Short answer. A dead leg is a branch of pipe leading to an outlet that is only used occasionally, so the water sitting in it stagnates between draw-offs. Redundant, capped-off pipework with no outlet at all is a different thing, properly called a dead end or blind end. Both let water stand still and warm up, which is exactly what legionella bacteria need, and both should be found and dealt with as part of a legionella risk assessment.
Somewhere in your building there is almost certainly a stretch of pipe that has not had water moving through it for weeks, maybe months. It might feed a basin in a room nobody uses. It might be a capped spur left over from a water cooler that was removed years ago. Either way, standing water is the one condition legionella needs above all others.
Dead legs and dead ends are not the same thing
HSE guidance HSG274 Part 2 draws a distinction that is worth keeping straight. A dead leg is a length of pipework that still leads to a working outlet, but one that is used infrequently, so water passes through it only on draw-off and stagnates in between. A dead end (also called a blind end) is pipework that no longer serves any outlet at all: it has been capped off, but not removed. A dead end is generally the worse of the two, because there is no draw-off to refresh the water at all.
Blind ends should be removed rather than managed. The commonly used rule of thumb, from NHS Technical Memorandum HTM 04-01 and BS 8558 rather than HSG274 itself, is that a capped branch should be no longer than about twice the internal diameter of the pipe. In practice, plenty of buildings have blind ends running several metres because cutting the pipe back to the main would mean opening up a wall or a floor.
Why stagnant water is a legionella risk
Legionella bacteria multiply most readily in water sitting between roughly 20°C and 45°C. A dead leg or dead end ticks every box: the water is not moving, its temperature drifts toward room temperature, and biofilm builds up on the pipe wall, giving the bacteria something to feed on.
The risk shows up when someone finally opens that rarely used tap. The first water out carries whatever has been growing in the stagnant section, and if it is dispersed as an aerosol, through a shower head or a spray tap, it can be inhaled. Dead legs and stagnation are consistently identified as contributing factors in legionella outbreak investigations.
How they get created
Almost all of them come from building work that did not account for the water system. Refurbishments where an outlet is capped but the pipe is left in place. Office reorganisations where a basin or kitchenette is decommissioned but the supply pipe stays connected. Equipment changes where a water cooler, coffee machine or dishwasher is removed but its feed pipe is not. Every one of these was probably done to save time; the side effect is a stagnation risk that sits there until someone finds it.
Infrequently used outlets need managing too
You do not need redundant pipework to have a stagnation problem. Any outlet used irregularly, the basin in a meeting room, the shower nobody uses because staff drive home, creates similar conditions to a dead leg. HSG274 Part 2 recommends flushing infrequently used outlets weekly, not every outlet in the building, and flushing each one until the water temperature stabilises, which typically takes several minutes rather than a fixed count. Each flush should be logged.
The record-keeping itself is not a direct requirement of L8, which is an Approved Code of Practice rather than a statute. The underlying legal duty to record significant findings, including control measures like flushing, comes from COSHH 2002 and the Management of Health and Safety at Work Regulations 1999, and applies in writing once you have five or more employees. In practice almost every commercial building keeps these records regardless, because it is the only way to show an inspection or audit that flushing has actually happened.
Finding dead legs and dead ends
The problem is that most of them are hidden: behind walls, above ceilings, under floors. A competent legionella risk assessor traces the water distribution through the building, identifies every outlet including redundant ones, compares the physical system to any available drawings, and flags sections of pipework no longer serving an active purpose. In older buildings the drawings often do not match reality, so a physical survey, lifting ceiling tiles and opening service risers to follow pipe runs, is usually necessary.
Fixing what you find
Removing a dead end is the best fix: cut the redundant pipework back to the distribution main and cap it properly, which removes the stagnation risk entirely. A genuine dead leg that still serves a needed outlet is managed with a documented weekly flushing regime instead. Either way, every building modification, refurbishment, extension or change of use should trigger a review of the water system for any new dead legs or dead ends it has created.
York Green's legionella risk assessment service includes tracing and testing dead legs and dead ends as standard. For the underlying guidance, see HSG274 Part 2: The control of legionella bacteria in hot and cold water systems and the L8 Approved Code of Practice.