A viewing in Grebaštica, a ground-floor flat in a four-storey apartment building. Nothing remarkable in the rooms. Then the bathroom, where the toilet was full of sewage that had dried where it stood.
Nobody had used the flat for months, and nothing inside the flat could account for it.
That is the point of this article. The cause was not in the flat. It was somewhere above the ceiling, and the owner had no way of knowing it.
Why the cause was four storeys up
Every flat in that building drains into one shared vertical pipe, the soil stack. Your bathroom is one opening in it. So is the bathroom directly above you, and the one above that.
A stack works because air can move in it. When a column of waste falls, air has to get in behind it and out ahead of it, which is what the vent at the top of the stack is for. Block the stack somewhere, or let the vent silt up, and the air has nowhere to go. Pressure builds. It then does what pressure always does: it leaves through the weakest opening it can find.
The weakest opening is almost never the flat that caused it. It is the lowest one. In a four-storey building that means the ground floor, and within the ground floor it means whichever fixture puts up the least resistance.
What normally puts up the resistance is the water sitting in the trap, the U-bend under every fixture. That water is the seal. It is also the only thing between you and the inside of the stack.
In an occupied flat the seal is topped up several times a day without anybody thinking about it. In an empty one, it evaporates.
Which is exactly the state the Grebaštica flat was in. Dry trap, no seal, nothing holding the line. When the stack found its weakest point, the weakest point was a toilet in a flat with nobody in it.
The seasonal curve nobody designs for
This coast puts a very particular load on a shared stack.
For eight months of the year a building like that one runs at near-zero. A handful of permanent residents, most flats empty, traps slowly drying out. Then in July and August every flat is occupied at once, often beyond its normal headcount, and the stack sees more in six weeks than in the whole rest of the year.
That is when a partial blockage that has been quietly building becomes a full one. And the flats most exposed when it does are the ones that have been standing empty, because they are the ones whose seals have gone.
The timing is cruel in a second way. If it happens in August, somebody is usually there and it gets dealt with. If it happens in October, after everyone has gone, nobody finds it until spring.
What the dried crust tells you
The state in which you find it is information.
Standing water means recent. Residue that has dried in place means the water came up, sat there, and then evaporated, which takes weeks or months in a closed flat. In Grebaštica it had dried out completely, so whatever happened had happened well before anybody opened the door.
It also means the smell had long gone, which is why no neighbour had raised it.
The second pressure: the boiler on your wall
The same logic runs through the other end of the flat, and here the pipe is yours alone.
Most flats on this coast have a small electric storage water heater. Water expands when it is heated. In an open system that expansion pushes back into the supply and nothing happens. In a system with a non-return valve on the inlet, which is the normal arrangement, it cannot go back, so the pressure rises inside the tank instead.
That is what the safety valve is for. It lets a little water out as the tank heats, and a slow drip from the discharge pipe during heating is the valve doing its job, not a fault.
Three things go wrong with that in a property that stands empty:
The valve seizes. Water on this coast is hard. A safety valve that is never operated for eight months scales up and stops lifting. The expansion then has nowhere to go at all.
The discharge goes nowhere useful. That drip usually runs to a nearby drain, and in an empty flat that drain has the same dried-out trap as everything else. At best you get smell. At worst the discharge pipe or the drain is blocked and the water that was supposed to leave arrives on the floor.
The element can be left dry. If mains pressure drops, which happens on seasonal supply, water can siphon back out of the tank and leave the heating element exposed. It survives that perfectly well until the moment the power and the water come back, and then it burns out on the first heat cycle.
None of these announce themselves. All of them happen with nobody in the building.
The awkward part of our own advice
This is where we have to be straight about a limit in what we do.
When we close a property down for the winter we shut the main stopcock, drain the pipework and empty the boiler. That deals with the boiler completely. No water in the tank means no expansion, no seized valve under pressure, no dry element, and nothing to freeze.
It does nothing at all about the stack.
Worse, it makes that side marginally harder, because a drained flat is a flat where nothing refills the traps. Closing your own water protects you from your own pipes. It does not protect you from the building’s.
The counter to that is small and easily skipped: when the flat is closed down, the traps are filled deliberately, and a film of cooking oil goes on top of the water so it evaporates far more slowly. Our winterising checklist lists that as a measure against drain smell. Grebaštica is the reason it is more than that. The seal is not an air freshener. It is the lid.
What owners should take from this
A dry trap is an open pipe. If your flat stands empty for months, every unused shower, basin, floor gully and toilet is a hole into the shared stack, held shut by water that is no longer there.
The flat that floods is rarely the flat that caused it. If you own on a lower floor you carry a risk created by the plumbing habits of everybody above you, and you have no way to see it. Owning higher up is not protection either, it just moves the consequence downstairs.
The stack is not yours, and that matters. Blockages inside the shared vertical pipe are a building matter, not a flat matter, and so is clearing them. Who carries what in a Croatian apartment building is a question about co-ownership, and it is a legal one. We do not answer it. If it becomes relevant we will put you in touch with a lawyer from our network. Our article on what a property manager actually is explains why the building manager and the person looking after your flat are two different roles.
Photograph it before anything is cleaned. The same rule as every other damage event: the pictures taken before the bucket comes out are the claim. See after the storm for the order that works.
Check what your policy says about water. Escape of water is one of the places where Croatian household policies differ most, especially on unoccupied property and especially on damage that was gradual rather than sudden. Our piece on what a standard policy leaves out covers the split.
How we handle it
Closing down and opening up are part of the ordinary round, not extras. Closing down means the stopcock shut, the pipework drained, the boiler emptied, and the traps filled and oiled rather than left to dry.
Opening up means the reverse in the right order, and it means looking at the things that only show themselves on the first refill: whether the safety valve lifts, whether the discharge runs away, whether anything has come up a drain since the last visit.
Where the cause sits in the shared stack we say so plainly, because that is a conversation with the building rather than a job we can quietly carry out in your flat. Where a trade is needed, we get quotes and instruct one after you have agreed.
The flat in Grebaštica had been through a full season of this without anybody knowing. That is the ordinary case, not the dramatic one. Nothing broke, nobody was burgled, no storm came through. A seal dried out, a pipe found a way, and the only reason anybody ever found out was that somebody finally opened the door.