Converted flats and shared soil stacks: whose blockage is it?
In a converted house the ground-floor flat sees the problem first and is rarely the cause of it. Sorting out who pays starts with knowing where the fault sits.
Why the lowest flat suffers
A soil stack collects discharge from every floor. When it is restricted, waste backs up to the lowest available opening, which is a ground-floor or lower-ground bathroom. The flat that floods is therefore the one furthest from the cause.
This produces predictable and unfair conversations, because the affected leaseholder is the one who calls, pays and then discovers the material came from upstairs.
Demised or communal
The dividing line is set by the lease, not by common sense, and it varies between buildings. Broadly: pipework serving only your flat, up to the point it joins a shared run, is usually yours. The shared stack and the drainage below it are usually the freeholder’s, maintained through the service charge.
That distinction decides who pays, so it is worth reading before accepting an invoice. Where the fault sits beyond the building boundary entirely, it may be the water company’s, which we cover in who is responsible for a blocked drain.
What actually causes it
- Wipes and sanitary items from any one flat, which affect every flat below
- Cooking fat from a kitchen that was moved during conversion, into a run with too little fall
- An original stack in rough cast iron, whose interior catches what a smooth pipe would pass
- A badly made connection added during the conversion, protruding into the stack
None of these are visible from inside a flat, which is why camera evidence settles arguments that talking does not.
Upper and lower conversions
Houses split into two flats have their own version. The kitchens sit above one another and discharge into one run, and the lower flat gets the backing up. Rear extensions built over the original drain line make access worse.
The pattern is heaviest in Balham, Clapham and Hackney, where this style of conversion is everywhere.
How to handle it as a landlord or agent
Get the fault located rather than cleared blind, so responsibility is established on the first visit. Ask for the finding in writing with position and cause. Where the stack is communal, that report is what supports a service charge item rather than a dispute.
For tenants, it is worth putting in writing what must not be flushed. A single leaflet prevents a lot of expensive call-outs. See landlord obligations for where the legal line sits.
For work in these areas specifically, see CCTV drain surveys in Islington.