Sewer Odors Inside

I was asked by a home inspector why there would be a bad smell inside a house that seems to be coming from the bathroom. Here is what I told him to look for:

Often, when a house has a few years on it, homeowners decide to upgrade their bathrooms. Many times, they change their flooring as a part of that process, and tile is a popular choice. When installing tile in a house built on a slab, typically you can just take up the original covering and cement the tile directly to the slab. It is a bit different when the house or bathroom is on a subfloor. You must first install a hard surface like HardieBacker cement board that adds roughly 1/2 inch to the height of the floor. Then you add 1/8–1/4 inch of cement and top it off with the tile, which is about 1/4-inch thick. When you are finished, you’ve added nearly a full inch to the height of the floor.

Why does this matter, and what does it have to do with odors?

When a house is first built, a flange is installed on the surface of the floor to which a toilet is bolted. A wax ring is used to seal the base of the toilet to the flange. Interestingly, toilets are different than any other drains in the house. All of the other drains like sinks, showers, tubs, washing machines, floor drains, etc., have a P-trap under them that always has a small amount of water resting in the bottom of a curved pipe. The purpose is to provide a vapor barrier to keep the sewer gases from coming back up through the drains.

The toilet has a P-trap built into it, and it’s a straight pipe once it hits the floor. Which translates to this: If the floor has been raised by an inch and the flange is still where it was originally, there will be a gap where vapors can escape. The toilet won’t leak at the base when you flush because it’s shaped like a funnel at the bottom and hovers over the flange. It will usually only leak if the plumbing backs up.

The remedy is to install a jumbo wax ring, which is on the same shelf in your hardware store and costs just a fraction more than the standard wax ring. This will fill in that extra gap and give you a good seal at the base of the toilet and should eliminate your odor problem.

Here’s another odor thought:

If your house has been vacant for a while, or if the kids have moved out and nobody has used their bathroom for a long time, the P-traps can dry out and allow sewer gases to escape into the house. This has an easy solution: Simply run water in all drains periodically to ensure there is always some water in the P-trap. You can also add a very small cap-full of vegetable oil to the drain. You only need to add enough oil to form a thin layer of oil on top of the water standing in the P-trap. The oil layer will protect the water from evaporating and water will easily break through it when the drain is used again. The key here is a thin layer. Don’t overdo it.

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