Smelly Guest Bathroom? It's Probably a Dry P-Trap

The bathroom you only use at Christmas

Every SA home with a guest bathroom, a second-bath en-suite, or a granny flat sometimes left empty has met this smell. You walk in, and it's like the municipal sewer just exhaled. You flush, run the tap, spray air freshener — gone for an hour, then back.

The cause is almost never a leak or a venting fault. It's a dry P-trap, and you can fix it yourself with a glass of water.

How a P-trap works

Every plumbing fixture in your home — basin, bath, shower, toilet, floor drain — has a U-shaped bend in its waste pipe called a P-trap. That bend always holds a small slug of water. That water is the only thing standing between you and the sewer gases (hydrogen sulphide, methane, ammonia) coming up from the municipal main.

The water seal evaporates. In dry SA conditions, a basin trap can dry out in 3–6 weeks with no use. A shower trap takes a bit longer. A floor drain in a rarely used room dries fastest.

How to find which trap is dry

• Sniff each fixture individually with your nose right at the drain opening. Whichever one is worst is the culprit.
• The floor drain in the shower or under the basin is the most common villain in en-suite bathrooms.
• A floor drain in a laundry that no longer has a washing machine plumbed into it is a classic source in older flats.

The fix

1. Pour roughly 1 cup of water down the offending drain.
2. Add a tablespoon of cooking oil on top. The oil floats on the water seal and slows evaporation by 80% or more — buying you 3–6 months instead of weeks.
3. For toilets in unused bathrooms, flush once a month, OR clingfilm the bowl to slow evaporation.
4. For floor drains you really don't need, a plumber can fit a waterless trap insert (a sprung silicone valve) for around R400 — fit-and-forget.

When it ISN'T a dry trap

If you can smell sewer gas in a bathroom you use every day, something is wrong — call a plumber. Real causes include:

• A cracked vent pipe in the ceiling.
• A toilet wax/silicone seal that has failed.
• A blocked or undersized vent (back-pressure pushes gas through the trap).
• A broken pipe under the slab (rare but serious).

A note on health

Sewer gas at the concentrations a dry trap produces won't kill you, but hydrogen sulphide gives headaches and nausea, and it's a marker that the same gases now have a path into living spaces. Don't ignore it for months — at minimum, do the water-and-oil trick.