Trained Eye


Sewer Gas


We are having an ongoing problem with a sewer gas smell in our en-suite. We have checked numerous things to find where it is coming from, including opening up walls in several places to check for cracked pipes or other points where this may be coming from. One place that we opened the wall, the smell moved into the adjoining bathroom, on closing the hole, the smell went back to the en-suite. There doesn't seem to be a smell in the wall cavity itself when it is open. The smell gets worse overnight. The smell has an ammonia/soapy odour, most often more ammonia like and occasionally, more soapy.

Our house is a 13-year-old bungalow built by Broadview. The smell started around the time that we finished the basement, including a third bathroom, which has shower, toilet and washbasin. The piping for this was already roughed in with the original construction.

This problem seems to be getting worse and I am at a loss in knowing what to do about this, what would you suggest?


The first suggestion I have for you is to contact a licensed journeyman plumber with many year of experience in residential plumbing installation and repairs. This sounds to me like there may be a venting problem, or some other issue with the plumbing drains. A good plumber may be able to look at the conformation of the drains in your home and identify a defect in the installation that was an oversight.

The cause or causes of smells in homes, especially sewer gas or other plumbing related smells, may be extremely difficult to identify. This is because of the complexity of air movement within the home, which is affected by several different mechanical systems. The plumbing drains may be the likely culprit, but it is also probable that the heating distribution system is to blame. The presence of forced-air heating and ventilation systems in our homes may create air pressure differentials and convective currents that can negatively effect the normal removal of sewer gas from the home. This sounds very complicated and difficult to understand, and it is, but it can be simplified for our discussions. This will not be the complete or proper story, but it should help to make the situation understandable.

The plumbing drains in a home, in this case ABS plastic in a newer house like yours, serve two main functions. The primary function is the removal of waste and water from our kitchens, bathrooms, and laundry areas. This waste is washed into the municipal sewer system and out of homes by a gravity system. The crude expression that is often used, for simple illustration purposes, is that “poop flows downhill”. A key component to the proper operation of these drains is proper venting, which has a dual role. The first function of the vents is to allow air in to the top of the drain pipes, which allows proper flow of waste though the pipes. The second function, which is not as commonly known, is to allow sewer gas to escape the drains to the exterior of the home. This is the reason that the drains have open vents that terminate above the roof.

The vents allow the sewer gas to escape the drains, but there is another component that prevents it from seeping into our living space. That is the “trap” that should be present under any plumbing fixture, with the exception of toilets that are self-trapping, in the home. The traps, if properly installed, should always contain water, which stops the sewer gas from backing up the pipes and escaping into the interior of the house through the sink and tub/shower drains. The water “traps” the gas inside the outgoing section of the drains after the faucet is turned off and the waste and water flows out into the main section of the drains.

When sewer gas is detected in a home it is normally a failure of either the venting to eliminate the noxious vapours from the home or a trap that is not doing it’s job properly. Newer homes, such as yours, are pretty well air-sealed and may be subject to depressurization, which can cause sewer gas to reverse its normal flow. Instead of exhausting properly up through the roof, the gas may stall and come back down the pipes as the pressure in the house drops. There are many factors that can cause this to happen, which will be very difficult to analyze, and should be checked after the actual drains are inspected by a plumber.

It is most likely that a trap was improperly installed or is missing altogether. The possible locations are in the bathroom affected or in the newer basement bathroom. If the vents or traps were not properly installed in the basement bathroom, the sewer gas may be escaping into the wall cavity near the plumbing stack. This is rising up into the area near the en-suite, by the same convective forces that are inside the drain, and seeping into the house through gaps in the wall or sink cabinet. That may be why the hole cut into the adjacent bathroom let the smell migrate there.

One word of caution is that the probable causes of your problem that I have suggested here are mere guesses, based on the limited information I have. This complex issue cannot not be fully remedied without a thorough visual inspection. A good plumber may be able to quickly identify the problem if a simple trap or vent was missed, but that is wishful thinking. Investigation by a qualified Home Inspector or indoor air quality specialist, who takes the “house as a system” approach, may be required to identify the full cause of the smell and suggest proper remedial action.




P.O. Box 69021
#110-2025 Corydon Ave
Winnipeg, MB
R3P 2G9