included here but not common is the odor causing effects of a slow drain or drainfield: sometimes there will be a partial clog, causing septic effluent to back up and then recede as the drain empties. In this case there may be no visible actual sewage backup into the building. The same thing could occur if the backup was caused by a failing drain field � the system may back up when there is a large volume of wastewater but then slowly recede. Or it might just create enough back pressure to push gas through the traps but no actual effluent.

How to diagnose partial drain blockages: If a partial drain blockage is the problem (watch out it could be due to a damaged pipe or tree roots in which case simply snaking the line is just a band-aid short-term fix), you might notice the problem more when the volume of wastewater surges, such as when doing laundry or emptying a bathtub. An increase in the number of occupants in the building (and thus using the bathrooms) will also cause a surge in wastewater volume and may disclose these conditions.

Septic Blockages and Sewer Odors in Wet Weather

A very common source of drainfield failure as well as an occasional source of septic tank back-flooding and occasional push of sewer gases (or sewage) back into a building is the failure to protect the septic tank and drainfield from surface runoff and ground water. An intercept drain may be needed. If this is the case septic problems (odors or backups) will be associated with heavy rainfall or wet weather.

Indoor Septic Odors with Outdoor Causes

Septic odors in the house could indicate that effluent is backing up into the house past the traps which normally intercept the odors, or entering at a building window or door which has had a plumbing vent terminated too close by. Such odors may or may not mean the septic system is failing. A backed up septic system does not always equal a failing septic system.

Outdoor Septic System or Sewer Gas Odor Sources and Causes

If you are smelling sewage odors outside in cold weather but not inside be sure to check these items:

• Is there a municipal sewer?: Make certain that your building and your neighbors are served by private septic systems as opposed to a municipal sewer. Gases from a municipal sewer are also explosive and can be present in large quantity.
• Is it a sewer gas or a fuel gas leak?: Make certain that what you're smelling is sewer gas and not a fuel gas leak such as LP gas or natural gas from a gas appliance, fixture, or gas line. There are serious explosion hazards with gas fuels. If you suspect a fuel gas leak leave the building and call your fire department from a safe location.

Sewer Gas Odor Tracking by Location and Season

Where on the property are odors strongest? you may be able to point to a waste line, building exit piping, leaks at a septic tank, drainfield failure, or even a neighbor's septic system problem.

Look for a wet area, possibly covered by snow in northern climates - kick the snow aside in a grid pattern over the septic system components (don't' fall into a collapsing septic system - it can be fatal). Look for areas where snow has melted to a thinner cover. This can occur in a normal system (bacterial action in the soil over the septic system and warm septic effluent carry heat out of the septic tank). But it can also be a clue of sewage effluent coming to the surface. Check such areas for effluent.

If a waste line is blocked or partly blocked and the odors are near the house, such as at the house wall at the waste line exit point, effluent could be running along the buried pipe but outside it, having leaked from a damaged pipe at the wall, between the wall and the septic tank, or at the tank itself there could be an effluent leak where the line enters the tank, or at the tank cleanout top cover (which would indicate a blocked tank outlet or blocked drainfield).

Effluent will follow a buried pipe because it runs in a trench dug in the soil - the pipe and backfill in the trench are less solidly packed than in the surrounding soil - the trench acts as a conduit to bring sewage effluent to the house if the trench is filling with liquid.

Broken pipe leaks may be mistaken for ground water leaks: At one property where basement paneling was removed following "a history of basement water entry from 'rising ground water' (according to the basement de-watering company)" a company had installed an expensive interior trench and drain system and sump pump to pump the "ground water" away. I saw an inverted "vee" of leak stains on the basemen wall extending from below the main waste line where it exited the building. It was obvious that the water entry had been not from rising ground water but from a broken leaking waste line outside the wall. Sure enough, our septic dye appeared in the new basement trench and drain system in just a few minutes. The basement de-watering system had not been needed at this home, and the owner still needed to have the broken waste line excavated and repaired.

Look for leaks at a waste line, perhaps first by having a plumber snake the line from inside the building to see if s/he feels evidence of a broken or collapsing or damaged pipe between the house and the septic tank.

If the drain field is saturated or blocked, expect to find abnormally high sewage level in the septic tank, possibly even backing up and flowing out when the tank is opened, and possibly also evident at the distribution box.

