A Legionella risk assessment cannot grow bacteria. The gaps in one can. When a sample comes back positive, an outlet runs lukewarm, or an inspector starts asking pointed questions, the trail almost always ends at something the assessment missed, recorded wrongly, or flagged and nobody acted on.
So treat a near miss the way a maintenance engineer treats a fault: as a symptom pointing back to a cause. The assessment is usually where that cause has been hiding in plain sight — signed off, filed, and quietly wrong.
Working back from the symptom on site to the error in the paperwork is the order things actually go wrong in, and it is the fastest way to stop them recurring.
Where the fault usually hides
The control framework all these errors quietly break is the familiar one set out in L8 and HSG274 — understand the system, write a scheme, monitor it, review it [1][2]. A handful of assessment errors account for most cases where that chain snaps far enough for Legionella to multiply. They are worth knowing in rough order of how often they bite.
The asset register doesn’t match the building. This is the big one. An assessment can only control what it lists, and the survey is where outlets quietly go missing. A capped dead leg still teed into a live main, a basement shower used twice a year, a redundant header tank in a warm loft, a hose union behind a cleaner’s cupboard — if the survey never recorded it, no written scheme flushes or monitors it, and it sits warm and still. The tell-tale is a positive result, or visible scale and sludge, at a fitting that isn’t on the asset schedule at all.
The assessment describes a building that no longer exists. A wing was added, a floor mothballed, a department’s hours cut, occupancy halved after a lease ended. HSE expects the assessment to be reviewed when the system, the way it is used, or the people exposed change [3]. A report dated comfortably within the usual review habit can still be wrong the morning after a refit, because the building moved and the paperwork didn’t.
It was a desktop job. Generic recommendations, no system schematic, no plant-room photographs, wording suspiciously identical to other sites on the same contract. The assessor never opened the riser cupboard. BS 8580-1 sets out what a competent Legionella risk assessment should actually cover — the system described, the assets listed, the responsibilities named [4]. A template that skips the survey is not that.
Risk was rated but never resourced. The assessment did its job and listed the remedial actions — remove the dead legs, lag the cold run that passes through a warm plant room, reinstate the failed TMV — and they have sat “open” across two review cycles. The error here is treating the assessment as the finished product. An open remedial is uncontrolled risk wearing a paperwork hat.
Scope and responsibility blur. On shared systems — a landlord’s communal cold tank, a leased unit, a managing agent and a tenant either side of a stopcock — each party assumes the other covers it, so nobody does. An assessment that stops politely at the demise line leaves the riskiest shared pipework unowned.
From symptom back to the assessment
When something looks wrong on site, trace it like this before reaching for the logbook. The pattern is always the same: confirm the symptom, find the assessment error behind it, then fix both the outlet and the assessment.
| Symptom on site | Likely assessment error | The check that confirms it | First action |
|---|---|---|---|
| Positive sample or visible scale at an outlet | The outlet was never surveyed | Walk the branch physically and compare it to the asset schedule | Add it to the scheme, then clean, disinfect and monitor |
| Cold water runs lukewarm at the tap | The risk of that pipe route was never assessed | Trace the run against the schematic — look for it crossing a warm void | Log a remedial, bring the temperature under control, reassess the route |
| A whole area sits outside the monitoring scheme | The scheme wasn’t built from the assessment’s findings | Cross-check sentinel outlets and frequencies against the report’s recommendations | Rebuild the scheme from the assessment, not the template |
| The same task is missed again and again | No named owner or escalation route in the assessment | Check who the report says owns the control and what happens on a failed result | Assign ownership, define the escalation, trigger a review |
| An inspector queries an asset you can’t explain | The assessment is out of date against the building as it stands | Date-check the report against the last refurbishment or occupancy change | Commission a review reflecting current use |
The discipline that makes this work is writing down the decision, not just the task: “this outlet is flushed weekly because use is intermittent; a missed flush escalates to the responsible person; repeated misses trigger a use-pattern review.” That single habit turns a logbook into evidence of a managed control rather than a list of ticks.
When to patch, and when to redo the assessment
One missed flush that you correct, record and explain is a fault. You close it and move on. But if the same weakness shows up in more than one place — two unsurveyed outlets, three remedials open past their date, a scheme that doesn’t follow the report anywhere — the assessment itself is the fault, and fixing outlets one at a time will never catch up.
That is the point to escalate. A review by the original author may be enough if the building has only nudged. If the report reads like a template, omits a schematic, or predates a significant change to the system, the safer call is a fresh assessment by a competent person — and the Legionella Control Association register is a reasonable place to start checking that competence [5]. Either way, remedials that need money go to whoever holds the budget, because the duty holder carries that accountability, not the contractor who wrote the report [3]. on why control plans fail covers that management side in more depth.
Where this guidance stops
The errors above are common, not universal, and spotting one on paper is not the same as judging your specific system. Which temperatures matter, which outlets count as sentinels, how often to monitor, and which remedial to fund first all flow from a site-specific assessment carried out by someone competent — not from a list like this one. If you read this and suspect your current assessment is wrong, the safe response is to have it reviewed properly, not to keep relying on it because it happens to be in date.
FAQ
How can I tell whether our Legionella risk assessment is any good?
Open it and look for four things: a system schematic, an asset list that matches a walk round the building, recommendations specific to your plant rather than generic phrasing, and named responsibilities. A report that could describe almost any building probably does, and that is the warning sign. It helps to know what a sound one contains first — see Key components of a Legionella risk assessment.
We have an in-date assessment, so are we compliant?
Not necessarily. A current date proves the document exists, not that it is correct or that its actions were carried out. Open remedials, an asset register that no longer matches the building, and a monitoring scheme that has drifted away from the report are all common on assessments that are technically in date.
If the assessment missed something and Legionella grew, who is responsible?
The duty holder — the business or person running the building. Commissioning a contractor to write the assessment does not transfer that accountability; you are expected to satisfy yourself the work was done competently [3].
Sources
[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Legionnaires’ disease - what you must do”. https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm [4] BSI, “BS 8580-1:2019 - Risk assessments for Legionella control. Code of practice”. https://knowledge.bsigroup.com/products/water-quality-risk-assessments-for-legionella-control-code-of-practice-1 [5] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/