What is a Legionella Risk Assessment? UK Requirements & Tank Specifications
Most UK businesses and landlords are aware that Legionella is a legal obligation, but far fewer understand that the design and specification of their steel water storage tank is one of the most significant factors determining their level of risk. A poorly specified tank, an uncovered tank, or a tank lined with the wrong material can create exactly the conditions that allow Legionella bacteria to multiply, regardless of how thorough the risk assessment process is.
This guide explains what a Legionella risk assessment is, who is legally required to carry one out, what the assessment process covers, and how correct cold water tank specification, including the right liner kit and a fully sealed roof kit, reduces Legionella risk at source before it becomes a compliance or health problem.
What is Legionella?
Legionella is a genus of bacteria found naturally in freshwater environments including rivers, lakes, and reservoirs. In low concentrations in natural water sources, Legionella bacteria in water pose little risk to human health. The problem arises when the bacteria enter a built water system and find conditions that allow them to multiply to dangerous concentrations.
Legionella bacteria in water systems thrive within a specific temperature range, between 20°C and 45°C, and are particularly associated with stagnant water, the presence of biofilm, sediment, rust, and organic matter, and water systems with sections of low or no flow. When contaminated water is disturbed and aerosolised into fine droplets that can be inhaled, exposure to Legionella bacteria becomes a serious health risk.
Legionnaires' disease is a serious form of pneumonia caused by inhaling those contaminated water droplets. It is a potentially fatal form of pneumonia, particularly for older people, those with weakened immune systems, and anyone with underlying respiratory conditions. A milder illness called Pontiac fever is also caused by Legionella bacteria, though it is less severe and rarely life-threatening. The Health and Safety Executive records several hundred confirmed cases of Legionnaires' disease in the UK every year, with many more going undiagnosed.
What is a Legionella Risk Assessment?
A Legionella risk assessment is a detailed and systematic inspection of a premises and its water systems to identify conditions that could allow Legionella bacteria to grow and spread. It is a water risk assessment specific to Legionella, designed to establish the risk level present in a water system and determine what control measures are needed to minimise the risk to anyone who may be exposed.
Carrying out a Legionella risk assessment means reviewing every water system in a premises, including cold water tanks, hot water systems, pipework, showers, cooling towers, and any other outlet or fitting where water is stored, distributed, or released as a droplet or aerosol. The assessment identifies foreseeable risk factors, evaluates the level of risk each presents, and recommends appropriate control measures to control the risk of legionella effectively.
A professional legionella risk assessment is typically carried out by a trained assessor with relevant training and experience in water hygiene and Legionella control. Whilst some simple, low-risk premises can be assessed by a competent person within the organisation, complex or higher-risk sites require a professional legionella risk assessment from a specialist water risk assessment provider.
Who Needs to Carry Out a Legionella Risk Assessment?
Under UK health and safety law, the legal obligation to conduct a Legionella risk assessment applies to any employer, landlord, or duty holder who has control of premises with a water system. This is not limited to large commercial operations. The duty of care extends to any premise where people may be exposed to risks from Legionella through a water system on the site.
The responsible person, who is the duty holder under health and safety law, is legally required to carry out the assessment and ensure appropriate control measures are in place. This applies across commercial premises, industrial sites, care homes, hotels, schools, hospitals, rented residential properties, and any other setting where a water system is present.
The legal framework governing Legionella risk assessments in the UK includes the Health and Safety at Work etc. Act 1974, the Control of Substances Hazardous to Health Regulations 2002 (COSHH), and the substances hazardous to health regulations framework that underpins it. The Health and Safety Executive's Approved Code of Practice L8, known as ACoP L8, and the associated guidance document HSG274 set out the specific requirements for Legionella control in water systems. UK health and safety law requires that the risk assessment be reviewed regularly, whenever there is a change to the water system, and whenever there is reason to believe the current assessment is no longer valid.
Landlords have a specific legal obligation under the Health and Safety at Work Act to assess and control legionella risks in rental properties. Even where a domestic hot and cold water system presents a low risk, the duty holder must still carry out the assessment and document the findings. There is no such thing as a legionella test certificate that replaces a full risk assessment, though legionella testing of water samples forms part of the ongoing monitoring and maintenance programme for higher-risk systems.
What Does a Legionella Risk Assessment Cover?
