Find a Sewer Gas Odor in a Bathroom

Dear James: Our house has several bathrooms with tile floors. One seems to have either a moldy or a sewer gas type of smell at times. What could cause this, and is it something that I can repair myself? — Bonnie D.

Dear Bonnie: As you have noticed, it is difficult to even distinguish the type of odor when it comes to plumbing. Don’t despair. Most plumbing repair jobs can easily be handled by the average do-it-yourselfer.

Odors in bathrooms can have many sources, so you will have to start checking the most likely sources first. The fact that you also have a moldy odor indicates that there is probably a leak somewhere, allowing the materials in the floor below it to stay damp and moldy.

Sewer gas odors may have many sources, but it often comes from a trap that is not staying full of water. The purpose of a trap in plumbing is to create a water seal to keep sewer gases, as well as small critters, out of your house.

Just looking down into the drains with a flashlight is not always a reliable method to determine if a trap is full of water. If there is just a little water at the bottom of the trap, it will reflect the light, even though there is not enough to seal the trap.

Pour a cup or two of water in all of the drains to make sure the traps are full. Push a thin stick down into the drain and measure the height of the water level from the top of the drain. Wait an hour or so and check the water heights again. Very little should have evaporated in an hour, so if the level is lower, there likely is a leaky trap.

This is very simple to repair under a sink. Just unscrew the fittings on the old trap and replace it with a new one. While you have the trap off, clean out the pipe above it. Soap scum, hair, etc. can build up in it and cause some odors even if the trap is functioning properly.

If the bathtub trap is leaky, it is much more difficult to repair. You will need to cut a hole in the ceiling in the room underneath the bathroom to gain access to the trap. Make sure to exhaust every other possibility before attempting this fix.

Another possible source for the combination of odors is the toilet. The wax ring seal, where the toilet fits over the floor drain, may be leaky. This does happen over time, especially if the toilet is not bolted down tightly to the floor. Try to rock the toilet. If it moves, you may have located your odor problem.

If the wax ring is leaky, each time the toilet is flushed and the gush of water flows through to the drain, some of it will leak out. This is dirty water that may seem like a sewer gas smell. The dampness that it creates would account for the moldy odor.

A new wax ring costs only a couple of dollars and it is not difficult to install yourself. The most difficult part of the job is handling the heavy toilet so you should have a helper. Turn off the water to the toilet and disconnect the feed pipe. Flush it and then use sponges to remove all the water from the tank and bowl to make it lighter.

Unbolt the toilet from the floor and lift it off the drain. Scrape off the old wax ring and insert the new one. Carefully place the toilet back over the drain so as not to damage the new wax ring. Give it a slight twist back and forth, secure it tightly to the floor and attach the feed pipe.

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