Remedies for Sewer Odors Caused by Plumbing Defects or Septic System Defects

• Inadequate plumbing vent size: If it's a plumbing vent diameter problem the solution would be to install a larger diameter vent pipe, eg 2" up through the roof and outside.
• Smelly dry plumbing traps indoors: Special plumbing traps which include a built-in seal against gas backups are available for use in problem locations. We just pour mineral oil into unused traps to prevent evaporation of the trap seal.
• Inadequate venting: if building vent piping is missing or inadequate or improperly installed get advice from a plumber on both stop-gap ("V-200 vacuum breaker vents) or proper (vent piping) repair alternatives.
• Bad Plumbing Vent location: if a building vent is too close to a window or otherwise is delivering gases to occupants, it may be possible to extend or relocate it.

Odors and Plumbing Drain Line Leaks or Blockage Repairs

Leaks and/or odors due to damaged drain piping

If a building drain is damaged and leaking the point of damage needs to be found and repaired. If it's an indoor drain the leak can usually be found by evidence of leaks into the building; if it's an outdoor drain leak, use of a plumbing snake, judicious careful probing, or excavation will be needed. Sometimes simply lifting a section of sidewalk over a drain or lifting a large stone will disclose a previously unrecognized wet area where a drain line has been broken or damaged.

Blocked Drains and Sewage Odors

If building drains are blocked or appear blocked hire a plumber to snake the drains. If all drains are blocked, the plumber might start by snaking out the main drain. During this process an experienced plumbing power snake handler can tell if the drain line is broken or damaged or clogged by tree roots. If this is the case an excavator and drain line repairs are needed.

Slow Drains and Septic Odors

If building drains are not blocked but drainage is slow, sluggish, smelly, or backing up into the building, an inspection of the septic tank can indicate (by abnormally high level) that the tank inlet baffle or outlet baffle are blocked (or missing entirely), or that the tank outlet to drainfield is backing up, or blocked, or the drainfield not working.

Pumping the septic tank to address an "odor problem": while periodic pumpout of a septic tank is important maintenance to protect the drainfield, will never "fix" one of these problems. The cause of blockage or failure needs to be identified and repaired.

How to Investigate & Repair Failed Drainfields and Septic Odors

If the septic system drainfield is blocked or failed it may be possible to reroute effluent to an un-used or under-used section of the drainfield (if effluent was not being distributed uniformly in the first place) - check the distribution box (if any). There will be more evidence of septic field condition in the D-box; if the box is flooded either these lines are blocked (such as by poor, uneven installation, tree roots, or a collapse, something not too likely unless you drove a truck over the fields), or the field has stopped percolating and needs replacement.

If on opening the D-box or excavating a drainfield line you see standing water in the leachfield line, either the field is saturated - lost perc - or the line has collapsed nearby and is not flowing. You can test this by running a hose into the leach line from the D-box or from your point where you've cut it open.

Sometimes you'll see that only one field line is saturated and failed - you can close it off in the D-box and just use the others to give the saturated one a rest, but at the end of the day, you probably need a new drainfield.

Septic Repair Shortcuts and Septic Treatment Products and "Magic bullet" septic repair products and procedures like chemicals, additives, root killers, or soil restorers are mostly ineffective, waste money, and in some cases are illegal as they contaminate the environment. If the septic system drainfield is blocked or failed, most often the property needs a new drain field.

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material.

• Daniel Friedman - principal author
• Herb Reed County Extension Director, Agricultural and Natural Resources Educator, Calvert County Maryland - private email to DF 9/5/2006 adding comments about odors and partial blockages.
• Technical reviewers are invited to comment or ask questions - contact us

Use links just below at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

Odors, Smells, Gases in Buildings-Diagnosis & Cure
ANIMAL ODORS IN BUILDINGS
CARBON MONOXIDE & GAS HEAT ODORS
CARPETS & PADDING ODORS IN BUILDINGS
GAS MEASUREMENT TOOLS
MOLD ODORS, Musty Smells in Buildings
OIL HEAT ODORS
OIL TANK LEAK ODORS
PAINTS & COATINGS ODORS IN BUILDINGS
SEWER GAS ODORS
SEWER GAS ODORS in COLD WEATHER
VINYL Siding or PLASTIC Window ODORS in Buildings
ODORS IN WATER
OZONE for MOLD OR ODORS

More expert information on this topic

More Information on Septic System Diagnosis, Maintenance, & Repair

• Sewer Gas Odors diagnosing, finding, and curing septic tank and sewer line smells.
• Drain Noises: may indicate defective or clogged plumbing: how to diagnose and cure drain sounds


• The Septic System Information Website Home Page: public information website about septic system design, installation, maintenance, and repair
• Ten Steps to Keeping a Septic System Working, suggestions from the U.S. EPA, edits and additions by DJF
• What is a Septic System An Engineer's View & Septic System FAQ's
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.


Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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