The assessment process for a professional legionella risk assessment is thorough and site-specific. A detailed assessment will typically include the following:
A full survey of all water systems on the premises, including cold water tanks, hot water cylinders and calorifiers, all pipework and dead legs, shower heads and taps, cooling towers, and any other water outlet or fitting.
Assessment of water temperature throughout the system, including cold water storage temperatures, hot water storage and distribution temperatures, and any areas of the pipework where temperatures may fall into the Legionella growth range of 20°C to 45°C.
Identification of risk factors including stagnant or infrequently used pipework, areas of low flow, materials within the system that may support biofilm growth, presence of sediment or scale, and any defects in tank covers, liners, or fittings that could allow contamination to enter the stored water.
Evaluation of the existing control measures in place and their adequacy for reducing the risk of legionella to an acceptable level.
A written record of the assessment findings, the risk level identified, the recommended simple control measures or more complex interventions required, and a programme for monitoring and maintenance going forward.
The responsible person must ensure the legionella risk assessment template and associated records are kept up to date, made available to the Health and Safety Executive or relevant enforcement authority on request, and reviewed at appropriate intervals.
Cold Water Storage Tanks and Legionella Risk
Cold water tanks are consistently identified as one of the highest-risk components in any water system for Legionella growth. The risks from legionella in cold water storage are well documented and arise from several specific conditions that tanks create if not correctly specified and maintained.
Water temperature is the primary risk factor. Cold water tanks should store water below 20°C. In practice, tanks that are poorly insulated, located in roof spaces or plant rooms where ambient temperatures are high, or that receive warm water back-flow from distribution systems can exceed this threshold. Once the stored water temperature rises above 20°C, the risk of exposure to legionella bacteria increases significantly.
Stagnation is the second major risk factor. Water that sits in a tank for extended periods without being turned over provides ideal conditions for Legionella bacteria to multiply and for biofilm to establish on the tank surfaces. Correctly sized tanks that match the actual demand of the premises reduce stagnation risk.
Contamination from the environment is the third risk factor. Open-top or inadequately covered cold water tanks allow debris, insects, birds, and airborne contamination to enter the stored water. Organic contamination provides nutrients for Legionella bacteria and encourages biofilm growth on the sides of the tank and pipework.
Liner material and tank construction directly affect the level of risk. Certain materials support biofilm formation more readily than others. A tank lined with a WRAS-approved, non-leaching liner material that does not support microbial growth is a critical factor in reducing the legionella risk level within the storage system itself.
How Correct Tank Specification Reduces Legionella Risk
Reducing the risk of legionella bacteria in cold water storage begins with specifying the right tank for the application. Butek Tanks' corrugated steel water storage tanks are designed to minimise Legionella risk through correct material selection, appropriate liner specification, and covered storage as standard.
WRAS-Approved Liner Kits
The liner is the surface that stored potable water contacts directly. A liner that leaches chemicals, supports biofilm, or degrades over time increases the risk of legionella bacteria establishing within the tank. Butek Tanks manufactures all liner kits in-house and offers WRAS-approved options for potable water storage:
Butyl rubber: the premier choice for potable cold water storage. Butyl is chemically inert, non-leaching, and does not support biofilm in the way that some lower-grade materials can. It carries a long service life and is the standard specification for Butek Tanks potable water tanks across commercial, industrial, and utility applications.
EPDM rubber: a cost-effective WRAS-approved alternative to Butyl, suitable for cold water storage where budget is a consideration and the chemical and biological performance requirements of the application are met.
Both liner materials are compatible with the Magnelis coated corrugated steel tank shell, which offers significantly greater corrosion resistance than standard galvanised steel, reducing the risk of rust contamination entering the stored water.
Tank Covers and Roof Kits
An uncovered cold water storage tank is a direct Legionella risk. Debris, insects, bird matter, and airborne contamination can all enter an open-top tank and introduce the organic material that supports Legionella bacteria growth. Our range of roof kits provides complete protection for cold water storage tanks:
Heavy-duty steel roof kits provide a fully sealed cover with lockable access hatches for inspection and maintenance access, preventing all environmental contamination of stored water.
The Aqua-Float floating cover sits directly on the water surface, preventing up to 98% of evaporation, eliminating the air gap above stored water in which contamination can settle, and significantly reducing the conditions that allow Legionella bacteria to establish.
Anti-algae covers block sunlight penetration that encourages algae growth on tank surfaces, which in turn provides nutrients for Legionella bacteria.
