Blocked drain outside a London terraced house

Most London terraces have at least two drains you can see outside. A gully, which is the small grid at ground level where the kitchen waste pipe and sometimes a rainwater pipe empty. And one or more inspection chambers, the square or rectangular metal or concrete covers in the yard, path or front garden. When one of these starts to fill and overflow, you have a blocked outside drain.

First, stop adding water

Ask everyone in the house to stop running taps, showers and washing machines. The water has nowhere to go, and everything you add ends up in the yard or coming back up inside.

What the outside drains do

Drain	Where it is	What it does
Gully	By the back wall, under a kitchen waste pipe	Takes waste from the kitchen sink and sometimes rainwater
Inspection chamber	Under a cover in the yard, garden or path	Gives access to the drain where pipes join or bend
Rainwater pipe	Down the outside wall	Takes water from the roof, sometimes into the same drain

In many older terraces, rainwater and household waste run into the same drain. That is why some outside drains only overflow in heavy rain.

Check the gully first

The gully is the easiest to deal with. Lift the grid wearing gloves and look inside.

• If it is full of leaves, food scraps or a layer of fat, scoop it out into a bag.
• Run some water down. If it drains, the problem was just the gully.
• If it stays full, the blockage is further along the drain.

Do not pour caustic chemicals into a gully. They rarely clear the drain and can splash.

Then look in the chamber

If you can lift the nearest inspection chamber cover safely, have a look. Covers can be heavy, so use proper lifting keys or ask a plumber.

• Chamber full of water: the blockage is downstream of this chamber, further towards the sewer.
• Chamber empty while the gully is full: the blockage is between the gully and this chamber.
• Chamber half full and draining slowly: a partial blockage, often fat or wipes, or roots in an old clay pipe.

Shared runs in Lewisham terraces

In Lewisham, 26.2% of households live in terraced houses, according to the 2021 Census (Nomis, TS044). In Catford, Lee, Ladywell and Forest Hill, many terraces were built with a single drain running behind several back yards before it turns towards the sewer in the street.

If your chamber is full and your neighbour’s is full too, the blockage is likely in a shared section. Since 1 October 2011, most shared private drains and sewers in London have been Thames Water’s responsibility. Thames Water advises asking your neighbours first. If they have no problem, the blockage is probably on your section. Our guide on who is responsible for a blocked drain explains how the 2011 transfer works.

Common causes in London

• Cooking fat and oil poured down the kitchen sink, which sets in the cold drain.
• Wet wipes flushed down the toilet. They do not break down.
• Leaves and silt washed into a gully from the yard or roof.
• Tree roots getting into joints in old clay pipes, common in streets with mature trees.
• Cracked or sunken pipes that catch waste. In older terraces, clay drains can move over the years.
• Limescale and soap narrowing waste pipes. Thames Water says all the water in its region is hard.

When to call a plumber

Call for help if:

• the gully is clear but the drain is still full
• the chamber is full and you cannot tell whether it is shared
• water or sewage is coming up inside the house
• the drain blocks again soon after being cleared
• there is a strong sewage smell

A plumber or drainage engineer will rod or jet the drain and check the flow. If it keeps blocking, a camera survey shows whether the pipe is cracked, sagging or full of roots. Our blocked drains page explains the visit.

If you rent

Drains that serve your home are part of the sanitation installations your landlord must keep in repair under section 11 of the Landlord and Tenant Act 1985. Report a blocked outside drain in writing. If it is a shared drain, your landlord or you can report it to Thames Water.

If it keeps coming back

A drain that blocks every few months usually has a reason. Common ones in older London terraces are a cracked clay pipe, a joint that has dropped so waste collects in the dip, roots finding their way in, or a long run with too little fall. Clearing it again only buys time. A camera survey shows what is going on inside the pipe and whether the fault is on your section or a shared one. If it is shared, the repair may fall to Thames Water.

Keeping it clear

Put fat in the bin, not the sink. Use a sink strainer. Keep the gully grid clear of leaves in autumn. Do not flush wipes. These four habits prevent most outside drain blockages in London terraces.