When you detect the smell of sewage in your shower's drain, you must immediately investigate the shower to determine the source of the smell. Several possible scenarios would produce a sewage smell from the shower's drain but not all involve a sewer gas leak as the source of the offensive smell.
The trap lies just below the surface of the shower and is made of a curving section of the shower's drainpipe. The trap literally traps water inside the pipe, keeping a wall of water present at all times. This water will block sewer gasses from entering your house through the shower's drain opening. If the trap dries out, either from a leak in the pipe or from nobody running water down the drain for a period of time, the sewer gasses will leak out of the shower's drain. Fixing leaks or running water down the shower's drain periodically will keep the trap full and the sewer gasses out.
A dirty shower drain can produce a smell that is very similar to sewage even though no sewer gasses or sewage is present in the shower. As you use the shower, you wash biological waste such as soap scum, skin oils and hair down the drain. Some of the waste will stick to the walls of the drainpipe, above the trap. Over time the waste will accumulate and become a ripe breeding ground for bacteria, which will produce an offensive smell. Each time you run water down the drain, the offensive smell is carried into the air. Scrubbing the drain with a large bottle brush, soap and hot water will clean off the waste and get rid of the smell.
Vent Pipe Problems
The shower and other plumbing fixtures in your home connect to vent pipes which protrude out of the house's roof. When a vent pipe becomes clogged by leafs, tennis balls or other debris, the plumbing fixtures can experience a drop in their drain water level, drying out the trap. The clog in the vent pipe can also push sewer gasses out through small leaks in the vent pipe. A vent pipe may have a major break or section missing behind a wall, creating the sewer smell in the shower. A plumber will send fake smoke through the house's vent pipes to locate obstructions or damage to the pipes.
Sewer gasses smell offensive, and they also pose a health risk to anyone who breathes them in for prolonged periods. While investigating a sewage smell in your shower, take precautions by wearing a protective mask and keeping others out of the bathroom. Be cautious of any open flames in the area of the shower since sewer gasses may contain methane, which is a highly flammable gas.