There are a number of causes of a bad smell from a shower drain. Clogs inside a drain typically composed of hair, soap film, and oils can amass and begin to decay inside the shower drain, resulting in a foul odor. Biofilms or a sheet of bacteria and mold that clings to the pipes may also produce a bad smell. A nasty drain smell may also be caused by sewer gas leaking back up the tub drain. When a bad drain smell is not caused by one of these problems, the offensive smell may only seem to be coming from the drain, but could actually be a separate bathroom issue.
Clogs form gradually over time as particles that pass through the shower strainer collect along the sides of the pipe. The organic matter in the clog will then begin to rot, giving off a foul smell from a shower drain that will become increasingly pungent. If there is a bad smell coming from the drain and the water takes long to run from the tub, then a clog is more than likely the cause of the bad smell. Removing the clog should eliminate the smell.
Biofilms can also be the cause of a awful smell from a shower drain. Composed of layers of bacteria, bacterial waste and mold, biofilms can grow in any aqueous environments that provide adequate nutrients, such as shower drains. When water has a small quantity of sulfate in it and some of the bacteria in the biofilm are capable of reducing sulfur, the biofilm will produce hydrogen sulfide. This chemical has a powerful and distinct odor of rotten eggs. Flushing bleach down the drain may destroy the noxious odor-producing bacteria.
Another cause of a bad smell from a shower drain can be the back flow of sewer gas into the drain. Below the shower drain is a pipe that is curved. This bend, know as the trap, is supposed to be completely filled with water to prevent sewer gas smell from passing up into the house. When a shower is not used, the trap can dry out and sewer gas can pass up through the drain. Running water to refill the trap can eliminate this smell.
Sewer gas smell emanating from a shower drain may also be caused by a block bathroom vent. When water passes into a drain, air is forced out of the pipe. The air in the pipe is resupplied by the roof plumbing vent rather than through the drain. If the roof vent gets clogged by a bird nest or something else, the air refilling the pipes can force the water from the trap, allowing the sewer gas to run back up the drain.
Occasionally, the drain can be mistakenly blamed for a smell that is actually coming from somewhere else in the bathroom. A leak in the shower or tub tile, allowing water to flow behind the tile, can create a warm moist environment for mold to grow. Mold can have an overpowering odor and is sometimes mistakenly traced to the shower drain. A dead animal decaying in between walls can also create a very overpowering smell that can be difficult to trace.