Why does my drain smell? Causes and fixes

A drain that smells is almost always telling you one of two things: either sewer gas is escaping into the house or garden where it shouldn't be, or waste is sitting somewhere in the pipe long enough to start rotting. Neither one is 'just how drains smell' — a healthy drainage system, indoors or outdoors, shouldn't smell of anything at all.

The smell itself often gives away the cause. A sharp, sulphurous, rotten-egg smell is classic sewer gas — that's methane, hydrogen sulphide and the rest of it working its way back up the pipe past something that should be sealing it off. A sweeter, staler, food-waste smell usually points to organic material sitting and decomposing further up the line — grease, food, hair, soap scum, that sort of thing. And a strong, damp, musty smell coming from under a sink or near an external wall can be a sign of an actual leak from a cracked pipe, where waste water is soaking into brickwork or subsoil.

In roughly the order we see them on jobs:

1. A dry water trap (U-bend). Every sink, basin, shower and toilet in the house has a U-bend that holds a small pool of water. That water is the only thing stopping sewer gas from coming straight up the pipe. If a fixture hasn't been used for a couple of weeks — a spare bathroom, a utility sink, a guest shower — the water can evaporate out of the trap, and the smell has a clear path into the room. Same story with a floor gully outside a rarely-used back door.

2. A blocked or dirty external gully. The gully is the little grated chamber outside where your kitchen waste pipe and sometimes your rainwater downpipes discharge. If it's full of leaves, silt, wet wipes and general debris, the water can't drain away properly and the whole thing turns into a stagnant pond. Lift the grate — if you see standing water with a scum on top, that's your smell.

3. A partial blockage building up in the pipe. A drain doesn't have to be fully blocked to smell. Grease, food waste, hair and soap gradually coat the inside of the pipe and reduce the diameter. Waste starts moving through more slowly, sits in the pipe longer, and rots as it goes. You'll usually notice the drain running a bit slow at the same time.

4. A damaged or cracked pipe. Older clay pipework — anything from the 1960s and earlier is very common in Northamptonshire — can crack, displace at the joints, or be broken open by tree roots. Once that happens, waste water leaks out into the surrounding ground and the smell either comes up through your lawn, through an air brick, or through a wall.

5. A venting problem. Every drainage system needs a vent (usually a soil pipe running up above the roof) so pressure equalises when water flows down the pipe. If that vent is blocked with a bird's nest, or a builder has capped it off during a loft conversion, the fixture traps end up getting sucked dry every time you flush and you get sewer gas coming through them. Classic sign: you flush the upstairs toilet and the downstairs sink starts to smell a minute later.

For a dry trap: run every tap, shower and unused toilet in the house for about 30 seconds each, then flush. That refills any traps that have dried out. If the smell disappears the next day, that was it — problem solved. For rooms you don't use often, running the taps for a minute once a fortnight is enough to keep the trap wet.

For a stinking external gully: lift the grate (a screwdriver as a lever works if it's stuck), pull out any leaves, wipes and gunge into a bucket you don't mind throwing out, and flush it through with a bucket or two of hot water and a bit of washing-up liquid. If the water drains straight away and the smell goes, that was the whole problem. If the water sits there and refuses to drain, the blockage is further down the line — see the next section.

For a slow, slightly-smelling kitchen sink: a kettle of boiling water followed by a plunger will often shift a light grease build-up. Don't use bleach or caustic drain cleaner — it rarely clears a real blockage, it's dangerous around your skin, and it makes life much harder for anyone (including us) who has to work on the drain afterwards.

If none of the above shifts it in a day or two — or if any of the following are true — stop and call someone in with a camera:

The smell keeps coming back after you refill the traps. That points to a venting problem, or a slow leak from a cracked pipe pulling gas back into the house every time water flows past it.

The external gully won't drain, or you've cleared it and it fills up again within days. That means there's a blockage or a collapse somewhere between the gully and the main sewer that a bucket of hot water isn't going to touch.

You can smell it outside — near a wall, in the garden, or coming up through the lawn — but you can't find where. That's a strong signal that waste is escaping from the pipe underground, which is a damaged-pipe job. Keep pouring water down and you'll just make the leak worse.

Sewage is visibly backing up anywhere inside the property. That's an emergency, not a smell.

For any of these, our first-hour visit is a fixed £300 — that covers diagnosis, rodding or high-pressure jetting where suitable, and a CCTV drain camera check within the first hour if we need one, with no separate camera fee. If the pipe turns out to be structurally damaged, we'll show you the footage and quote for a patch repair or lining before we do any further work.

Once the immediate smell is gone, a few habits keep it from coming back. Don't pour fats, oils or food waste down the kitchen sink — they solidify further down the pipe and start the whole cycle again. Only flush the three P's (pee, paper, poo); wet wipes, even the 'flushable' ones, are one of the biggest single causes of drain smells we deal with. Clear leaves and moss out of gutters and external gullies at least once a year — autumn does most of the damage. And if you've got a large tree within about 10 metres of your drain run, especially on an older property with clay pipework, a one-off CCTV survey is a lot cheaper than dealing with a collapsed root-damaged pipe later.

If you'd like the full DIY walkthrough for clearing an outside drain yourself before calling anyone, our engineers wrote a step-by-step guide covering the tools you need and the exact process — see how to unblock an outside drain pipe.