A sewer gas smell that is present only after taking a shower may indicate a problem with your drain or drain vent. One way to diagnose the problem is to eliminate potential sources of the odor by checking for common drain issues. A basic knowledge of plumbing is all that is necessary to diagnose many common household plumbing problems.
The first thing you should check for when presented with a drain with a sewer odor is a clogged drain. A drain clog can prevent water from draining, allowing mold and mildew to grow and an odor to emanate from the problem drain. Ensure that all of the drains in your bathroom are functioning properly by running the water in the sink, flushing the toilet, and double-checking that your shower is draining properly.
Your shower drain, and any other household drain, should have what is called a drain trap. A drain trap is basically a small elbow piece of pipe that is installed between a household drain and the sewer. This elbow is situated in such a way that it remains filled with water, sealing off sewer gases and preventing them from backing up in to your home. Check to make sure that your shower has a drain trap and it is properly installed.
All household drains need to be tied in to a drain vent. A drain vent is a stack pipe that typically goes through your attic and vents outside through your roof. This allows air to flow into the household drain system to prevent draining water from forming a vacuum in the pipes and backing up the drains. These vents can become clogged with debris, and can easily be checked and cleaned using an ordinary garden hose if you have access to your roof. The vent should look like a small piece of metal pipe, approximately 2 inches in diameter. Run the hose down the vent pipe and turn on the water. Ensure the water drains and does not back up.
Cracked Drain Line
A cracked drain line can cause a sewer gas odor to emanate from the drain system when the shower is run. If the crack is situated along the top of the drain pipe, sewer gas would escape before any water would, so there would be no obvious signs of a leak. Check the top of your drain pipe for any cracks that would allow the sewer gas to escape.