Toilet overflowing into the flat below: what to do

An overflowing toilet is bad enough. An overflowing toilet in a flat is worse, because the water does not stay on your bathroom floor. It runs under the pan, through the floorboards and into the ceiling of the flat below. In a converted house, it may come through within minutes.

Stop it overflowing

1. Stop flushing. Every flush adds more water.
2. Stop the cistern refilling. Take the lid off and lift the float, or push the float valve arm up. Or close the small isolation valve on the pipe to the cistern with a flat screwdriver.
3. If water is still coming, turn off the stop tap for the flat.
4. Mop up fast. Towels around the base of the pan, then a bucket and mop.
5. Go downstairs. Tell your neighbour, and ask them to switch off electrics near any water coming through their ceiling.

Two different kinds of overflow

It helps to know which one you have.

Type	What you see	Usual cause
Pan overflow	Water and waste rise and spill over the rim after flushing	Blockage in the pan, branch pipe or soil stack
Cistern overflow	Clean water runs constantly into the pan or out of the overflow	Stuck fill valve, worn seal, scale

A cistern that overflows into the pan does not usually reach the flat below unless the pan is also blocked. If yours has an internal overflow and the pan is clear, the water just runs away. If the overflow pipe runs to an outside wall and it is blocked or disconnected, water can spill inside instead.

A pan overflow is the one that causes damage downstairs.

The shared soil stack in Islington conversions

In Islington, 21.3% of households live in part of a converted or shared house, according to the 2021 Census (Nomis, TS044). Many of these are Georgian and Victorian terraces in Barnsbury, Canonbury and Highbury split into two to four flats. The toilets in every flat usually drain into one cast iron soil stack at the back of the house, which runs down to the drain under the garden or the basement.

If that stack blocks near the bottom, waste from the flats above has nowhere to go. It comes back up in the lowest toilet, usually in the lower ground or ground floor flat. That flat’s owner may find their toilet overflowing when they have not flushed at all.

So if you live upstairs and your toilet is fine, but the flat below says their toilet is filling up, stop flushing. The problem is probably in the shared stack or drain below both of you.

Who is responsible

The Leasehold Advisory Service says each leaseholder is usually responsible for pipes serving their own flat, even if part of the pipe is outside it, and the freeholder for communal pipes that serve more than one flat. In a conversion, the soil stack and the drain under the building usually serve every flat, so a blockage there is often the freeholder’s to clear. Check your lease, as leases vary.

If the blockage is in the drain beyond the building and serves neighbours too, it may be Thames Water’s. It became responsible for most private sewers and lateral drains on 1 October 2011. Our guide to blocked drains and Thames Water explains how to tell.

If you rent, your landlord must keep sanitation installations, including toilets, in repair under section 11 of the Landlord and Tenant Act 1985.

Helping the flat below

If the water has gone through, your neighbour should:

• switch off the lighting circuit if water is near a ceiling light
• keep out from under a sagging ceiling
• catch the water and move belongings
• take photos of the damage

Water from a toilet may be contaminated. Clean hard surfaces with disinfectant and throw away anything soft that cannot be properly washed. Insurers will want photos and a note of what was damaged.

What the plumber will do

The plumber first works out whether the blockage is in your pan, your branch or the shared stack. They may rod the pan, open an access cap on the stack or lift the drain cover in the garden. If it is the stack and the freeholder has a contractor, they may tell you to contact the managing agent. Either way, you get a written note of where the blockage was. That note helps decide who pays. Our blocked toilet page explains the visit in more detail.

Stopping it happening again

Most stack blockages in conversions come from wipes, fat and sanitary products from one or more flats. If your building has had more than one, it is worth the freeholder having the stack and drain checked with a camera. A cracked or sagging pipe will keep catching waste. Agreeing with the other flats what does and does not go down the toilet costs nothing and saves everyone a ceiling.