What Causes a Sewer Smell in the Bathroom?

One of most common causes of sewer odors in the bathroom and any area in the house with a drain is simply water evaporating from water traps. Simply called a “trap,” these devices are the U-shaped pipes commonly seen in plumbing fixtures, particularly in pipes that drains waste water to the sewage system. Water traps are designed to hold or “trap” a certain amount of water that creates a simple barrier that effectively prevents sewage gas and critters from seeping up a drain pipe and into a room.

The basic design of a water trap was invented in the 1880s by Thomas Crapper. Its effectiveness in sealing out sewage gases made the invention a standard plumbing feature in bathrooms, sinks and toilets. Water traps may either be external, such as in kitchen and bathroom sinks, or internal like in floor drains.

Trapped water in water traps may evaporate over time due to lack of use. Once this happens, sewage gas and odor, as well as vermin, will be able to enter through the drain pipes. The following shows one simple way of dealing with the problem.

  1. Find the source
  2. Locate the drain where the sewage smell is coming from. It will be a good idea to start with drains that are seldom used.

  3. Refill
  4. Once the source or sources of the sewage smell is located, pour about a pitcher of water into the drain. To help keep the water from evaporating quickly, add a few tablespoons of cooking oil into the drain.