What separates your bathroom from the sewer is 5 centimetres

You put the heating on for the first time this year, walk past the spare bathroom — the one nobody has used since July — and catch a smell of sewer. Nothing in there is blocked: what is missing is five centimetres of water that evaporated on its own while nobody turned on a tap.

This is the only thing standing between your bathroom and the sewer in the street: a bend of pipe with water in it. It is called a water seal, and it is the U-shaped elbow under the basin, inside the shower trap and at the bottom of every floor drain. Gravity keeps the water sitting there and it works as a plug: the gases from the sewer push up from below and get nowhere, because pushing water costs effort. Spain's building code, the norm that governs anything built in the country, requires a seal depth of 50 millimetres on drains in continuous use and 70 on drains used only now and then, with a maximum of 100. Five centimetres of tap water, and seven in the bathroom nobody uses, which is precisely the one that runs out of them.

And that water leaves in three different ways, all three of which you can diagnose without tools and without lifting anything.

It evaporates. This is the case of the closed-up flat, the bathroom of the spare room nobody uses, the garage floor drain, the terrace drain in summer. If nobody turns on a tap, those five centimetres go on their own within weeks, and faster with central heating or dry heat. Symptom: the smell is there from the first minute you walk in and it does not shift.

Suction takes it away. When a column of water drops suddenly down the stack — an upstairs neighbour's toilet, a washing machine emptying — it drags air behind it and creates a vacuum. If the stack is not properly vented, that vacuum sucks the plug out of your trap. Symptom: the bathroom smells in bursts, and the bursts coincide with the sound of water in the wall. You can almost always hear the basin gurgle just before.

There is no trap at all. This happens more often than you would think with washing machines and dishwashers plumbed any old way into a drain with no bend. Symptom: it smells all the time, in winter too, with the house full of people as well.

What to do, in this order and without calling anyone yet:

1. Pour a litre or two of water down every drain in the house, including the ones you never use. A glass is not enough: it has to fill the whole bend and spill over.

2. On top of the water, pour a tablespoon of the cheapest cooking oil you have. It floats, it does not evaporate, and it buys the seal several months of life. It is what caretakers do in buildings with empty commercial units.

3. Smell again ten minutes later. If the smell goes and does not come back, it was evaporation and you have fixed it yourself.

4. If it comes back within two or three days, evaporation is ruled out: you have suction, or a stack that vents badly, and that is the building's business now. That is when you call, and you call with the fact in your hand: "it smells exactly when someone flushes."

5. For a drain you are never going to use again, there are waterless trap valves, which seal with a membrane instead of with water and therefore cannot evaporate. And if you really are never going to use it, you can simply cap it.

So far this is a domestic nuisance. In Hong Kong, twenty-three years ago, this same plumbing detail ended up in an epidemic report.

During the SARS outbreak of 2003, a housing complex in Kowloon called Amoy Gardens had recorded 321 cases by 15 April, and around 41% of them were concentrated in a single block, Block E. Investigators found that residents cleaned their bathroom floors with a mop instead of sluicing water down the floor drain, so those water seals were dry. On 17 April 2003 the Hong Kong health authorities described a very simple check: they taped paper strips over the floor drains in those bathrooms — rooms of about 3.5 square metres — and the air coming up the stack blew them into the flat, more so with the extractor fan on. The World Health Organization team that went in afterwards to take samples found neither genetic material from the virus nor live virus. The dry trap was not the only cause of that outbreak, but it was on the list.

Do it this afternoon, not next summer: two litres down every drain in the house and a tablespoon of oil on top. Forty seconds per drain.