Shower Drain Smells: Trap, Hair Build-Up or a Venting Fault?

What kind of smell, and when?

The nose does most of the diagnosis. Stand over the plughole and then step back into the room, and note when the smell is worst.

• Musty, sour or like old cheese, strongest at the grate, there all the time: hair, soap scum and skin cells have formed a slimy mat in the trap. This is the most common cause by a distance.
• Sewer or rotten eggs, arriving just after the shower drains or after a basin, bath or toilet nearby empties: the trap’s water seal is being pulled out. That is a pipework or venting problem, not a dirty trap.
• Sewer smell after the shower has not been used for a week or more: the seal has evaporated. Run the shower for a minute and it should go.
• Damp, mouldy smell in the room or the cupboard next to the shower, not at the grate: water is escaping round the tray or the trap and sitting underneath.
• Rotten eggs only when the hot water runs, from every hot tap as well as the shower: that is the water heater, not the drain. See hot water that smells of eggs.

If you are not sure whether you are smelling drains or gas, treat it as gas: open windows, turn off the gas at the meter if it is safe to, avoid switches, then leave and call the National Gas Emergency Service on 0800 111 999.

Clean the trap from above

Most shower traps are designed to be cleaned from inside the tray. Lift or unclip the grate, often with a flat screwdriver in the slot at the edge. Underneath you will usually find a removable cup, dip tube or hair catcher that lifts straight out. Wear gloves: what comes out is unpleasant.

Pull out the hair by hand or with a plastic drain snake, then scrub the cup and the inside of the trap body with an old toothbrush, washing-up liquid and hot tap water. Flush plenty of water through before you refit.

Refitting matters more than people expect. The cup or dip tube is what creates the water seal in many shower traps. Leave it out, or push it back without its rubber seal seated, and the trap holds no water at all, so the drain breathes straight into the room. A trap that smells worse after cleaning has usually been put back wrong.

Skip the bleach and the caustic granules. Bleach only masks the smell. Caustic can eat into hair, but in a slow shower trap it sits in the water and can splash on to your skin and eyes, and there is no need for it when the cup lifts out and the hair comes away by hand. Mixing bleach with an acid-based cleaner releases chlorine gas in a small, closed room. If water is also leaving slowly, see a shower draining slowly for clearing a blockage further along.

Why shower traps lose their water seal

A shower trap is shallow by design. Building Regulations Approved Document H gives a 50mm seal for a bath or shower trap, against 75mm for a basin or sink, and where the waste discharges straight to an outside gully the seal may be as little as 38mm. Every trap should still hold at least 25mm of water while the system is working. With so little water to start with, a shower trap has less margin than anything else in the bathroom.

Low-profile trays and wet-room floors make this worse, because the trap has to fit in a shallow space under the floor. Several things eat into the seal.

• Evaporation in an en suite or guest shower that is rarely used, helped along by underfloor heating or a warm airing cupboard next door.
• Self-siphonage: the shower’s own water rushing down a long or steep waste pipe pulls the trap empty behind it. You hear a gurgle as the last water leaves.
• Induced siphonage: the shower shares a waste pipe with a basin or bath, and when that empties it drags water out of the shower trap as it passes.
• A surge down the soil stack from a toilet flush, where the stack vent cannot let enough air in.

To test the seal, fill the trap, wait until you would normally notice the smell, then shine a torch into it. Water that has visibly dropped, or a trap you can see down into, confirms a seal problem.

Gurgling as well? Look at the venting

Gurgling from the shower when the toilet flushes or the basin empties is air being drawn through the trap because the pipework cannot get it anywhere else. Every waste system needs air in behind moving water. It comes from the open top of the soil stack above the roof, or from an air admittance valve: a plastic cap on the end of a pipe in the boxing, loft or under the basin that opens to let air in and closes to keep sewer air out.

If there is such a valve near the bathroom and the smell is strongest beside it, its membrane may be stuck open with dust or split with age. If the stack vent on the roof is blocked by leaves or a nest, the whole system gurgles and several traps smell. Either way, cleaning the shower trap will not stop it.

The fixes are pipework ones. A plumber may replace the valve, fit an anti-siphon trap, shorten or re-fall the shower waste, give it its own connection to the stack instead of sharing with the basin, or fit a waterless self-sealing valve trap, which closes against drain air without relying on a water seal and suits showers that are rarely used. For the same fault on a basin or kitchen sink, see a smelly sink, and if the whole house smells rather than just the shower, a sewage smell in the bathroom works through every source in order.

When the smell is under the tray, not in the drain

A damp, mouldy smell that does not come from the grate is usually water where it should not be. The joint between the trap and the tray has a rubber washer and a large nut; if the tray has flexed or the nut was not tightened, a little water escapes on every shower and soaks into the floor. Failed silicone at the wall or round the tray edge does the same, and so does a cracked tray on a floor that is not solid.

Signs are black mould along the silicone, a tray that moves or creaks when you stand on it, loose or hollow-sounding tiles near the tray, a stain on the ceiling of the room below, or a skirting board that stays damp. The smell is a slow leak rotting timber or chipboard, and it gets worse the longer it runs.

Stop using the shower if there is a stain below, and use a bath or another bathroom. A leaking shower explains how to tell a waste leak from a tray or sealant leak, and a ceiling leak below a bathroom covers protecting the room underneath.

What a plumber does, and what it costs

A visit for a smelly shower drain starts by checking which of the causes above it is: lifting the trap, checking it holds its seal while the basin and toilet are run, testing any air admittance valve, and, if a leak is suspected, taking off the bath or tray panel or the access hatch to look at the trap-to-tray joint.

Most jobs are small once the cause is known: a new trap or cup seal, a replacement valve, an anti-siphon or self-sealing trap, or remaking a leaking waste joint. A tray that has to come out because it has cracked or leaked for months is a bigger job, since tiles and sealant go with it.

Our emergency leak repairs service covers leaks under trays and baths at any hour, and the same visit deals with trap and venting faults. The £60 call-out applies day or night, and the repair price is agreed with you on site before any work starts. Full details are on our pricing page.