Sewer smell in the restroom or breakroom — the dry P-trap

What this error means

The most gratifying diagnosis in facilities: that awful smell is usually an EMPTY TRAP, not a plumbing failure. Every drain has a P-trap holding a plug of water that blocks sewer gas; in floor drains and rarely-used fixtures the water evaporates — Oatey documents 2–4 weeks of non-use as enough — and sewer gas strolls in. The documented fix: pour water down the drain (about a gallon), plus a tablespoon or two of mineral oil, which floats on top and slows future evaporation. Monthly, for drains nothing ever uses.

Symptoms

• Sewer smell strongest near a floor drain or unused sink
• Smell appears after vacations/slow periods
• Comes and goes with weather (pressure changes push gas past dry traps)

Try this first (safe DIY steps)

1. Find every floor drain and rarely-used fixture (mop sinks, that shower nobody uses)
2. Pour ~1 gallon of water down each, plus 1–2 tablespoons of mineral oil
3. Calendar it monthly for unused drains — or fit trap-seal inserts (Oatey/SureSeal style) for a permanent fix
4. Smell persists at a toilet specifically: suspect the wax ring — that one's a repair

Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.

When to call a technician

Persistent smell after trap-filling = wax ring, failed trap primer, or a vent problem — plumber territory ($150–400 typical for a wax ring).