Q: My husband and I purchased a home, built in 1900, which needed some work. What we didn’t know was that a previous owner was well known for thinking he could do all the “fix-up” projects himself. Soon after moving in, we noticed a sewer smell in the bathroom tub/shower. There was a similar smell in the attic. We installed a Studor vent and the attic smell went away, but it didn’t get rid of the one in the shower…
— Alice Stauffer
A: Any number of reasons can cause bad odors to seep out of drains. In some cases, just plain lack of use may be the culprit. Every fixture that has a drain connected to a waste line must have a “trap” that holds water, which acts as a seal to keep sewer gas from coming up the line. If a fixture is not used at least once a week, the water in the trap may evaporate and break the seal, allowing sewer gases into the house.
Another possibility is that the tub is inadequately vented or that the vent itself is blocked. Plumbing vents, also called stacks, typically rise up through the roof so that the gases can float away. In an old house, the stacks may have been damaged by construction over the years, or blocked by falling debris.
When water or other fluids flow down an undersized or blocked vent line and completely fill it, air in the drain line can create enough suction to siphon water out of the trap, allowing sewer gases to enter the drain and consequently the bathroom.
If the problem is caused by inadequate ventilation, try adding a second air-admittance valve (i.e., Studor vent) in the line, as close to the tub as possible. That will allow enough air to be pulled into the line to improve the flow in the drain.
Last but not least, check the P-trap that serves the tub and shower. It may be damaged—or, in the cause of a house with a lot of sketchy DIY projects—missing altogether. Replacing the P-trap should cost less than $100.