Blocked toilet in an apartment building: your flat or the shared stack, and who deals with it?

Do these checks in order and note down the result of each one. The notes will help later when you explain the situation to neighbours or the building manager.

A blockage inside the flat is usually limited to one point and starts with an event: a wet wipe flushed down the toilet, a toy a child dropped in, too much paper in one go. The water rises only when you flush and stays calm when you are not using it. The neighbours notice nothing.

In this case the problem is your flat's problem, and you do not need to wait for anyone to solve it. If you are a tenant, it is a good idea to tell your landlord, because repeated blockages are sometimes a sign of an old pipe or a connection that has lost its slope, and the landlord will want to know that too.

A stack blockage does not start in one flat, it builds up in the whole building. Grease, wet wipes and paper pile up over the years at the bottom of the stack, at the bend where it turns into the horizontal line. The symptoms show first on the lowest floors and usually get worse at times when everyone uses water at once, such as morning and evening.

For someone living on the ground floor or garden level, the typical picture is this: dirty water appears in the toilet even though nobody has been in the bathroom, a smell comes from the floor drain, and sometimes water spills onto the bathroom floor. If this is what you are seeing, nothing you do inside your own flat will fix it for good.

With a stack problem, what wastes the most time is missing information. If you can explain clearly what is happening when you call the manager, the work gets organised much faster. Approach neighbours with information rather than blame: it is rarely possible to know who blocked the stack, and focusing on that only slows things down.

Until the unclogging is done, ask the neighbours on the upper floors to cut back on the washing machine, dishwasher and showers. This small request can stop the flat below from flooding.

In most buildings, stack unclogging is organised by management or by a resident on the manager's behalf. When the team arrives at the building, the first decision is where to open the stack from. Each stack usually has a cleaning hatch. It is sometimes in the basement, sometimes in the bathroom of a ground-floor flat or behind a cupboard. The manhole in the garden is another access point. If the hatch is inside a flat, the resident of that flat needs to be at home and give permission, and talking about this beforehand makes the job much easier.

During the work, a camera is put into the stack to see at what level the blockage is and what it is made of. Then the blockage is broken up with a motorised drain snake or a robotic head, the stack is flushed with high-pressure water jetting and checked again with the camera. The recorded footage is passed on to management. This footage lets everyone see with their own eyes whether the blockage is in the shared line or in the connection of a single flat.

The line between the shared stack and the plumbing inside a flat is also the starting point for the conversation about responsibility. In practice, this is mostly settled according to the building's own arrangements, its management plan and the agreement between residents. We do not give legal assessments, but showing the exact location of the blockage with camera footage removes most of the argument.

When stack blockages keep coming back, it helps if management makes an announcement to all residents: simple rules such as no wet wipes or sanitary pads in the toilet and no kitchen grease down the sink protect the whole building. Some buildings prefer to have the stack checked at regular intervals, and if the ground-floor flat often floods, fitting a backflow valve on that flat's connection can also be discussed.