What is a Legionella Risk Assessment

Understanding the Purpose of a Legionella Risk Assessment

A Legionella risk assessment is a structured review of the water systems within a building to identify conditions that could support the growth of Legionella bacteria. These bacteria thrive in water that is warm, stagnant or contaminated with scale or biofilm. When droplets from contaminated water become airborne, they can be inhaled and potentially lead to Legionnaires disease. For anyone responsible for a building, understanding how and why these risks occur is essential to maintaining a safe environment.

A good assessment looks at the design of your water systems, how they are used, how they are maintained and whether any parts of the system may allow water to sit still or reach temperatures that encourage bacterial growth. It is a crucial part of a wider water safety plan and helps ensure you meet your legal duties as a duty holder. If you want a deeper look at basic water hygiene responsibilities, you may find the information in our water hygiene services section useful as a follow up.

For facilities managers, the assessment is not just about compliance. It gives you a clear picture of how well your building is managing water safety and what practical steps are needed to stay on top of things.

Who Needs a Legionella Risk Assessment and When It Should Be Reviewed

Duty Holders and Building Responsibilities

Any organisation or individual responsible for a building with hot and cold water systems has a legal duty to keep occupants safe. This includes offices, commercial facilities, schools, healthcare settings, rental properties and any other premises where water is stored or distributed. Your role as a duty holder involves making sure the risks associated with Legionella are identified, assessed and managed appropriately. The assessment must be carried out by someone competent to evaluate the risks.

This could be a trained member of your team or an external specialist. The key requirement is that they understand water systems, know how to recognise risk factors and can recommend suitable control measures.

When to Update or Revisit the Assessment

A Legionella risk assessment is not a one time document. Circumstances change, and your assessment should be reviewed whenever:

• The building layout or plumbing system is altered

• New water tanks or outlets are added

• Occupancy patterns change

• Water systems are unused for extended periods

• There is reason to believe existing control measures are no longer effective

• An incident or near miss highlights potential weaknesses

Many facilities managers choose to set a review schedule, typically every two years, although some environments require more frequent checks. Read more in our blog post How Often Should Legionella Risk Assessments Be Carried Out? | Titan Water

What the Assessment Should Include

Building the System Overview

A thorough risk assessment begins with mapping your entire water system. This includes hot and cold water storage, distribution pipework, tanks, calorifiers, pumps, outlets, thermostatic mixing valves and any equipment that produces aerosols. The goal is to understand how water moves through the building so potential problem areas can be identified.

During this stage, the assessor will consider whether any pipes are redundant, whether storage tanks are oversized or whether circulation is inadequate. Dead legs, low flow sections and under insulated pipe runs are common causes of temperature stratification and stagnation, both of which increase risk. You can read more about dead-legs and the risk they pose at our dedicated dead leg piping page.

A clear schematic is an important part of your records.

Identifying Key Risk Factors

Several factors increase the likelihood of Legionella growth. These include:

• Water temperatures between 20 and 45 degrees

• Areas where water remains still for long periods

• Build up of scale, rust or sediment

• Systems that generate fine water droplets

• Inadequate flushing routines

• Poorly maintained valves, strainers or tanks

A competent assessor will check these conditions carefully, using temperature readings, visual inspections and a review of how the system is used day to day.

They will also consider whether the building hosts vulnerable occupants, such as older adults or people with compromised immune systems, as this affects the overall risk rating.

Evaluating Current Control Measures

Once the system has been reviewed, the assessor examines whether existing controls are working effectively. This includes looking at:

• Temperature control regimes

• Storage tank inspection frequency

• Shower head cleaning and descaling

• Flushing routines for low use outlets

• Water treatment processes

• Monitoring and record keeping

For anyone managing multiple buildings, having consistent documentation is essential.

Why a Legionella Risk Assessment Matters

Protecting People in the Building

Legionnaires disease is serious, and outbreaks can be avoided through straightforward, sensible control measures. A risk assessment helps highlight where issues might develop so that remedial work can be planned before problems arise. This keeps everyone in your building safer, from staff and tenants to visitors and contractors.

Supporting Legal Compliance

UK regulations are clear about the duty to assess and control risks associated with Legionella. A documented, suitable and sufficient risk assessment helps demonstrate compliance and protects your organisation should an inspection, audit or investigation occur. It also supports your internal governance and shows that water safety is being handled responsibly.

Strengthening Professional Standards

A well maintained water system reflects positively on any facility. Whether you manage a commercial building, residential block or public site, having clear controls in place enhances confidence among those who use the premises. Good documentation also helps in the event of staff turnover, ensuring continuity in how water safety tasks are handled.

What Happens After the Assessment

A risk assessment is only valuable if the findings lead to practical action. After reviewing the report, you may need to schedule remedial work, improve flushing routines, update your logbook or tighten monitoring. This could involve adjusting tank sizes, improving insulation, removing dead legs or increasing the frequency of cleaning and descaling. Our blog on prioritising remedial works explains strategies you can employ to deal with any recommendations.

Consistent follow up is what keeps the system under control. Many facilities managers build these tasks into their planned maintenance routines. If you are unsure where to start with planning this work, our water hygiene services page gives a clear overview of typical tasks and how they fit into ongoing compliance.

Final Guidance

If you want to go further in understanding how your building’s water systems should be managed, you may find it useful to read our detailed guidance in the Learning Centre where you will find more material on water hygiene, monitoring and maintenance.