Why does my outside drain smell?

Drains smell for exactly one reason: sewer air is reaching the outside where it should not. Everything in the system is designed to stop that — every gully and every waste has a water-filled trap, and the whole drain is vented at high level so pressure changes are relieved above the roof rather than through a seal. So the diagnosis is a short list of ways that arrangement can fail.

Safety

What you'll need

• Waterproof gloves
• Bucket and a hose
• Trowel or a scoop for clearing the gully
• Torch
• Drain rods (optional)
• Replacement gully grating (optional)

Step by step

1. Find where the smell actually is.Walk round in still air and put your nose close to each gully, each chamber cover and the base of any soil pipe. A smell that is strongest at one gully is a local problem; a smell that is general and worse in certain wind directions is often a vent issue.
2. Lift the gully grating and look.A trapped gully has a water seal in a U or a dip below the grating. If you can see the pipe outlet with no water in front of it, the seal has gone. If you cannot see anything for leaves and silt, that is the first job.
3. Clear the gully out completely.Scoop out the leaves, silt, moss and grease. Rinse it through with a hose. A gully packed with debris cannot hold a proper seal and the rotting organic matter has its own smell entirely separate from sewer air.
4. Reset the water seal.Pour a bucket of clean water in. The trap should hold it. Come back in an hour: if the water has gone, the trap is damaged, cracked or bypassed and it needs replacing rather than refilling.
5. Consider whether the trap simply dried out.A gully that receives water only from a downpipe will lose its seal in a long dry spell through evaporation. So will any waste outlet that has not been used for weeks. Refilling it is the whole fix, and it recurs every dry summer unless something discharges into it regularly.
6. Check the downpipe shoe position.The shoe should discharge inside the grating, into the gully. A shoe that has slipped out or discharges onto the grating splashes, does not keep the trap topped up and leaves debris on the grate.
7. Look for a blockage.A partially blocked drain holds standing waste, which is exactly what a sewer smells of. Lift the chamber covers and check whether anything is standing. A smell plus slow drainage anywhere in the house is a blockage until proved otherwise.
8. Think about the vent.The system is vented through a soil and vent pipe taken to above eaves level. If a vent terminal is blocked with a nest, if an internal air admittance valve has failed, or if a vent pipe has been altered during building work, pressure changes pull traps out and push air out at low level. Persistent smells with no local cause point here.
9. Investigate properly if it persists.A strong sewage smell in still weather with a clear, sealed gully and no blockage suggests a cracked pipe or a defective joint below ground, sometimes with a wet patch or unusually green grass above it. A camera survey locates that; guessing does not.

Tips

• A bucket of water is the cheapest diagnostic you have. Pour it in, come back in an hour, and the trap has told you whether it holds.
• Clear gullies twice a year, in autumn for the leaves and in spring for what the winter left. It prevents blockages as well as smells.
• A back-inlet gully, where the downpipe enters below the grating, keeps leaves out of the trap entirely and is worth retrofitting where debris is a constant problem.
• If a smell started after building work, look at what changed. Extensions, new bathrooms and moved kitchens frequently disturb vents and traps.
• Do not mask it with drain fragrance products. The smell is information, and covering it delays finding a defect that may be getting worse.

Common mistakes

• Pouring bleach down and calling it fixed — Bleach does not restore a broken water seal or clear a blockage, and it kills the bacteria in a treatment plant if you have one. The smell returns within days.
• Refilling a trap that will not hold water — If it emptied once it will empty again. A trap that does not hold has a crack, a bypass or a leak, and it needs replacing.
• Sealing a gully grating to stop the smell — It stops the gully doing its job entirely, so water cannot get in, and the air finds another route out. It also makes clearing the trap impossible.
• Assuming a smell outside is harmless — A persistent sewage smell with no local cause can be a damaged underground pipe, which undermines ground, attracts rodents and gets worse. It is worth locating.

If it doesn't work

Questions people ask