Sewer smell in the house? Drain smells but is not blocked

Every drain in the house has a trap: a bend (the trap) or a chamber in which a layer of water always remains. That water blocks the air from the sewer. Sewer odour in the house almost always means that this water seal is missing or broken, or that there's a leak somewhere in the pipework allowing air to escape.

First: is it really a sewer smell?

Sewer smell is musty or like rotten eggs. If you are unsure whether it might be gas, do not investigate. Open windows and doors, do not use naked flames or light switches, leave the property and call the national gas and electricity emergency number: 0800-9009.

The six most common causes

1. A dried-out trap

A drain that's rarely used will eventually dry out: the water in the trap evaporates, especially in a warm room. Think of the guest bathroom, a floor drain in the utility room, storage room or garage, a washbasin in the loft or the drain of a washing machine that's located elsewhere. Once the water is gone, the sewer smell has free rein.

2. A siphoned trap

When a lot of water drains away at once – for example when flushing the toilet, a bath emptying or a washing machine pumping – negative pressure can build up in the pipe. This pulls the water out of another trap. This happens especially when the drain venting is missing or not working properly. You'll smell it shortly after using another drain, often accompanied by gurgling. You can read more about this in the article on a gurgling drain or toilet.

3. A dirty trap or drain

Sometimes the smell does not come from the sewer, but from the trap itself. Soap residue, hair and grease form a slimy layer that starts to rot. That quickly smells like a sewer, even though the water still drains perfectly.

4. A loose or leaking connection

A WC connector (pan collar) that doesn't seal properly, a trap that isn't tightened correctly, a loose washing machine drain hose or a leaking connection in the crawl space lets sewer gas through, even without you seeing water. The smell is then particularly strong close to that spot, for example behind the WC or in a cupboard.

5. The ventilation

The soil stack runs up to above the roof, so that sewer air can escape there. If that vent pipe is blocked (leaves, a nest) or damaged, the air finds another way. Some drains have an air admittance valve instead: a valve that lets air in but does not let air out. If that valve is broken, it does let sewer air through.

6. A leak or subsidence under the floor

A broken or subsided pipe in the crawl space, a connection that has come loose or an access point whose cover is not closed: the smell then rises through joints in the floor. You often smell it most strongly near the floor hatch. Also read about a subsided sewer.

How to find the cause: step-by-step plan

1. Smell carefully. In which room is the smell strongest, and near which drain?
2. Run water everywhere. Turn on the tap at every drain for half a minute, including forgotten drains, and flush a rarely used toilet. If the smell is gone within a day, it was a dried-out trap.
3. Clean the trap of the washbasin or kitchen sink where the smell is coming from.
4. Check visible connections: under the washbasin, behind the WC and at the washing machine. Is the drain hose properly inserted in the trap or standpipe?
5. Note the timing. Does it smell especially after flushing or after the washing machine? Then think of a siphoned trap or the ventilation. Does it smell in strong wind? Then the vent on the roof probably plays a part. Is it continuous and stronger near the floor hatch? Then the cause is more likely in the crawl space.
6. Look into the crawl space with a torch, without going in if it is not safe to do so. Do you see water, wet ground or a loose pipe?

What you'd better not do

• Masking the smell with air fresheners or blocks. The cause remains.
• Sealing off a drain or removing a trap. The air in the pipe must be able to go somewhere.
• Using chemical products. They don't solve a leak or dry trap and damage rubber seals.

With a camera inspection we look inside the pipe and see if there's a break, subsidence or loose connection. For The Hague and Delft: also see camera inspection in The Hague and in Delft.

How we find the source of sewer odour (and what it costs)

Sewer smell almost always has a clear cause, but it's not always visible. We start with the easy points: dried-out traps and drains, a loose cover plate or a sealant joint that has come loose. If that's not it, we look at the sewer ventilation on the roof and at damaged pipes.

For difficult cases we use a smoke test or a camera inspection. With a smoke test we blow harmless smoke into the drain and see exactly where it enters. With the camera we see cracks, loose connections or subsidence in the sewer.

Call us immediately if the smell is suddenly very strong, if water is coming up from a gully or if you are unsure whether it is gas or sewer. If you smell gas, call your grid operator first.