Start by filling dry fixture and floor-drain traps and checking waterless urinals, toilet seals and vents. Map which restrooms report the odor. One room can have a local or shared cause; several rooms justify checking common piping and airflow. Persistent odors need a site-specific inspection.
Step 1: Fill every trap
Restrooms usually have a floor drain, sink traps, and toilet or urinal traps. The 2024 International Plumbing Code, as adopted in Phoenix, Section 1002.4, calls for a water seal of 2 to 4 inches in each trap. The floor drain is the usual culprit, since nothing runs into it. Pour in water until it stands full, and do the same for rarely used sinks. Give it a day.
Step 2: Check urinals
Flush urinals rely on their trap like any fixture. Waterless urinals use a sealing liquid or a cartridge to block gas, and those need maintenance on the maker's schedule. See waterless vs flush urinals. A neglected cartridge is a common source of a restroom smell.
Step 3: Look at the toilet base and the wall
A failed seal at the toilet flange lets gas up around the base. Signs are a wobble, stains or a smell strongest at floor level. Also check for an open cleanout cap or a missing floor drain grate.
Step 4: Think about air movement
Exhaust fans, air handlers and open suite doors can pull gas through a dry trap or a poor seal and spread it. If the smell appears only when the ventilation runs, note that for the plumber, because it points to a small opening that air movement is exploiting.
A smoke test when the cheap checks fail
If full traps do not end the smell, a smoke test can help reveal otherwise hidden air paths. HQ uses professional liquid-smoke equipment that forces air into the vent system, so smoke can enter accessible drain and vent paths and may escape through an open pipe, fitting or seal. Smoke that appears inside the restroom, in a wall or at a floor drain marks the opening.
Step 5: Vents and the main
If traps are full and the smell returns, a blocked or damaged vent can pull water out of traps as fixtures drain. Look for gurgling and slow drains. Then a plumber can camera the drain and check the vent path. Chapter 7 of the code also sets cleanout and testing rules for the drain system, which matter if the building was altered recently.
How to tell local from building-wide
One restroom smells: work that room. Restrooms on several floors smell, or the smell is in the lobby: suspect the shared stack, vent or building sewer. Our apartment building guide has the same mapping method.
A safety note
ATSDR points out that the rotten-egg smell can vanish at high levels as the nose adapts. So a "smell gone" after a few minutes is not proof of safety. HQ offers 24/7 service, is licensed and insured, and gives free estimates with upfront pricing.
Hotels and lodging have the same problem in guest rooms and back-of-house areas. See hotel sewer smell in rooms and floor drains.
A smoke result identifies an air path under the test conditions. Obstructions, soil and concealment can limit visibility, so no visible smoke does not rule out every defect. The crew confirms the cause before assigning a repair or responsibility. See HQ’s plumbing smoke testing.
For a related odor investigation, see sewer odor in a school or clinic: what is the risk and the fix.
For a related odor investigation, see why does a suite smell like sewer after a remodel.
For a related odor investigation, see sewer smell in a strip mall: how do you find which tenant.