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Drain Smells Of Sewage
A foul sewage smell around an outside drain, a downstairs room or an appliance, with nothing visibly blocked.
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What's happening?
The whole drainage system is designed so that no sewer gas ever reaches you. Every fitting has a water trap, the stack is vented above the roof so pressure changes cannot pull those traps out, and the pipework itself is sealed. A sewage smell therefore means one of those three defences has failed: a trap has run dry or been siphoned empty, a seal or joint has come apart, or the venting is not working so the traps are being sucked out every time something discharges. Smell that comes and goes with use points at venting. Smell in a room nobody uses points at an evaporated trap. Smell outside near a chamber points at a lifted or broken cover, a damaged gully or a blockage starting to build.
What usually causes it?
- A trap that has dried out in a rarely used shower, basin, WC or floor gully
- A washing machine or dishwasher hose pushed into a waste pipe without a trap
- A soil stack air admittance valve that has failed and is no longer sealing
- A cracked or displaced joint on a soil pipe inside a boxing or a void, or a broken chamber cover outside
- A blocked or silted gully outside, so waste stands and decomposes rather than flowing away
- A wrongly connected appliance waste discharging into a rainwater downpipe
- A partial blockage in the run, holding sewage in the pipe between uses
Is it dangerous?
Sewer gas is unpleasant and can cause headaches and nausea in an enclosed space, and it is a sign that a barrier between you and foul water has failed. It is not usually a serious health hazard in a domestic setting at the concentrations involved, but it should never be masked and left. What it more often signals is a defect — a broken seal or a partial blockage — that will eventually present as an overflow. Ventilate the room and find the cause rather than using an air freshener.
Can I fix it myself?
Several easy tests. Run every tap, shower and floor gully for a minute, and flush every toilet, including in rooms you never use — refilling dried traps solves a surprising share of smells. Check that appliance waste hoses go into a proper trapped standpipe or spigot rather than being pushed down a pipe. Look at the outside gully: lift the grating, clear leaves, silt and grease, and flush it with a bucket of water. Check inspection chamber covers sit properly and are not cracked. If the smell only appears when a bath or a washing machine drains, that is venting and needs an engineer.
When should you call a drainage engineer?
Call a drainage engineer if the smell persists after refilling the traps, if it appears whenever something upstairs drains, or if you can smell it inside a boxed-in soil stack. Persistent smell in a downstairs room with no obvious source is worth a smoke or dye test rather than guesswork, because a cracked underground pipe under a floor slab shows up no other way until damage is done.
What will the drainage engineer actually do?
- Test each trap and check the seal depth, refilling or replacing traps that cannot hold water
- Carry out a smoke or dye test to find where the system is leaking to atmosphere
- Inspect the soil stack, its joints and any air admittance valve, and replace a failed valve
- Run a CCTV survey where the smell points below ground and there is no visible defect
- Clear and reset a silted or damaged gully, and reseat or replace chamber covers
- Correct a wrong connection into a rainwater or surface water pipe
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