---
title: "Wet and dry risers and firefighting mains: managing Legionella in fire water systems"
source_url: https://legionella.io/articles/wet-and-dry-risers-legionella/
canonical_url: https://legionella.io/articles/wet-and-dry-risers-legionella/
pillar: "Legionella Risk Assessment"
summary: "Dry risers sit empty; wet risers hold static water year-round. See how each one's Legionella risk differs and who owns the water-hygiene duty in your block."
primary_keyword: "dry riser Legionella"
date_published: 2026-06-17
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Wet and dry risers and firefighting mains: managing Legionella in fire water systems

A dry riser sits empty for most of its life, so its routine Legionella risk is low. A wet riser is a permanently charged firefighting main holding water that never moves — a genuine stagnation point. Both belong in a high-rise water risk assessment; what differs is how much control each one needs and who actually carries it out.

That last part is where building-safety managers get stuck. The fire contractor tests the riser for fire performance. The water-hygiene contractor never mentions it. So the charged main in the stairwell — full of warm, static water, with a landing valve on every floor — sits in the gap between two contracts, owned by neither. Closing that gap is a water-hygiene duty, and it starts with knowing which type of riser you actually have.

## What a riser is, and why the two types behave differently

A firefighting riser is the vertical main that lets the fire service get water to upper floors without dragging hose up the stairs. There are two kinds, and they hold water in opposite ways.

A **dry riser** is normally empty. It runs from a breeching inlet at street level — where a fire appliance connects and pumps water in — up to landing valves on each floor. Water only enters during a fire or during a test. Broadly, dry risers serve buildings up to around 50 metres tall, where the fire service can pump from the ground.

A **wet riser** is permanently charged. It is fed from a storage tank and pump set, often through a break tank from the mains, and holds water under pressure at every landing valve all year. Wet risers are required in taller buildings, broadly above about 50 metres, where street-level pumping cannot reach the top.

The fire performance of both is governed by fire-safety law — the Regulatory Reform (Fire Safety) Order 2005 — and the design and testing code for these systems, BS 9990, not by Legionella guidance. The water-hygiene question runs alongside that regime, not instead of it. And that one design difference, empty by default versus charged by default, drives everything in the table below.

## Dry riser versus wet riser: the Legionella comparison

Use this to place each riser in your building before you decide what control it needs. The two columns diverge most on the rows that matter to a risk assessor: how much water stands in the system, and when a person could be exposed to a spray from it.

| Decision axis | Dry riser | Wet riser |
|---|---|---|
| Normal water state | Empty; charged only when pumped in | Permanently charged and pressurised |
| Standing water | Residual only, in low points after wet testing | Full static column, all year round |
| Stagnation profile | Intermittent — matters mainly after a test | Continuous — a warm, undisturbed main |
| Routine aerosol risk | Low; there is nothing to aerosolise when empty | Higher; water is present whenever a valve opens |
| When exposure can occur | During wet or flow testing, and in actual use | During testing, use, and any landing-valve operation or leak |
| Link to the wholesome supply | Usually none in the normal empty state | Check the tank or break-tank feed and its backflow protection |
| Position in the Legionella RA | Scope in; control centres on draining down after a test | Scope in; treat as a charged main needing defined control |

The mechanism behind the table is the one HSG274 keeps returning to: water that stays warm and undisturbed long enough lets the bacteria establish, and dead legs and infrequently used parts of a system are repeatedly named among the features that create that risk [1][2]. A wet riser is close to a textbook example. A dry riser, while empty, is not.

## Which one gets which control

For a **dry riser**, the live issue is not the empty pipe — it is what is left behind after a wet test. Flow and pressure testing puts water into the system, and if it is not fully drained afterwards, residual water sits in low points and the inlet cabinet until the next test. The control is simple to state: confirm the test procedure includes draining down, scope the riser into the assessment, and record the low-risk reasoning. No invented interval here — the fire test schedule and your risk assessment set the timing.

