Blocked soil stack: who pays to clear it and how to stop it happening again

It usually starts the same way: the neighbour on the first floor notices that the shower is not draining, or that water that is not theirs is coming up through the kitchen sink. They call the president, the president calls the plumber, and a few days later the usual question arrives: who pays for this?

The answer almost always depends on one very specific detail: where the blockage is.

Where yours ends and everyone's begins

In a building under the Spanish horizontal property regime (propiedad horizontal), the main soil stacks and the collector drains that take the water from every flat are common elements. The Spanish Civil Code, in article 396, expressly lists drainage pipes and conduits among the common elements.

The pipework running from each flat's fittings (washbasin, toilet, kitchen sink) to the point where it joins the main stack is, as a general rule, a private installation, because it only serves that flat.

With that, the starting split is simple:

• If the blockage is in a flat's branch pipe, its owner pays.
• If it is in the main stack or the collector drain, the community pays, even if the water only came out in one flat.

Water showing up on the first floor does not mean the problem belongs to the first floor. When a stack is blocked, the water rises through the lowest outlet it can find, and that neighbour is usually the victim, not the culprit.

What the plumber writes down matters (a lot)

Since everything depends on where the blockage is, always ask for the report or the invoice to state where it was, what caused it and what was done. If the company uses an inspection camera, ask them to send you the footage or at least a written description.

That piece of paper settles most arguments. Without it, it comes down to the neighbour's word against the president's.

What if someone caused the blockage?

Wet wipes, building debris, cooking oil, nappies. If it can be shown that a specific neighbour caused the blockage in the stack, the community can claim the cost from them: the Spanish Horizontal Property Act (Ley de Propiedad Horizontal) requires every owner to use the shared installations with due care and to answer for any damage they cause.

In practice, proving it is hard except in clear-cut cases, such as mortar residue right after a renovation in a particular flat. It is best not to throw accusations around in the neighbours' group chat without proof: it creates more conflict than it recovers money.

The damage the water leaves behind

A blockage usually comes with damage: a flooded bathroom, a stained ceiling in the shop premises below. This is where insurance comes in.

• If the cause is in a common element, the claim goes to the community's insurance, which normally covers water damage to third parties.
• If the cause is in a private installation, it goes to that neighbour's home insurance.

Check your policy: many cover the damage caused by the water but not the unblocking itself, which is considered maintenance.

What to do on the day it happens

• Ask the neighbours not to use water while the work is being done, especially on the upper floors.
• Call a company with a camera if the blockage keeps coming back or cannot be located the first time.
• Take photos of the damage before cleaning anything up.
• Notify the relevant insurer as soon as possible.
• Keep the invoice stating where the blockage was.

How to stop it happening again

• Preventive maintenance: in buildings with a history of blockages, regular cleaning of the stacks and collector drains works out cheaper than weekend emergency call-outs.
• A clear notice to the neighbours: what must not go down the toilet or the sink. Once a year, with concrete examples, works better than a permanent sign that nobody reads.
• Renovations kept in check: ask anyone doing building work not to tip debris down the drains, and put it in writing in the house rules if it has become a problem.
• Check the inspection chamber and the collector drain before autumn, when the rain adds water to the system.

Every blockage leaves a lesson: where it was, what caused it, how much it cost and who paid. If that history is kept, next time the decision is immediate and nobody has to piece it together from memory. That is what we use Dommiva for: incidents, invoices and reports in the same place, visible to every neighbour.