A musty smell from a sink, tub, shower, or basement floor drain is one of the most common household complaints in St. Louis, especially in older homes and during warm, humid months. The smell usually comes from sewer gas or from a film of bacteria and debris inside the drain. It rarely comes from the water supply itself. Work through the steps below in order, since the simplest cause is the most likely one.
Start with a few observations
Before you open anything, write down what you notice. A short list helps you spot patterns later.
- Which drain smells: the bathroom sink, the tub, the shower, a basement floor drain, or the kitchen sink.
- Whether the smell is strongest at the drain opening or throughout the room.
- Whether it appears only after the fixture has sat unused, such as after a trip or a dry spell.
- Whether the smell is present on cold water only, hot water only, or both.
A smell that shows up only on hot water points toward the water heater, not the drain. A smell at a drain that rarely gets used usually points toward the trap.
Check for a dry trap
Every drain has a curved section of pipe under it, called the trap, that holds a small amount of standing water. That water blocks sewer gas from rising into the room. If a fixture goes unused for weeks, the water can evaporate and let the odor through.
To test this, run the tap for a short time and smell the drain again. If the odor fades after the water runs, a dry trap is the likely cause. In a basement floor drain, pour in enough water to refill the trap, then keep the drain used or add a trap seal product sold for that purpose. Floor drains in St. Louis basements are often neglected, so this is a frequent fix.
Look for film and buildup
If the trap is full and the smell persists, the problem is often inside the drain. Hair, soap scum, and bacterial slime collect on the pipe walls and in the stopper. Lift the stopper or remove the pop-up assembly, then clean it with a brush and hot water. A mild dish soap is enough for routine cleaning.
Avoid pouring different drain cleaners in sequence. Mixing chemicals can create fumes and damage pipes, and most musty odors respond well to brushing and rinsing alone.
Check the vent
Drains are connected to a vent pipe that runs up through the roof. The vent lets air move so the traps can drain and refill correctly. If a vent is blocked by leaves, debris, or a nest, a drain can start to gurgle, drain slowly, or pull water out of its trap, which lets gas through.
If you notice gurgling from one fixture when another is used, or slow drainage across several fixtures, look at the roof vent cap from the ground with binoculars if you cannot safely climb. Do not climb onto a steep roof to check it yourself.
Test before you treat
- Pour a small amount of water down the unused drain, wait a few minutes, and smell the opening.
- Run the tap at full flow for a short time, then smell the drain again.
- Clean the stopper and the visible drain opening, then smell the drain once more.
- Compare the results. A fix after step one points to the trap, a fix after step three points to buildup, and a smell that persists through all three steps points to the vent or the main line.
Keep notes on which step helped. If the smell returns, you will know which part of the system to check next.
Rule out the water supply
A water treatment system, including a salt-free system, does not usually produce a drain odor. If your smell is limited to the drain and does not appear at the tap, the water supply is an unlikely cause. If the smell is present at the tap itself, check the water heater for hot-water-only odors, and consider a water quality test for cold water so you can compare the results against your local water report.
When to call a licensed plumber
Stop guessing and call a licensed plumber if you see sewage backing up, wet floors around a drain, odors from several fixtures at once, or gurgling that continues after the simple checks. Many older St. Louis homes have aging sewer lines, and a plumber can inspect the line with a camera to find problems that a homeowner cannot see from above. Ask for a clear written explanation of what was found before you agree to any repair.