Sink Smells: Trap, Vent or Drain?

Where and when does it smell?

Two questions do most of the diagnosis. First, is it one sink, or several fittings at once? A smell at a single sink, basin or bath points to that fitting’s trap, overflow or connections, and a shower tray has its own version, covered in the smells that come up a shower drain. If a kitchen sink also empties slowly, the trap is probably part-blocked with fat; a blocked kitchen sink covers clearing it. A smell at several fittings, from outside gullies, or with slow drainage everywhere points to the drain, which is a different job, covered in a blocked drain and who deals with it.

Second, when is it worst? A smell that is constant and musty is usually rotting waste in the trap or overflow. A sewer smell that appears after the sink drains, or after the washing machine or a nearby toilet empties, is a trap losing its water seal. A smell that is strongest inside the cupboard, not at the plughole, points to a valve or joint under the sink. And a smell of rotten eggs only when the hot tap runs is a hot water problem, not a drain one; see hot water that smells of eggs.

Clean the trap first

Most sink smells are the trap itself. Grease, food, soap and hair collect in the bend and rot. Put a bowl underneath, undo the trap nuts by hand and take it off. A bottle trap has a base that unscrews; a P or U trap comes off in one piece. Clean it out, scrub the inside, check the washers, and refit hand-tight.

While it is off, look into the waste pipe going into the wall with a torch. Grey sludge coating the first part of the pipe will keep the smell going, and a flexible drain brush pushed a short way in clears it. Avoid pouring caustic cleaners into a trap you are about to take apart, and never mix drain products.

On a basin or bath, the overflow is the other common source. The channel behind the overflow hole never gets flushed, so it grows a slimy film that smells musty. A bottle brush and hot water with washing-up liquid down the overflow hole usually sorts it.

Is the trap losing its water seal?

A trap works because the water sitting in its bend blocks air from the drain. Building Regulations Approved Document H gives a minimum seal of 75mm for sinks and washbasins and says that under working conditions traps should keep at least 25mm of water. When the seal drops below that, drain air comes straight into the room.

Signs of a lost seal are gurgling as the sink empties, a smell that arrives right after draining, and a trap that looks low when you shine a torch down the plughole. The usual causes are these.

• Self-siphonage: a basin waste pipe that runs too far or too steeply pulls the trap empty behind the water. Approved Document H limits an unventilated 32mm basin branch to 1.7m at a fall of 18 to 22mm per metre, with shorter lengths allowed at steeper falls.
• Induced siphonage: another appliance on the same pipe, often a washing machine or a toilet, empties and sucks the seal out of the sink trap as its water rushes past.
• Evaporation: a sink in a spare room or a holiday let that is not used for weeks simply dries out. Running the tap for a minute restores it.
• Appliance discharge: a washing machine or dishwasher emptying into the sink waste sends a surge past the trap that can pull its seal down. Without a high loop in the drain hose, dirty water from the sink can also run back into the machine and rot there. Fix the hose in a high loop under the worktop, as the appliance instructions show.

An anti-siphon trap, which has a small valve that lets air in when a siphon starts, is often the simplest fix for a sink that gurgles and smells after draining.

The valve in the cupboard, and hidden leaks

Many modern kitchens and extensions have an air admittance valve, a grey or white plastic cap on top of a short vertical pipe under the sink. It lets air into the waste to stop siphoning and should snap shut to keep drain air out. When its membrane sticks open with dirt, or cracks with age, the cupboard fills with a sewer smell even though the trap is full. Replacement valves are cheap, typically £10 to £30, and push or screw onto the pipe. The valve needs to stay upright, reachable for replacement and in free air, so do not seal it inside boxing without an access panel.

A musty rather than sewer smell inside the cupboard often means a slow leak has soaked the chipboard base. Dry everything and put dry paper under each joint to find it. A drip that appears only while draining is a waste joint, covered in a leaking waste pipe; on a bathroom basin the waste itself may be the culprit, covered in a leaking basin waste.

When the smell is not the sink

If you have cleaned the trap and overflow, the seal holds, the valve is sound and the smell remains, look wider. Sinks near an outside wall often drain to a gully outside; a gully with a blocked grid or a dry seal smells through the pipe. A cracked soil pipe boxed in behind a kitchen unit or a toilet pan connector that has shifted can also be mistaken for a sink smell; our checks for a sewage smell in the bathroom cover those in order.

At that point a plumber is the quicker route. They check each trap seal, test the valve, look at the branch length and fall of the waste against what the fitting needs, and fit an anti-siphon trap or reroute the pipe where needed. Our 24-hour plumber service covers these visits. The £60 call-out applies at any hour and the repair price is agreed with you on site before any work starts, as set out on pricing.