For a **wet riser**, treat the charged main the way you would treat the permanently stagnant water in [Fire sprinkler systems and hose reels: managing permanently stagnant water](https://legionella.io/articles/fire-sprinkler-systems-and-hose-reels-managing-permanently-stagnant-water/). The water sits warm in a heated stairwell core, undisturbed for months — the exact condition described in [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/). The controls that matter are knowing the water source and whether it can draw from or return to the wholesome supply, checking backflow protection on any mains feed, and deciding — jointly with the fire side — whether the static water is monitored, periodically turned over, or recorded as an accepted, isolated risk. Aerosol exposure is the route that counts [3], and it is most likely when a landing valve is opened, so note who operates the system and where the discharge goes.

In a tall residential block both risers sit alongside the other high-rise water problems — long riser runs, stored cold water that warms on upper floors, flats left empty for weeks — set out in [Legionella challenges in high-rise buildings](https://legionella.io/articles/legionella-challenges-in-high-rise-buildings/). The firefighting main is one more vertical asset to bring into that picture, not a separate world.

## Resolving the fire-contractor-versus-water-contractor question

The honest answer is that the riser's fire function is the fire contractor's, and its water-hygiene status is yours to assign — usually to the Legionella risk assessor, and to the water-hygiene contractor where there is a cross-connection or an actual control task. The two regimes run in parallel; neither automatically covers the other.

What goes wrong is assuming the fire test covers Legionella. A periodic wet test proves the riser will deliver water to fight a fire. It says nothing about the bacteriological state of the water standing in a wet riser, or the drain-down of a dry one. Equally, a water-hygiene contractor engaged to survey the hot and cold water systems may never have been asked to look at the firefighting main at all. The result is a charged main that two competent contractors each assume the other has handled.

The fix is a scope decision in writing. The Legionella risk assessment should name each riser, state its type, and record who owns the water-hygiene aspect — even where that record reads "dry riser, drained after test by the fire contractor, no further water-hygiene control required." It is the assessment, not the fire schedule, that defines which systems and components are surveyed and judged [4]. In a higher-risk building that reasoning also belongs in the building's live, retrievable records, in the spirit set out in [The Building Safety Act, the golden thread and water-safety records in higher-risk buildings](https://legionella.io/articles/building-safety-act-water-safety-records/). An unexplained silence is the gap an investigation finds; a recorded, reasoned decision is defensible.

## A caveat to keep this in proportion

This is general guidance to help you ask better questions on site, not a design standard or legal advice. The riser's fire-safety design, testing and maintenance sit under fire-safety legislation and BS 9990, and nothing here should override them or be used to alter a firefighting system without the agreement of whoever owns the fire risk assessment. The Legionella side is risk-assessment-led: the actual scope, controls and any frequencies come from a competent, site-specific assessment under L8 and HSG274 for your building, not from this comparison [1][3]. Keep it proportionate, too — a dry riser is empty most of the time and is a lower routine concern than a permanently charged wet main.

## What to do this week

Find out, for one building, which type of riser you have. It is marked on the inlet cabinet and named in the fire strategy. Then check whether that riser appears anywhere in your current Legionella risk assessment. If it does not, you have found the gap. Add it with its type, its water source, and a named owner for the water-hygiene decision — so the charged main in your stairwell stops being nobody's job.

## Frequently asked questions

### Do dry risers need Legionella treatment?
A dry riser is empty in normal use, so routine treatment is rarely the point. The water-hygiene issue is the residual water left after wet testing: confirm the test procedure drains the system down, scope the riser into your risk assessment, and record the low-risk reasoning. Your assessment sets any action, not a fixed rule [1].

### Is a wet riser a Legionella risk?
A wet riser holds a permanently charged column of static, often warm, water — the kind of stagnation the bacteria favour [2]. That does not make it an automatic source, but it should be scoped in as a charged firefighting main, with attention to its water source, backflow protection on any mains feed, and the aerosol released when a landing valve is operated.

### Who is responsible for Legionella in a firefighting riser, the fire or water contractor?
Both, at different points. The fire contractor maintains the riser's firefighting function under fire-safety law and BS 9990; the water-hygiene status is the duty holder's to assign, typically to the Legionella risk assessor and, where a control task exists, the water-hygiene contractor. The risk assessment should name who owns the water-hygiene decision in writing [4].

## Sources

[1] HSE, "Legionnaires' disease: Technical guidance (HSG274)". https://www.hse.gov.uk/pubns/books/hsg274.htm
[2] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[3] 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
[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