Sewer Smell from Shower Drain after Flushing Toilet – How to Fix a Broken Trap Seal Yourself

A sewer smell from the shower drain after flushing the toilet is a rather unpleasant and common problem many shower cabin owners face. In this article, I will explain why such a smell occurs, what may cause it, and how to deal with it.

What is a Shower Drain and Why is it Needed?

A shower drain is a device installed on the floor of the shower cabin to drain water into the sewer system. It consists of several parts: a body, a grate, a P-trap, and a trap seal. The grate is meant to catch large particles like hair, sand, or soap scum that may clog the pipes. The P-trap is a curved pipe that connects the drain body to the sewer line. The trap seal is the part of the P-trap that holds water and prevents sewer gases from coming back into the room.

Why Can a Sewer Smell Come From the Shower Drain?

A sewer smell from the shower drain can appear for various reasons, but the most common is a malfunctioning trap seal. If the trap seal fails to perform its function, air from the sewer can freely enter the shower cabin and create an unpleasant odor. Here are some possible causes of a malfunctioning trap seal:

  • Trap drying out. This can happen if you haven’t used the shower for a long time or if the air in the room is too dry or hot. In this case, the water in the trap seal can evaporate leaving it empty. To prevent this, flush water down the drain regularly or add some vegetable oil to the drain.
  • Leaks. If there are cracks or loose joints in the pipes or traps, sewage water can seep through them and cause a foul smell. To detect leaks, check all plumbing parts for signs of corrosion, mold, or grime.
  • Clogs. If too much grease, hair, paper or other waste has accumulated in the pipes or traps, it can obstruct proper water drainage and create a favorable environment for bacteria and fungus growth. This can also lead to a sewer smell. Use special chemical or mechanical means to clear out clogs.
  • Trap seal loss. This can happen if flushing the toilet creates a vacuum in the sewer system that sucks the water out of the shower drain’s trap seal. It can be caused by improper installation or selection of the toilet or shower drain, as well as defects in the sewer system.

Video with a detailed explanation of some trap seal loss causes and sewer smell in an apartment:

How to Eliminate a Sewer Smell From the Shower Drain?

To eliminate a sewer smell from the shower drain, you need to determine its cause and take appropriate measures. Here are some ways to solve the problem:

  • Flush the shower drain and P-trap with hot water mixed with baking soda or vinegar. This will help soften and remove grime from the pipes.
  • Use special sewer cleaners like Roto-Rooter, Tirex, or Mr Muscle. They contain active agents that break down organic buildup and eliminate odors. Follow instructions on the package and be careful when working with chemicals.
  • Use a drain auger or drain snake to clear the pipes. This mechanical device can be fed down the pipes to pull out clogs. Be gentle not to damage the pipes or traps.
  • Install a vacuum breaker valve before the shower drain P-trap. This device opens when pressure drops in the pipe and lets in air to compensate for the vacuum. Thus, water will not be sucked out of the trap seal when the toilet is flushed.
  • Install a backwater valve on the toilet outlet. This device closes when pressure rises in the pipe and doesn’t let water backflow. Thus, water from your toilet will not create excessive pressure when flushed. However, this will not solve the problem if your trap seal gets sucked out by your neighbors’ toilet flushes.
  • Install an air admittance valve on the sewer stack. This device is connected to the upper part of the stack and provides constant air access to the sewer system. Thus, no vacuum will form in the stack when the toilet is flushed.
  • Call a professional plumber. If you cannot fix the problem yourself or don’t know how to properly disassemble and clean the shower drain and P-trap, it’s best to trust this job to a specialist. They will quickly and efficiently perform the necessary work and advise how to prevent the problem from reoccurring.

If the Smell Occurs After Flushing the Toilet or Using the Sink

In this case, it’s possible that a faulty drain is installed or the installation is improper, resulting in the toilet flush or sink use pulling the water out of the drain trap seal, which should stay there to act as a barrier and prevent sewer gases from entering the apartment.

To fix this problem, try the advice from this video:

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