All Butek Tanks roof kits include lockable access hatches, allowing the responsible person and appointed assessor to carry out inspections, water temperature checks, and legionella testing in accordance with the monitoring and maintenance programme required by ACoP L8.
Control Measures for Legionella in Cold Water Tank Systems
Once the risk assessment has been carried out and the risk level established, the responsible person must implement and maintain appropriate control measures. For cold water storage tank systems, the key control measures recommended by ACoP L8 and HSG274 include the following:
Temperature control: maintaining cold water storage below 20°C at all times. This requires correct tank siting, insulation where appropriate, and regular temperature monitoring as part of the ongoing monitoring and maintenance programme.
Tank inspection and cleaning: carrying out regular inspections of the tank interior, liner condition, cover integrity, and inlet and outlet fittings. Tanks should be cleaned and disinfected at appropriate intervals, typically annually as a minimum for higher-risk systems.
Legionella testing: regular water sampling and testing for legionella bacteria in water taken from the cold water tank and distribution system. Positive results require immediate remedial action to control the risk of legionella and minimise the risk of exposure to legionella bacteria.
System review: ensuring that pipework connected to the tank does not include dead legs or infrequently used sections where stagnation can occur. Appropriate control measures include flushing programmes for low-use outlets and removal or capping of redundant pipework.
Record keeping: maintaining a full written record of all risk assessments and testing, temperature readings, inspection findings, cleaning and disinfection activities, and any remedial actions taken. The risk assessment must be reviewed whenever the system changes or the responsible person has reason to believe the current assessment is no longer valid.
Frequently Asked Questions About Legionella Risk Assessments
What is a Legionella risk assessment?
A Legionella risk assessment is a systematic inspection of a premises and its water systems to identify conditions that could allow Legionella bacteria to grow, spread, and cause harm through exposure to contaminated water droplets. It establishes the risk level present in the system and identifies the control measures needed to minimise the risk and comply with UK health and safety law.
Who is legally required to carry out a Legionella risk assessment?
Any employer, landlord, or duty holder with control of premises containing a water system has a legal obligation to carry out a Legionella risk assessment under UK health and safety law. This includes the Health and Safety at Work etc. Act 1974, COSHH Regulations 2002, and the Approved Code of Practice ACoP L8 issued by the Health and Safety Executive.
How often does a Legionella risk assessment need to be reviewed?
The risk assessment must be reviewed at regular intervals, typically every two years for most premises, and immediately whenever there is a significant change to the water system, a change of use of the premises, or whenever there is reason to believe the existing assessment no longer reflects the current risk level. Legionella testing and monitoring and maintenance activities should take place more frequently in higher-risk systems.
Does a cold water storage tank need a Legionella risk assessment?
Yes. Cold water tanks are specifically identified in ACoP L8 and HSG274 as a high-risk component in any water system for Legionella growth. The assessment process must include an inspection of cold water tank temperature, cover integrity, liner condition, and water quality, and appropriate control measures must be put in place to control the risk.
How does tank specification reduce Legionella risk?
Correct tank specification reduces Legionella risk by ensuring cold water is stored below 20°C, that the tank is fully covered to prevent environmental contamination, and that the liner material does not support biofilm or leach substances that encourage bacterial growth. WRAS-approved Butyl or EPDM liner kits, combined with a fully sealed roof kit, significantly reduce the conditions that allow Legionella bacteria to establish in cold water storage.
What is ACoP L8?
ACoP L8 is the Approved Code of Practice issued by the Health and Safety Executive for the control of Legionella bacteria in water systems. It sets out the legal requirements and practical guidance for carrying out Legionella risk assessments, implementing control measures, and maintaining water systems safely. Compliance with ACoP L8 is the recognised standard for demonstrating that the duty holder has met their legal obligation to manage the risk of Legionella in a UK premises.
Conclusion
A Legionella risk assessment is a legal requirement for any UK business, landlord, or duty holder with a water system on their premises, and cold water storage tanks are consistently one of the highest-risk components in any assessment. Understanding what the assessment process covers, who carries it out, and what control measures are required is the foundation of effective Legionella control.
At Butek Tanks, we have been designing and manufacturing corrugated steel water storage tanks since 1965. Our potable water tanks are available with WRAS-approved Butyl and EPDM liner kits and a full range of sealed roof kits, providing cold water storage that is correctly specified to minimise Legionella risk from the point of installation. Contact our team today to arrange a free site survey and discuss the right tank specification for your premises.