Somewhere in your building, there's almost certainly a stretch of pipe that hasn't had water flowing through it for weeks. Maybe months. It might be serving an outlet that was capped off during a refurbishment. It might feed a basin in a room that's been locked since the last reorganisation. It might be a spur that once served a water cooler that's long since been removed.
In plumbing terms, this is a "dead leg." In legionella terms, it's a five-star hotel for bacteria.
A dead leg is a section of pipework that is no longer part of the actively flowing water system. The water in a dead leg has nowhere to go — it just sits there, gradually warming to ambient temperature, accumulating biofilm and providing ideal conditions for legionella bacteria to multiply.
The HSE guidance (HSG274 Part 2) recommends that dead legs should be kept as short as possible and ideally eliminated entirely. The general rule is that a dead leg should not exceed the internal diameter of the pipe — so for a 22mm pipe, the dead leg should be no more than 22mm long. In reality, many buildings have dead legs running several metres.
Legionella thrives in stagnant water between 20°C and 45°C. Dead legs tick every box: the water isn't moving, the temperature is uncontrolled, and biofilm builds up on the internal pipe surfaces, providing nutrients for bacterial growth.
When someone eventually does open that forgotten tap, the first water out carries whatever has been growing in the dead leg. If that water is dispersed as an aerosol — through a shower head, a spray tap, or even agitation in a basin — the bacteria can be inhaled.
The risk isn't theoretical. Dead legs and stagnation are consistently identified as contributing factors in legionella outbreaks.
They're usually the result of building modifications done without considering water safety. Common scenarios include refurbishment works where outlets are capped off but the pipework is left in place because removing it would mean opening up walls or floors. Office reorganisations where basins or kitchenettes are decommissioned but the supply pipes remain connected. Equipment changes where water coolers, coffee machines or dishwashers are removed but the feed pipe stays. Building extensions where the original distribution was modified but old spurs were left connected.
In each case, the intention was probably to save time and money. The unintended consequence is a stagnation risk that persists until someone identifies it and removes the redundant pipework.
You don't need redundant pipework to have a stagnation problem. Any outlet that isn't used regularly creates similar conditions. The basin in the meeting room that gets used once a month. The shower in the changing room that nobody uses because everyone drives home. The kitchen tap on the floor that's currently unoccupied.
L8 guidance recommends that all outlets should be flushed at least weekly. For outlets that aren't used regularly, a documented flushing regime needs to be in place. Run the water for at least two minutes (long enough to replace the standing water in the pipe) and record that it's been done.
This sounds tedious, and it is. But it's considerably less tedious than dealing with a legionella outbreak.
The challenge is that dead legs are often hidden. They're behind walls, above ceilings, under floors. You can't see them without looking, and most people aren't looking.
A competent legionella risk assessor will trace the water distribution throughout the building, identify all outlets (including redundant ones), compare the physical system to any available drawings and identify sections of pipework that are no longer serving an active purpose.
In older buildings, the original drawings may not exist or may bear little resemblance to the current installation. In these cases, a physical survey of the system is essential. This might involve lifting ceiling tiles, opening service risers and following pipe runs to identify where they go and whether the outlets they serve are still in use.
The solution is straightforward in principle, if sometimes disruptive in practice:
Remove dead legs. The best option. Cut the redundant pipework back to the distribution main and cap it properly. This eliminates the stagnation risk entirely.
Implement a flushing regime. For infrequently used outlets that are still connected and may be needed in future, regular flushing prevents stagnation. Weekly flushing is the standard recommendation.
Record everything. Temperature checks, flushing records, maintenance actions. Documentation is a legal requirement under L8 and the first thing an enforcing authority will ask for.
Review after any building works. Every time the building is modified — refurbishment, extension, change of use — the water system should be reviewed to identify any new dead legs or redundant pipework.
Dead legs are one of the most common and most preventable legionella risk factors in commercial buildings. They exist in far more premises than most building managers realise, and they're created every time a building modification doesn't account for the water system.
Finding and eliminating them is a core part of any legionella risk assessment — and it's one of the most impactful things you can do to protect the people in your building.
Worried about dead legs in your water system? Book a Legionella Risk Assessment →
York Green Safety Partners identifies and addresses dead legs and stagnation risks as part of every legionella risk assessment. Based in Cheshire, covering the whole of the UK.