Why Does My Drain Smell?

A smelly drain is your plumbing telling you that sewer gas is getting somewhere it should not. The good news is that the causes run on a ladder from trivial to serious, and the trivial ones are far more common. Work up the ladder in order and you will usually find the answer before anyone needs to lift a tool.

The whole system depends on one simple device: the trap. That U-shaped bend under every sink, basin, shower and floor waste holds a plug of water that seals the drain, so waste can go down but sewer gas cannot come up. Almost every drain smell traces back to a trap that has lost its water seal, or to gunk sitting on the house side of the trap.

If the smell comes from a drain you rarely use, this is almost certainly it. The water in a trap evaporates over a few weeks, especially in warm weather or under a heating vent. Once the seal is gone, sewer gas drifts straight up into the room. Classic culprits are the floor waste in a second bathroom, the laundry trough in a house where the washing machine empties into a standpipe, a guest ensuite, and the shower in a holiday home that sat empty over winter.

The fix costs nothing: pour a jug of water down the drain to refill the trap. If the room is used rarely, do it monthly, and a teaspoon of cooking oil floated on top slows the evaporation. If the smell comes back within days despite refilling, water is leaving the trap another way, and you are further up the ladder.

If the smelly drain is one you use every day, the trap is wet, so the smell is usually coming from the fixture itself. Hair, soap scum, skin oils and food residue coat the inside of the waste pipe above the trap and grow a layer of bacteria called biofilm. It smells sour or sulphurous, and because it is above the water seal, the trap cannot block it.

In kitchens, the overflow passage and the underside of the sink strainer are favourite hiding spots. In bathrooms, the pop-up waste and its linkage collect an impressive amount of hair and slime. Remove and scrub the pop-up or strainer, brush the visible section of the waste with a bottle brush, and flush with hot (not boiling) water. Bicarb followed by vinegar helps loosen the film. What we would steer you away from is caustic drain cleaner, particularly in older homes, and we have written separately about why it and old pipes do not mix.

Sometimes the trap keeps losing its water even though you use the fixture. Two mechanisms do this. The first is siphonage: when a large volume of water rushes down a shared line, for example a washing machine emptying, it can pull the water out of a nearby trap behind it, like drinking through a straw. The second is a partial blockage downstream, which causes gurgling as air is dragged back through the trap, burping sewer gas into the room with each gurgle.

Gurgling is the key symptom to notice. A drain that gurgles when another fixture empties is telling you the vent or the line itself is restricted. In Bayside homes this is where tree roots enter the story, because a root mass in the sewer creates exactly this kind of partial restriction long before it causes a full blockage.

If the smell persists after the traps are wet and the fixtures are clean, or if you can smell sewage in the garden, under the house or in a wall, the problem is structural. Common versions we find across Brighton and surrounding suburbs include a cracked earthenware sewer pipe leaking under or beside the house, a failed rubber pan seal where the toilet meets the floor, a corroded section of old cast iron or galvanised waste pipe, and a damaged or blocked vent pipe that should be carrying sewer gas above the roofline.

The earthenware issue is the local speciality. Much of the sewer pipework under Edwardian and Californian bungalow blocks around here is glazed clay laid in short sections with mortared joints. After a century of ground movement and thirsty street trees, those joints open up. Roots grow in through the gaps, and the same gaps let wastewater seep out into the soil, which is why a persistent damp sewage smell in one part of the garden is worth taking seriously. A wet patch that never dries, unusually lush grass along one line, and drains that block repeatedly are the same fault seen from three angles.

This rung of the ladder is not a DIY diagnosis. A CCTV drain camera finds the fault precisely, and you see the footage before anything is quoted. Most plumbing repair work in Victoria legally requires a licensed plumber in any case, and underground drainage always does.

Before you call anyone, five minutes of checking will make the conversation far more useful.