You hop in the shower to get fresh and clean, not to smell something gross coming from the drain. Smells in a shower drain can be caused by odor-causing bacteria that feed on debris in the pipe. Some of these anaerobic bacteria live in fetid water in the P-trap and produce hydrogen sulfide gas, which smells like sewage. Other odors can also be caused by the debris itself, like hair or soap scum build-up. Mold can also grow on the soap and give the drain a musty odor. Thoroughly cleaning the drain or ensuring the P-trap vent is clear can help that smelly shower.
Remove the Drain Strainer
The drain strainer itself has to be removed for a deep clean. Most shower strainers are attached with a single Phillips screw. After removing the screw with a screwdriver, you may have to pry the strainer up -- do this with a flat-head screwdriver. If you don't see a screw, all you have to do is pry off the strainer.
Clean Both the Drain and Strainer
The first thing you'll notice after removing the strainer is a layer of hair and scum on the underside. Pull on a pair of rubber gloves and pull off the hair and scum, using a sponge and warm, soapy water or a mix of disinfectant and water. If you look in the drain you'll see the same scum coating the drainpipe. Pull out as much hair as you can, using your fingers, an old toothbrush or a plastic drain cleaning tool with zippered teeth. You can also use a foam paint roller -- without the roller attachment -- to clean the pipe. Soak the roller in soapy water, insert it in the drain and rotate it like a sponge. Rinse and clean again until the roller picks up no more scum.
Disinfect and Deodorize the Drain
You can disinfect the drain by pouring a 50-50 solution of household bleach and water into it, but the bleach will simply flow down the sides of drainpipe and may not kill all the odor-causing organisms. As an alternative, pour one cup of baking soda in the drain, followed by a cup of white vinegar. Cover the drain and let the mixture fizz inside the pipe like a science fair project for a few minutes, then flush with hot water. Repeat if needed to get rid of any lingering smells.
Preventing Drain Odors
When it comes to shower odors, the main culprit is soap. You can't stop using soap in the shower, but you can prevent it from building up and becoming a problem by pouring boiling water into the drain once a week. If you suspect that soap is building up, dissolve it with an enzyme-based drain cleaner, and then add the water.
Vent-Related Sewer Odors
If you detect a strong sewer smell from the shower, especially after flushing a toilet or using the washing machine, your plumbing vents may be blocked. When this happens, suction in the pipes from flowing water can empty the P-trap. Clearing debris from the roof vent usually solves this problem. If the blockage occurs in winter, the roof vent may be blocked or iced over in winter months. If this is the case, go into the attic and point a hairdryer at the vent at the point where it exits the roof to melt the ice. Cleaning the blockage may solve the problem.