Bathroom Smells of Sewage | Find the Cause Fast

A sewage smell in the bathroom is one of those things that gets into your head. You clean every surface, bleach the toilet pan, re-grout the tiles, and the smell comes back the moment you close the door. Nine times out of ten, the first instinct is 'blocked drain'. Sometimes that's right. Often, it isn't.

Before you phone anyone, spend five minutes working through these checks. Most of them cost nothing. One might cost a tenner at the DIY shop. None of them require lifting a slab.

Check 1: The Dry Trap (Start Here)

Every waste fitting in your bathroom, the basin, the bath, the shower tray, sits on a U-shaped or P-shaped trap beneath it. That trap holds a small plug of water that physically blocks sewer gas from rising back up the pipe. No water in the trap, no barrier. Simple as that.

Traps dry out when a fitting isn't used for a while. A guest bathroom that nobody touches for three weeks. A basin in an en suite that the tenant never uses. A shower tray that's become ornamental storage for shampoo bottles.

What to do: Run the cold tap for 30 seconds into every basin, bath and shower in the room. Pour a litre of water down any floor gullies too. Wait two minutes, then smell again. If the stench drops noticeably, a dry trap was your culprit. To keep it sealed longer, pour a small amount of cooking oil into the trap after the water. It floats on the surface and slows evaporation.

If the smell doesn't shift, keep going.

Check 2: The Toilet Wax Seal

Under your toilet pan, where it meets the floor, there's a wax ring (or a rubber gasket on more modern fits) that creates a gas-tight seal between the pan and the soil pipe. These last years, sometimes decades, but they don't last forever. A toilet that rocks slightly when you sit on it is a good sign the seal is starting to fail. So is a toilet that was re-set or moved at some point and not properly bedded back down.

A failed wax seal lets sewer gas seep out at floor level. Because it's low down and the smell disperses upward, it can feel like the whole room stinks rather than one corner of it.

What to check: Sit on the toilet and shift your weight left and right. Any rocking at all? Look at the base of the pan where it meets the floor. Discolouration, a slight dampness, or a gap in the sealant bead are all signs. You don't need to pull the toilet up to diagnose this, but fixing it properly means re-setting the pan with a new seal, which is a plumber's job rather than a drain job.

Check 3: The Overflow Pipe on the Basin

Most basins have a small overflow slot near the top, the one that stops you flooding the floor if you leave the tap running. That overflow connects to a short internal channel that joins the main waste below the trap. The problem is that channel never gets cleaned, and it sits above the water line in the trap, so it's permanently exposed to air from the drain side.

Old soap scum, hair and bacteria build up inside it and it can smell absolutely rank, out of all proportion to its size.

What to do: Get a bottle brush or an old toothbrush. Pour a tablespoon of bicarbonate of soda into the overflow slot, follow it with a dash of white vinegar, and scrub the slot opening while it fizzes. Then flush with boiling water. Not glamorous, but effective.

Check 4: The Soil Vent Stack

Your soil pipe doesn't just carry waste away. It also vents sewer gases safely above roof level, which keeps the pressure in your drainage system balanced and stops gases from being pushed back into the house. That vent pipe is usually a 100mm or 110mm plastic or cast-iron stack running up an internal or external wall.

Two things go wrong with it. First, the stack itself can crack, split at a joint, or come loose from a fitting, especially in older properties in areas like Cove or South Farnborough where cast-iron stacks from the 1960s are still doing their job. A crack part-way up means gases vent inside the wall cavity or floor void before they reach the atmosphere. Second, birds or debris can block the vent cowl at the top, which backs up the pressure and pushes gas in at every fitting in the building.

What to check: Walk around the outside of the house and look at the soil stack from ground level. Any visible cracks, displaced joints, or sections that look bowed? Inside the bathroom, put your hand near the inspection cap at the base of the stack (if there is one). Any smell there specifically? These are starting points. Confirming a cracked stack usually needs a CCTV survey or a smoke test, but you can rule out the obvious external damage first.

Check 5: The Bath or Shower Waste

This is separate from the dry-trap check, because the issue here isn't a lack of water. It's a loose or poorly seated waste fitting. Shower trays in particular are prone to this: the waste body sits in a hole in the tray, held by a threaded ring underneath. If the sealant around it has failed or the fitting is slightly loose, there's a gap between the fitting and the pipe below, and gas seeps through.

What to check: Press firmly around the shower waste grate while the tray is dry. Any movement or flex? Can you see a hairline gap between the grate surround and the tray surface? A loose fitting can sometimes be tightened from below if you have access under the tray. If not, it needs re-seating with a new rubber gasket and fresh silicone.

When It Actually Is the Drain

If you've worked through all of the above and the smell is still there, or if it's worse when you flush the toilet or run a lot of water, then yes, the drain itself is worth looking at. A partial blockage can trap decomposing organic matter and produce hydrogen sulphide gas. A collapsed section can do the same. A CCTV survey will tell you exactly what you're dealing with in 20 to 30 minutes, and there's no point guessing.

Properties in older parts of Farnborough, Aldershot and Ash Vale especially, often have clay drain runs from the 1950s and 60s that have shifted at the joints over the decades. A bellied pipe (one that's sagged and holds standing water) produces a persistent smell without ever fully blocking. You'd never know without the camera.

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If you've done the checks and you're still stuck, Drainsco covers Farnborough and the GU postcodes, engineers available 24 hours a day. There's no call-out fee, and you'll get a clear price before any work starts. A CCTV survey is usually the fastest way to settle the question for good, and the footage is yours to keep.