What is line stopping?
Line stopping is a method of temporarily isolating a section of a live, pressurised water main without shutting down the wider network. A mechanical stopper is inserted through a tapping sleeve bolted onto the main, blocking flow for the duration of the repair, valve installation, or modification work, then removed to restore full flow. Line stopping avoids the widespread customer disruption that would follow a full network shutdown, and it is one of the core techniques UK water contractors use on burst main repairs and planned network work.
Line stopping is also known as flow stopping. The two terms are used interchangeably across the UK water industry and both appear in contractor literature, trade body publications, and water company specifications. Some sources also use line plugging, particularly in older standards documents. This page uses line stopping throughout as the primary term.
Line stopping shares its insertion mechanism with hot tapping. Both techniques begin with a tapping sleeve bolted onto a live main and an under-pressure drilling operation that creates the access point without releasing the contents. What happens next is what distinguishes them: hot tapping inserts a new permanent branch connection, line stopping inserts a temporary mechanical stopper that gets removed when the work is finished. The two techniques often run back-to-back on the same job.
How line stopping works
Line stopping follows a defined sequence on every job. The detail varies with pipe size, pipe material, and network conditions, but the core stages are consistent across UK water network practice.
The access pit
Line stopping starts with an access pit dug down to the main. The pit is sized for the tapping sleeve plus the working space around it, shored and managed under HSE excavation guidance. Pits in the highway fall under statutory reinstatement rules for the backfill and surface works after the job is complete. Before the crew goes in, a services check confirms no other utilities conflict with the excavation, and the main is exposed and cleaned at the line stop location.
Fitting the tapping sleeve
A tapping sleeve is bolted or welded around the main at the line stop location, sized and specified for the pipe material. WRAS approval is required for the sleeve and any coatings that will come into contact with drinking water. A temporary isolation valve is fitted above the sleeve. An under-pressure drilling machine is then mounted on the valve, and a cutter drills through the sleeve and the wall of the main while the main stays at full pressure. The cutter is withdrawn, the temporary valve is closed, and the drilling machine is removed.
Inserting the line stop
The line stopping equipment is mounted on the temporary valve in place of the drilling machine. A mechanical stopper (the line stop head) is lowered through the valve and into the main, where it expands or pivots to seal against the pipe wall. Flow past the line stop point is now blocked. Water in the isolated section can be drained through downstream access points, and the repair, valve installation, or network modification proceeds on a dead section of pipe with no flow and no pressure.
Restoring the flow
When the work is complete, the line stop head is withdrawn through the temporary valve, which is then closed. A completion plug is fitted inside the tapping sleeve to seal the access hole permanently. The temporary valve and the line stopping equipment are removed. A blind flange is fitted over the completion plug. The main is returned to full flow and full pressure, and the access pit is reinstated under SROH 4th Edition if it sits in the highway.
When line stopping is used
Line stopping is used where a section of a live water main needs to come offline for work, and a full network shutdown would cause unacceptable disruption, cost, or risk to downstream customers. Most uses fall into three patterns.
Burst main repairs without network shutdown
The headline use of line stopping in the UK water industry is burst main repair. A large-diameter main failure in a residential, commercial, or hospital supply area would cause hours of customer disruption if the wider network were shut down to repair it. Line stopping isolates the burst section alone, keeps the surrounding network supplied, and meets the response and restoration targets that water companies build into their contractor agreements. McFadden line stops burst mains on the Affinity Water contract on a regular basis.
Valve and hydrant installation on live mains
Many older UK water networks do not have isolation valves at useful intervals. Where a new valve or a new hydrant needs to go into a live main, line stopping creates a temporary isolation long enough to cut the main, install the valve or the hydrant tee, and return the main to service. The line stop is removed, and the newly installed valve can then provide permanent isolation for future work on that section.
Scheme connections and network modifications
Line stopping supports larger network modification schemes where a section of pipework needs to come out and new pipework needs to go in. Examples include main diversions for road or rail schemes, new connection tees for large-scale developments, and network reconfiguration work driven by AMP capital investment cycles. Where the isolation needs to hold for hours or days rather than minutes, line stopping is often paired with bypass pipework to keep downstream customers supplied throughout.
Types of line stopping kit
Line stopping has evolved since the 1930s, and several kit variants are now in UK water industry use. Each suits a different combination of pipe size, pipe material, and site conditions.
Pivoting head line stops
The pivoting head line stop is the oldest and still most widely used method. The line stop head enters the main through a full-bore tapping sleeve (the access hole is the same size as the main itself), and pivots inside the pipe to seal against the pipe wall. Pivoting head stops handle large diameters and high pressures reliably. TDW STOPPLE equipment, the long-running industry standard, works on this principle.
