5 things that help with a sink or shower drain that smells like sewage only sometimes — not all the time

The p-trap holds a small plug of water that physically blocks sewer gas from rising into the room. When a drain goes unused for even a few days, that water evaporates. A guest bathroom, a basement shower, a utility sink you only use on washing day — all of them are quietly losing their seal whenever the weather is dry and the heating is running. The intermittency is the tell: the smell correlates with how recently you used that drain, not with any defect in the pipe itself. Most people clean the drain endlessly and the smell comes back a fortnight later, baffled, because cleaning was never the problem.

Every bathroom basin has an overflow — that small oval slot near the top inside the bowl. Most people have never cleaned it. That slot connects via a channel moulded into the ceramic directly to the drain below the plug, bypassing the p-trap entirely. Over months, biofilm of decomposing hair, soap residue, and skin cells builds up inside the channel and produces hydrogen sulphide. The smell is intermittent because it triggers on turbulence: run hot water, warm air rushes up past the overflow opening, and suddenly the whole room smells like a drain field. People reclean the drain. The smell persists. They reclean it again. The overflow is the culprit and it is almost never discussed.

The smell people attribute to the drain is often coming from the stopper — specifically the greasy ring of biofilm that accumulates where the stopper meets the drain body. This biofilm is a combination of soap scum, skin cells, hair product, and anaerobic bacteria, and it produces hydrogen sulphide in pulses as water disturbs it. That is why the smell hits when you first turn on the water, then fades. The trap is fine; the stopper is a petri dish. Because the stopper sits above the waterline, no amount of pouring things down the drain reaches it.

Shower drain strainers — the chrome disc you step on — are held down by two screws, and the gap between the strainer body and the shower pan is sealed with a thin bead of silicone. That silicone shrinks and cracks within a few years in a warm, wet environment. When it cracks, you get a gap that leads directly to the drain cavity below the shower floor, and that cavity connects to the drain-waste-vent stack. The smell is intermittent because pressure in the stack fluctuates: hot showers elsewhere in the house create convection that pushes gas upward, while cold periods let the gap sit dormant. People clean the drain obsessively and wonder why nothing improves, because no amount of cleaning closes a physical gap in the seal.

Most homes over 20 years old have a clean-out access point — a threaded cap on the drain-waste-vent line — that allows a plumber to rod out blockages without opening the wall. These caps are often PVC or ABS plastic, and they degrade. A hairline crack in a clean-out cap inside a wall cavity leaks sewer gas episodically: when water moves through the main stack nearby, the turbulence forces gas past the cracked seal, and it finds its way out through gaps in drywall, under vanity cabinets, or around the toilet flange at floor level. The smell seems to come from a drain because it enters the room near the floor. People miss this because the cap is hidden and because it looks fine until you put a light on it.