Sewer Smell in Your House? Get the P Trap Fundamentals

It’s mind baffling. When that smell starts to come into your home, and you’re looking for the unflushed toilet, or the doggie accident, or the poopy diaper you may have forgotten, and you just can’t figure it out…

Where is that smell coming from?

Often, this means your home has a sewer drainage and/or ventilation system problem.

It’s important to do something about these smells. When sewer gases enter your home, they bring a noxious smell and possible headaches and respiratory ailments to those who smell the gases.

Still, there’s no reason to panic. Often, the problem can be easily fixed with a bucket of water, and/or by clearing the home’s ventilation exit on the roof.

Home Sewer and Ventilation Systems

Take a look under any of the sinks in your home – in the kitchen, bathroom, or basement laundry. As you follow the drainpipe, you’ll see it comes down, forms a “U” and then continues along into the wall. This is the P-trap.

As the name suggests, it vaguely resembles the letter P turned on its side. (No, it does not refer to pee!) Normally P-traps exit into the wall to the ventilation system. If not, you’re likely looking at an “S-Trap” which does not ventilate.

What does a P-Trap do?

A P-trap ensures that there is always a little bit of water trapped in the drain pipe in order to make sure smells from the sewer don’t enter into your home through your drains.

Smooth water and airflow

As you can see from the diagram above, in order for water to flow down into the drainpipe, there must be an air passageway behind the water. Vent pipes go up toward a building roof to provide a source of air and carry smells out of the house. The down pipe goes to the sewer.

NOT CLEAR? Ever bought a family sized bottle of detergent? As you open the valve to get the liquid out, it stops flowing if you don’t open the cap a little, because there is not enough airflow behind the liquid. You may have experienced this problem with a ketchup bottle. Your home sewage system works on the same principle.

Blocking Sewer Smells from travelling back up the pipe

The P Trap plays an important role in ensuring sewer smells stay in the sewer. The water in the trap acts as a stopper, making sure sewer smells don’t travel back up through the pipes and enter into your home. It’s as simple as that.

– But when there is no water in the P Trap, you will get a smell. Stinky indeed!

Basement or Main Drain P Trap

Basement P trap

Your basement P Trap is larger than other P Traps. It might be made out of a porous material, and can dry out quickly. When dry, it lets the smells travel in to your home.

The smell may come and go – just when you want to get help and get someone else to smell the problem, it will go away.

There’s a reason the smell isn’t always there. Most floor drain odours are caused by “negative pressure” which means your building is pulling or sucking in the odours from outside the building. Often, the problem is not consistent, only occuring on certain days, say when the city’s main sewer system is a little more backed up up due to increased rain, or ice blockages in the city.

Pour Water in your P-Trap

Make sure there is always water in your building’s main drain P-trap to block the smells. Simply take a bucket and pour water in. To slow the water from evaporating, add a half a cup of mineral oil, which you can purchase from the local pharmacy. This will naturally keep the water from evaporating too quickly.

If this doesn’t help your sewer smell problems, call a plumber to look for possibilities of cracked pipes, a blockage in one of your pipes, or perhaps there’s a drain in your home without any P Trap at all.

All drains need a P Trap: even the dishwasher!

In the course of inspecting kitchens, we have noticed a really bad smell coming out of some dishwashers. Same problem: dishwashers are sometimes installed without a P-Trap, so there’s nothing to stop the sewer smells from entering.

In addition to the sinks, bathtub and main drain, Make sure your dishwasher and washing machine have a P-Trap to avoid stinky dishes and laundry.


Mose Home Inspection Services
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Lay Chan Wee
Lay Chan Wee
01:19 31 Aug 21
I was impressed with the service provided by Rui Su. He was organized, explained in detail area that I am not familiar... with. He provided a good report. I will recommend him if you need home more
Mary Jane Caro
Mary Jane Caro
14:25 27 Aug 21
I am always pleased to recommend Carol Mose from Mose Home Inspection Services to my clients who have had offers... accepted on residential real estate properties. She is knowledgeable, thorough, provides clear explanations, and delivers comprehensive more
Massimo Pannunzio
Massimo Pannunzio
22:05 25 Aug 21
Michael has got to be on of the best home inspectors in all of Quebec. It was amazing working with him and he had my... best interest at heart when making a life changing decision. He made sure I had all the answers and made it easy to work with him. I will be using Mose inspection for any moment that I need an inspection done that's for sure!read more
Amanda Lee
Amanda Lee
19:14 16 Jun 21
Amazing experience with Carol Mose who helped me with a pre-purchase inspection! She was very approachable and... observant, and informed me of all her first impressions on-site. This gave me greater understanding of any conditions as I was able to ask any quick questions on the spot. Her reports are the most thorough that I've seen and include diagrams to help explain what any physical signs mean and potential consequences. Could not have asked for a better inspector!read more
Maeghan Banks
Maeghan Banks
15:38 14 Jun 21
Carol was wonderful to work with for our pre-purchase inspection. She was professional, thorough and communicative,... taking the time to explain her findings as we went through the house. After the inspection, we felt we had clear knowledge of the condition of the house (both pros and cons), allowing us to move forward with the purchase with confidence. Highly recommend!read more
Ryan Chalupovitsch
Ryan Chalupovitsch
13:43 01 Jun 21
Carol Mose is the best home inspector in the business. Approachable and extremely knowledgeable. She did an... inspection for me and really took the time to explain her findings and help me understand the home I was looking to purchase. Cannot recommend her enough. Don’t more
Elisabeth Faure
Elisabeth Faure
21:16 29 Apr 21
This is the second time I have used MOSE, and they are always really amazing. Very nice, very professional. they came... to have a look at my roof and let me know if we were due for a repair - I was very satisfied with everything and would recommend them to anybody!read more
Hannah S
Hannah S
22:05 16 Apr 21
I had a great experience with Mose home inspection. My husband and I are first time homebuyers and so had a ton of... questions about the process. Everyone we talked with was really informative and helpful. Michael was our inspector and was phenomenal. The inspection was very thorough and he explained everything in detail with realistic next steps for us to take. We came out of the inspection feeling really informed. Plus, he is just an absolute delight! He had very contagious enthusiasm and made the inspection a really positive experience. Would highly recommend!read more
Gabriel Lukuc
Gabriel Lukuc
03:05 16 Apr 21
Michael was amazing. He took the time to explain to us what’s really important and the reasoning behind it. We’re not... very handy nor savvy when it comes to home construction so we really appreciate him taking the time to explain to us all aspects of the inspection. Also, Michael was friendly, professional and timely; he provided the report over a holiday and a more
Stephanie Ballard
Stephanie Ballard
14:24 19 Mar 21
MOSE is the best in the business. Once again we were happy with their inspection services - they are professional,... friendly and knowledgeable. Would not hesitate to recommend to friends and family (and have)!read more
Rodrigo Morales
Rodrigo Morales
18:27 10 Sep 20
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