Drains

Reading the smell coming out of a shower drain

Sewer gas and biofilm are both common, and neither is hard to identify once you know what each one smells like. This covers dry traps, the layer growing on hair inside the pipe, and the gurgle that means the vent is the real fault.

Updated August 20, 2026 ·Drain Cleaning & Hydro Jetting

Reading the smell coming out of a shower drain

Two different faults make a shower drain smell, and they smell different. A sharp, sulphurous, unmistakably sewage odour means gas from the drain system is getting into the room, which almost always means the water seal in the trap is gone. A musty, sour, slightly sweet smell that is strongest in the first minute of a hot shower is biofilm, the soft grey layer growing on hair and soap trapped inside the drain and the arm behind the wall. Identify which one you have and the fix is usually short.

The trap seal, and how it disappears

Under every shower is a P-trap, a U of pipe that stays full of water and seals the drain system off from the room. Nothing holds that water in place except the shape of the pipe. If the water evaporates, the seal opens and the drain system vents straight into the bathroom.

Evaporation takes weeks rather than days, which is why this turns up in a guest bathroom nobody uses, a second shower in a converted garage, or any house after a long trip. It also turns up in summer, when the air in the room is warm and dry. The test costs nothing but time: run the shower for two minutes and see whether the smell is gone by the evening.

For a shower that genuinely sits unused, pour a jug of water down it once a month and follow it with a spoonful of cooking oil. The oil floats on the trap water and slows the evaporation considerably.

Biofilm, the layer on the hair

The other smell is organic and lives inside the drain. Hair mats across the strainer and just past it, and skin oil, conditioner and soap residue collect on the mat. A soft grey-brown film grows over that. It is not dangerous, but it smells sour and stale, and it is at its worst during a hot shower because heat and steam lift the odour up out of the drain.

Houses here make it worse than it needs to be. Costa Mesa is slab-on-grade and mostly built around 1972, so the shower trap and the arm behind it are frequently cast iron, cast into the concrete. The inside of a fifty-year-old cast iron trap is not smooth; it is scaled and rough, and a rough surface holds film in a way that a smooth ABS pipe does not. That trap cannot be replaced without opening concrete, which makes cleaning it properly matter far more than it would in a newer house.

Telling the two apart

Smell at the drain opening with a torch: sewer gas is sharp and sulphurous and reaches you before you get close, while biofilm is sour and you have to be near the drain to notice it.

Run the shower for two full minutes, leave the room for an hour and come back, because a refilled trap will smell of nothing while biofilm will smell exactly as it did before.

Note when it is strongest: biofilm peaks during and just after a hot shower, sewer gas peaks when the room has been shut up and still.

Watch for the shower drain gurgling when a toilet is flushed or a washing machine empties, which points at a venting fault rather than either of the two.

Check whether the smell is confined to that room, since an odour drifting through several rooms is not a shower drain problem.

Gurgling points at the vent

Drain pipes need air behind the water. That air comes down vent pipes that run up through the roof. If a vent is blocked by a nest, leaves, a builder’s rag left in it during construction, or a section that has corroded shut, then a large slug of water going down the line pulls a vacuum behind it and takes the easiest air it can find. Often that is the water sitting in a nearby trap.

That is siphoning, and it means the shower loses its seal every time the washing machine drains. Cleaning the shower will not touch it. The check is on the roof and inside the line, and the fix is at the vent, not at the drain that smells.

Cleaning the drain without damaging the pipe

Lift or unscrew the strainer, which is usually two screws or a quarter turn.

Pull the hair out with a barbed plastic drain tool rather than pushing it further down with a straightened coat hanger.

Scrub as far into the drain and the top of the trap as a long bottle brush will reach.

Flush through with several litres of hot tap water and washing-up liquid, not boiling water, which can crack an old fitting.

For the film you cannot reach, use an enzyme drain product overnight, repeated over several nights, because enzymes digest the organic layer slowly rather than burning at it.

Leave caustic drain openers out of this. They sit on top of a hair mat instead of passing through it, they generate heat inside an old cast iron trap, and they turn the water in that trap into something nobody should be opening a pipe into afterwards.

When the shower is not the source

A failed wax ring under a nearby toilet leaks gas at floor level, and the smell drifts to the lowest, coolest opening in the room, which is usually the shower. A laundry standpipe with too short a trap arm does the same thing in a hallway. Under a slab, a cracked cast iron waste line can release gas into the fill beneath the concrete, and it finds its way up wherever the slab has been penetrated.

Those are found by elimination and, where it is worth doing, by putting a camera in the line rather than guessing at it. A smell that survives a refilled trap and a genuinely cleaned drain is worth investigating properly instead of covering with a scented block.

Keeping it from coming back

A hair catcher over the drain removes most of what feeds the film in the first place. A jug of water down every drain that goes unused for a month keeps the seals full, and the same applies to a garage floor drain, which the article on floor drain backups goes into. And if the shower is slow as well as smelly, deal with the slowness first: a drain that empties in a few seconds does not hold enough standing water to grow anything worth smelling.