What is a dead leg? How to spot one in your building

By the Germ3 Water team. Updated 20 October 2026.

What a dead leg is

A dead leg is a length of pipe still connected to the water system that no longer leads to anything in use. Water goes in but never comes out. So it sits still.

Most dead legs are left behind by building work. A sink is taken out and the pipe is capped instead of cut back. An office is refitted and a branch to an old kitchen stays in the ceiling.

Why it matters

Still water loses its temperature control. Cold pipes warm up and hot pipes cool down. Legionella grows best between 20°C and 45°C and a dead leg can sit in that range for months.

Bacteria also form a slimy layer called biofilm on the inside of the pipe. Biofilm protects legionella from heat and chemicals. When the water in a dead leg mixes back into the live system it can carry bacteria with it.

“An assessment that missed a tank or a dead leg because the person who wrote it did not know to look for one is not a defence.”

Where to look

Walk the building with your water schematic if you have one. Check these places:

• Under sinks and in kitchens where appliances have been removed.
• Capped pipes in ceiling voids and risers.
• T pieces where one branch ends in a cap.
• Valves that feed nothing.
• Rooms that have changed use since the last refit.
• Old washing machine and dishwasher connections.
• Showers and basins in rooms nobody uses.

A cold pipe that feels warm is another clue. So is a drawing that no longer matches the building.

Dead legs and little used outlets

A little used outlet is a tap or shower that still works but is rarely run. It is not quite a dead leg. Water can still flow if someone opens it. HSE guidance says these outlets should be flushed at least once a week or removed if they are no longer needed.

What to do if you find one

1. Note where it is and take a photograph.
2. Tell the responsible person for your building.
3. Add it to your risk assessment action list.
4. Leave the pipe alone until an engineer has looked at it.

What removal involves

An engineer isolates the supply and cuts the redundant pipe back as close to the live pipe as possible. The joint is sealed and the system is flushed. Where the system has been disturbed it may need disinfecting. Finally the schematic and risk assessment are updated so the change is on record.

HSE covers dead legs in HSG274 Part 2. You can read more about the wider duty on the HSE legionella pages.