Folding head line stops
The folding head line stop enters the main through a reduced-branch tapping sleeve. The access hole is smaller than the main itself, which reduces the footprint of the sleeve, the cost of the fitting, and the excavation size. The line stop head folds open inside the main to seal against the pipe wall. Folding head stops suit smaller and medium diameter mains where a full-bore sleeve is oversized.
Inflatable bag stops and specialist equipment
Inflatable bag stops use a reinforced fabric bag that enters the main uninflated, then inflates to seal against the pipe wall. Bag stops suit older cast iron mains, variable internal conditions, and situations where a rigid mechanical head cannot guarantee a seal. On polyethylene mains, squeeze-off equipment provides a simpler alternative by mechanically compressing the pipe wall from outside. Specialist kit such as WASK’s Hydrant Wizard range provides compact line stops for smaller diameter pipes at working pressures up to 8 bar.
Line stopping compared with other live-network methods
Line stopping is one of four practical methods for working on a live water main without shutting down the wider network. The right choice depends on pipe size, pipe material, flow condition, and whether the isolation needs to be temporary or permanent.
Live-network isolation methods at a glance
| Method | What it does | Best for | Permanent or temporary |
|---|---|---|---|
| Line stopping | Mechanical stopper blocks flow through a tapping sleeve | All pipe materials and sizes, burst main repairs, network modifications | Temporary |
| Hot tapping | Creates a new live connection through a tapping sleeve | Adding a branch, service tee, or access point to a live main | Permanent |
| Pipe freezing | Liquid nitrogen creates an ice plug inside the pipe | Smaller pipes with minimal or no flow, no hot tap available | Temporary |
| Valve insertion | Installs a permanent valve on a live main under pressure | Where a new isolation point is needed for all future work | Permanent |
UK standards and industry bodies
Line stopping on UK potable water mains sits inside a specific regulatory and industry framework. Four documents and bodies cover the ground that matters on site.
BS EN 805:2025 is the current British Standard for water supply systems outside buildings. Published by BSI on 30 April 2025, BS EN 805 sets the general requirements for the design, installation, testing, and commissioning of potable water mains, service pipes, service reservoirs, and associated components. The standard applies to the mains that line stopping is performed on, and to the completion of the line stop work once the flow is restored.
WRAS Approvals (the Water Regulations Approval Scheme) provides the UK certification for plumbing products and materials in contact with drinking water. Tapping sleeves, line stop fittings, completion plugs, and any coatings on those components that touch the water must hold WRAS approval or equivalent demonstrated compliance with the Water Supply (Water Fittings) Regulations 1999. WRAS has been UKAS-accredited since August 2021, which places the scheme inside the national formal accreditation framework.
British Water is the leading UK membership body for businesses in the water and wastewater supply chain. Founded in 1993 through the merger of BEWA and BWIG, British Water represents contractors, manufacturers, and consultants working with the UK water companies. McFadden operates inside this supply chain as a contractor to Affinity Water and Thames Water and the line stopping work on those contracts runs under the standards and best practice that British Water supports.
The access pit for every line stop is an excavation, and excavations carry their own statutory framework. HSE HSG150 covers health and safety in construction, including the shoring, edge protection, confined-space management, and daily inspection that a live main access pit requires. Legal duties sit under the Health and Safety at Work etc. Act 1974, the Construction (Design and Management) Regulations 2015, and the Confined Spaces Regulations 1997.
Line stopping in practice
Line stopping is a core capability for McFadden on the Affinity Water repair and maintenance contract and for water company, civils, and developer clients across Hertfordshire, London, Surrey, Essex, Cambridgeshire, and Berkshire. The technique sits in the under-pressure workstream alongside hot tapping, valve insertion, and pipe freezing, and is the method of choice when a section of a live water main needs to come offline for repair or modification without shutting down the wider network.
Kit and crews
McFadden crews carry line stopping kit on the Affinity Water standby contract, paired with under-pressure drilling equipment and tapping sleeves sized for the main being worked on. Line stops on small-to-medium mains are installed by a two-person under-pressure team. Larger jobs, 12 inch and above, are run by a dedicated crew working alongside the excavation team preparing the access pit. All crews are in-house: nothing on the under-pressure workstream is subcontracted out.
Typical delivery
A McFadden line stop follows a typical pattern, illustrated by the Palace Way Woking job in 2023. The scheme was a 15 inch burst main repair on the Affinity Water network, with the main running through swamp conditions. A double 15 inch line stop isolated the burst section, the repair was completed, and the line stops were removed to restore full flow. The job was delivered within Affinity Water’s Key Performance Indicator window for burst main response. Similar patterns apply across the Weybridge and Birchway schemes, where line stopping supported valve installation and construction-phase network works.