Drain smells in the house: where they come from and how to stop them
A drain smell indoors means sewer gas is getting past something that should be sealing it. There are only a handful of candidates, and they are easy to work through in order.
Start with the water seal
Every waste fitting has a trap holding a small plug of water that blocks sewer gas. If that water evaporates, the barrier goes with it, and the fitting behaves like an open pipe into the drain.
The usual culprits are the ones nobody uses: a spare bathroom, a utility sink, a shower in a rented room standing empty, or a floor gully in a basement. Run the tap for thirty seconds and wait a day. If the smell goes, that was it, and the fix is to run the water occasionally.
Seals that have failed rather than dried
If the trap is full and the smell persists, look at what surrounds it. A toilet pan connector that has slipped, perished, or was never seated properly leaks gas at floor level, usually worst in warm weather.
Same for a shower waste sitting slightly proud of the tray, a washing machine hose pushed into a standpipe without a trap below, and silicone that has pulled away from a bath. All of these smell like a drain because, functionally, that is what they have become.
The vent pipe nobody thinks about
A soil stack vents above the roofline so that pressure changes do not siphon traps dry. Block that vent, with a bird’s nest, a leaf pack, or a slipped cowl, and every flush pulls water out of the nearest trap. The smell then moves around the property, which is the giveaway.
In converted houses this shows up as a smell that appears in the flat below whenever the flat above uses the bathroom. See converted flats and shared soil stacks.
When the pipe itself is the problem
If the seals are sound and the vent is clear, the smell is coming from somewhere it should not be able to reach: a cracked underground run beneath a floor slab, a broken connection behind a boxed-in section, or a redundant pipe capped badly during a refit.
You cannot find that by looking. A CCTV survey locates it, and a smoke or dye test confirms where the gas is escaping. This is also the version that quietly damages the building, so it is worth settling rather than living with.
Outside smells are a different question
A smell in the garden near a gully usually means debris sitting in the trap rather than a fault. Lift the grid, clear it, flush it through.
A smell around an inspection chamber, particularly after rain, more often means the run is partly blocked and holding waste. That one is worth acting on before it becomes a backing-up drain: see blocked or broken, or coverage in Hackney, Islington and Southwark.
For work in these areas specifically, see CCTV drain surveys in Islington, CCTV drain surveys in Southwark and French drains in Southwark.