Dry traps in fittings you rarely use
Every sink, bath, shower and floor gully has a trap: a bend that holds a small amount of water to stop sewer gases coming back up. In a spare bathroom, a utility sink or an outside gully that hardly gets used, that water slowly evaporates. Once it is gone, there is nothing between the room and the drain.
This is the easiest cause to rule out. Run each little-used tap for a minute and flush any spare toilet. If the smell fades over the next day, a dry trap was the cause.
Fat and food build-up
In kitchens, grease coats the inside of waste pipes and the drain beyond, trapping food scraps that rot. The smell is often strongest around the kitchen sink or the gully where the kitchen waste pipe discharges outside. Wiping pans before washing them and keeping fat out of the sink helps, but an established layer usually needs proper cleaning.
Food businesses see the same thing on a larger scale; our page on commercial drainage covers how planned cleaning keeps it under control.
Slow or blocked outside chambers
If an inspection chamber outside is partly blocked, waste sits in it instead of flowing away, and the smell comes up through the cover. You might also notice slow drainage indoors. Our guide to a blocked outside drain explains how to check a chamber safely.
Broken seals and damaged pipes
A smell close to a toilet, especially one that rocks slightly, can come from a failed seal where the pan meets the soil pipe. Under the floor, a cracked or disconnected pipe can leak waste into the ground beneath the house. That kind of smell often lingers in one room, may be worse in warm weather and does not go away with cleaning.
Vent problems are another cause. The vent pipe lets air into the system so water can flow freely. If it is blocked or missing, traps can be sucked dry each time a toilet is flushed, letting smells back in.
Simple checks you can make safely
Work through these before calling anyone. They are quick and may solve the problem outright.
Refill traps
Run water in every rarely used fitting, including floor gullies and spare sinks.
Check gully grids
Lift leaves and debris off outside gullies with gloves on and rinse the grid.
Look in the chamber
Carefully lift an outside cover and see whether it is full or flowing. Replace it securely afterwards.
Note the pattern
Is the smell worse after rain, in hot weather, or when a particular fitting is used? That helps narrow it down.
Safety first
Never climb into a chamber, however shallow it looks; drain gases can build up in confined spaces. Keep children and pets away from any open cover, and do not leave one off unattended. Avoid mixing drain chemicals, which can give off harmful fumes.
When the smell means a fault
If traps are full, gullies are clear and chambers are flowing, but the smell persists, the cause is probably in the pipework itself. A CCTV drain survey shows cracked pipes, displaced joints, disconnected sections and areas where waste is sitting. For a sense of what a survey involves price-wise, read our guide to CCTV survey costs.
If a chamber is full of standing water, the drain is blocked and needs clearing first; see drain unblocking. Smells together with rats or scratching noises are covered in our guide to rats in drains.
Get help with a smell
Tell us where the smell is and when it is worst, and we will suggest the most sensible next step.
Frequently asked questions
Why does my drain smell only in hot weather?
Heat speeds up evaporation from traps and makes rotting build-up smell stronger. Topping up traps and cleaning the drain often helps.
Can a smell come from a neighbour's drain?
Yes, particularly on shared runs. If the smell is outside and your chambers are clear, the problem may be further along.
Is a sewage smell dangerous?
A faint smell is mainly unpleasant, but do not ignore a strong or lasting one, and never enter a chamber to investigate.
Will air fresheners or bleach fix it?
They mask the smell for a short time. They do not deal with a dry trap, a blockage or a damaged pipe.
My smell is in one room only. What does that suggest?
A dry trap, a failed toilet seal or a leaking pipe under that floor are the likeliest causes.