Nine times out of ten it is a dried-out P-trap in a drain nobody uses, and the fix is to run the tap for thirty seconds. If the smell survives that, the next most likely cause is a failed wax ring under the toilet, then a cracked or blocked vent stack.
Start with the free fix
Every drain in your house has a U-bend called a P-trap that holds a plug of water. That water is the only thing between your bathroom and the sewer. In a guest bathroom, a rarely-used tub, or a floor drain, the water evaporates — faster in Florida than almost anywhere — and the barrier is simply gone.
Run every fixture in the room for thirty seconds, including the tub and the shower. Pour a jug of water down any floor drain. Wait an hour. If the smell goes, that was it, and it will come back every few weeks in a room nobody uses. A cup of mineral oil in the trap slows the evaporation right down.
If the smell is worst near the toilet
The toilet sits on a wax ring that seals it to the drain flange. Wax rings fail, especially if the toilet has ever rocked. The tell is a smell that is strongest at floor level near the base, sometimes with discoloured or lifting flooring around it.
This is a genuinely doable afternoon job if you are handy — drain the tank, unbolt, lift, scrape, new ring, reset. It is also the job people most often do badly, because setting the toilet down twice squashes the new ring and you are back where you started. One placement, straight down, no wiggling.
If it smells worse when it rains, or only sometimes
That points at the vent stack — the pipe that runs up through your roof and lets the drain system breathe. Blocked with a nest or debris, or cracked in the attic, and the system pulls its air from the nearest trap instead, siphoning it dry and venting sewer gas into the room.
That one is not a DIY job, mostly because it involves your roof. It is also not usually expensive to diagnose — a plumber with a camera will find it in under an hour.
When to worry
A sewer smell that comes with a damp patch, a soft floor, or a spike in your water bill is worth treating as urgent. Persistent sewer gas in an occupied room is a health issue, not just an unpleasant one.
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