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.