How to Carry Out a Legionella Risk Assessment – Step-by-Step

A Legionella risk assessment is a legal requirement for many landlords, employers and building managers in the UK. But what does it actually involve? In this guide, we’ll walk you through the key steps that a competent assessor follows when carrying out a Legionella risk assessment, based on the Health and Safety Executive’s ACOP L8 guidance.

Step 1: Review the Premises and Water System

The assessor begins by understanding how water is stored, distributed and used throughout the premises. This includes identifying:

• Hot and cold water storage tanks
• Pipework layout and dead legs (redundant pipework)
• Taps, showers, outlets, and appliances
• Any areas where water may stagnate

They may ask questions about occupancy, building use, and past water system maintenance.

Step 2: Identify Potential Risks

The next step is to evaluate whether the current system could support the growth of Legionella bacteria. This involves looking for:

• Water temperatures between 20–45°C
• Stored or recirculated water
• Biofilm or scale buildup
• Areas where water is not regularly flushed
• Any risk of aerosol generation (e.g. showers or spray taps)

Step 3: Assess Who Might Be at Risk

The assessor considers who could be exposed to contaminated water droplets. This often includes:

• Residents, tenants or employees
• Vulnerable people (elderly, immune-compromised)
• Contractors or visitors

Step 4: Evaluate Current Controls

The assessment must review whether appropriate precautions are in place. These might include:

• Regular temperature checks
• Flushing of little-used outlets
• Tank cleaning and descaling routines
• Written Scheme of Control
• Training for the responsible person

Step 5: Record the Findings

The risk assessor will provide a written report showing:

• Identified risks and their severity
• Areas of non-compliance
• Photographs and notes from the site visit
• Recommended actions to reduce risk

This report is a legal document and must be retained.

Step 6: Recommend and Prioritise Actions

Any actions needed to reduce the risk of Legionella must be listed clearly and prioritised. Examples include:

• Removing dead legs or unused pipework
• Raising or lowering water temperatures
• Implementing regular flushing schedules
• Installing thermostatic mixing valves (TMVs) where needed

Step 7: Schedule a Review Date

Legionella risk assessments should be reviewed at least every two years — or sooner if:

• The building’s water system is altered
• There is a change in use or occupancy
• Legionella is detected in the water

After your risk assessment, you’ll need to develop a written scheme of control.

Don’t forget to review your legal duties: What the Law Says About Legionella Risk Assessments.

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