What Is A Dead Leg

If you are responsible for managing a building’s water system, understanding what a dead leg is and why it matters is essential. Dead legs are closely linked to water stagnation, loss of disinfectant protection, and conditions that increase the risk of Legionella bacteria developing within pipework.

Put simply, a dead leg is a section of pipe where water does not flow often enough, or at all. The water inside becomes stagnant, meaning it sits for extended periods without being refreshed. Over time, this stagnant water can deteriorate in quality and create an environment where bacteria are more likely to grow.

For facilities managers, dead legs are rarely an isolated issue. They often sit quietly in the background, unnoticed, while undermining the effectiveness of otherwise well managed control measures.

Dead legs and blind ends explained

What is meant by a dead leg

A dead leg usually refers to a run of pipework that supplies an outlet which is used infrequently. Water may only move through it occasionally, leading to poor turnover and stagnation.

This commonly occurs in buildings with low occupancy areas, seasonal use, or spaces that are technically still connected to services but rarely accessed.

What is a blind end

A blind end is pipework that has been capped or sealed at one end. It often results from alterations, refurbishments, or the removal of fixtures where the supply was left in place rather than removed back to the main.

Although blind ends no longer supply an outlet, they can still trap water and remain hydraulically connected to the system.

Why the difference matters

From a practical point of view, blind ends are often easier to address because they serve no ongoing purpose. Dead legs can be more complicated, particularly where an outlet is still required occasionally or where future use is anticipated.

Facilities managers should be aware that both can contribute to risk, but blind ends are frequently prioritised for removal due to their redundancy.

For a straightforward explanation of how dead legs are treated in remedial works, this overview of dead leg piping provides useful background.

Why dead legs increase Legionella risk

Stagnation and disinfectant decay

When water remains static, disinfectant residuals diminish over time. Without that residual protection, microorganisms are more likely to survive and multiply. The longer water sits undisturbed, the greater the potential loss of protection.

Biofilm development

Stagnant conditions also encourage biofilm formation on the internal surfaces of pipework. Biofilm provides nutrients and shelter for bacteria, making them harder to control and increasing the likelihood of recurring issues.

Once biofilm is established, simply improving temperatures or flushing occasionally may not be enough to resolve the problem.

Temperature drift

Dead legs are more prone to temperature drift, particularly in mixed use buildings. Hot water can cool down and cold water can warm up, allowing temperatures to sit within a range that favours bacterial growth.

In practice, dead legs often act as the enabling factor that allows other weaknesses in a system to become more significant.

For a broader view of how dead legs fit into the wider management process, this guide on what facilities managers must do after a Legionella risk assessment is a useful reference.

How to identify dead legs in a building

Focus on change and history

Dead legs frequently arise following refurbishments, changes in occupancy, or reconfiguration of spaces. Buildings with a long history of alterations are particularly susceptible.

If areas have been repurposed, downsized, or partially decommissioned, there is a higher likelihood that redundant or low use pipework exists.

Look for low use outlets

Common indicators include:

• Outlets used infrequently or seasonally

• Long pipe runs serving a single fitting

• Converted spaces such as former plant rooms or storage areas

• Capped branches hidden above ceilings or within service voids

Improving control around infrequently used outlets can help reduce stagnation risk. This guidance on how to flush little used outlets explains how to structure flushing in a way that is practical and proportionate.

Use the risk assessment properly

A Legionella risk assessment should act as your roadmap. It should identify schematic layouts, sentinel outlets, infrequently used outlets, and recommended actions relating to dead legs and blind ends.

If dead legs are identified, they should be prioritised based on both likelihood and consequence, not simply listed and left unresolved.

This service overview of a Legionella risk assessment helps clarify what should be covered and why it matters.

What should be done about dead legs

Removal is the preferred option

Best practice guidance consistently favours complete removal of redundant pipework back to the live main. Simply capping a section closer to the outlet can still leave a stagnation zone connected to the system.

Where possible, physical removal provides the most robust and permanent solution. Take a look at one of our case studies showing the removal of multiple dead legs - Dead Leg Removal Project Leeds | Titan Water

Managing dead legs that cannot be removed

There are situations where immediate removal is not feasible, such as access restrictions or planned future use. In these cases, risk management typically involves a combination of:

• Minimising pipe length and volume

• Implementing a documented flushing regime

• Monitoring outlet temperatures

• Reviewing controls after any building works or changes in use

Understanding what should be included in routine checks can be helpful. This page on water hygiene monitoring services outlines the typical elements involved in ongoing monitoring.

Consider related risks

Dead legs often coexist with other contributing factors such as scale, sediment, and poor tank condition. Addressing pipework alone may not fully resolve risk if other system components are deteriorating.

If storage tanks form part of your system, reviewing inspection and cleaning arrangements is sensible. This overview of tank cleaning explains what the process typically involves.

Dead legs within the wider compliance framework

Alignment with ACoP L8 and HSG274

Facilities managers operating under ACoP L8 and HSG274 are expected to identify and manage foreseeable risks. Dead legs are well recognised within guidance as a preventable contributor to Legionella risk and commonly appear within recommended actions.

A plain English explanation of these expectations is covered in Understanding ACoP L8, essential guidance for facilities managers.

Sampling and investigation

Where water sampling results are unexpected or repeat positives occur, dead legs and blind ends are often reviewed as part of the investigation. Removing or managing them can play a key role in improving long term control and system stability.

If you are responsible for interpreting results, this guide on how to test for Legionella bacteria in water provides useful context.

Bringing it all together

Dead legs are rarely obvious, but their impact can be significant. Identifying them, understanding their role in stagnation and temperature control, and addressing them proportionately forms an important part of effective water system management.

A practical starting point is to review areas that have changed in the last two years, list infrequently used outlets, and compare them against your schematics and risk assessment actions.

For a wider checklist of how dead legs fit into overall control measures, you may find it helpful to explore the water hygiene services overview as a reference point for the typical components of a managed system.