Underground Water Mains Leak Detection in Kensington

Symptoms

A property in W10 was losing water through a mains leak that had gone undetected beneath the building's structure. The external water meter on the public footpath was registering approximately one litre every five minutes with no water in use, confirming an active loss on the incoming supply.

The internal isolation at the rear of the property in the kitchen extension was still functional, while a secondary isolation near the front door appeared already isolated and possibly disconnected from the mains.

The property presented several complicating factors from the outset. Engineered wooden flooring sat over a suspended floor throughout, with the supply pipework running through the voided space beneath.

This configuration meant any tracer gas introduced to the system would disperse across the void rather than concentrate directly above the fault, making pinpoint location more demanding. The combination boiler in the kitchen was holding pressure normally on the heating circuit, confirming the leak was confined to the incoming cold mains rather than the heating system.

For water mains leak detection jobs of this type — concealed pipework beneath a suspended floor with a concrete step at the threshold — the challenge is not just confirming a leak exists but establishing precisely where within that voided structure the failure has occurred. The meter reading gave a clear external rate of loss, but the exact fault location required methodical pressure testing and gas tracing to identify.

Methods Used

Pressure testing was carried out with a connection made under the kitchen sink to the incoming mains. With 2 bar applied, pressure dropped to zero within five minutes, confirming a significant active leak on the cold mains supply rather than a slow seep.

Tracer gas was then introduced through the same connection point and readings were monitored across the floor areas. Signals began emerging around the inside of the front door, which directed attention to that zone of the suspended floor.

Acoustic equipment was used alongside the gas tracing, and the leak noise was audible through the microphone in the front door area. A large floor panel behind the mat at the front door was accessed by removing the skirting board, which allowed the panel to be lifted.

Once inside the void, the acoustic signal became stronger but indicated the fault was situated slightly further under the concrete step immediately outside the front door. This combination of tracer gas concentration and acoustic confirmation is a standard and reliable approach in water mains leak detection, and on this job it narrowed the location to within a very small area before any excavation began.

Investigation Process

1. 1
   Site assessment and meter check

   The external water meter on the public footpath was confirmed to be running at approximately one litre every five minutes with no water in use. The locations of the external stop tap, secondary isolation near the front door, and internal isolation in the kitchen extension were all identified.

2. 2
   Pressure test on incoming mains

   A test connection was made under the kitchen sink and 2 bar of pressure was applied to the cold mains. Pressure fell to zero within five minutes, confirming a substantial active leak on the incoming supply.

3. 3
   Tracer gas introduction and surface survey

   Tracer gas was introduced through the same under-sink connection and readings were taken across the suspended floor area. Concentrations began appearing around the inside of the front door, directing attention to that part of the structure.

4. 4
   Acoustic microphone used to refine location

   The acoustic microphone picked up audible leak noise in the front door area, corroborating the gas readings and indicating the fault was close to or beneath the front threshold.

5. 5
   Access panel lifted at front door

   The large floor mat at the front door was removed to reveal a screwed access panel. Skirting board was taken off to allow the panel to be lifted, giving access to the suspended floor void beneath.

6. 6
   Leak confirmed beneath concrete step

   With the void exposed, the acoustic signal was stronger but indicated the leak was positioned slightly further under the concrete step just outside the front door. Water staining was also visible on the joists in the void area.

7. 7
   Excavation through tile and concrete

   The customer was informed that the concrete step would need to be broken to reach the fault and gave consent to proceed. The tile was broken and concrete excavated to expose the pipework beneath the step.

8. 8
   Leaking 32mm fitting identified and replaced

   A leaking 32mm fitting was found at the excavated location. The faulty fitting was removed and replaced using 32mm and 25mm fittings with 32mm pipework.

9. 9
   Post-repair pressure test and meter check

   Pressure was re-applied to the mains and held without loss. The external water meter was checked and confirmed completely stationary, indicating no further active leaks on the supply.

10. 10
    Site made safe and handover

    The access panel was replaced and the joists were screwed down to make the floor safe. The customer was advised that a tradesperson would need to return separately to reinstate the concrete step.

Result & Outcome

The leak was located beneath the concrete step immediately outside the front door of the property in W10. The fault was a failing 32mm fitting on the incoming cold mains supply, concealed within the structure of the step and not accessible without breaking through the tile and concrete above it. The combination of tracer gas concentration and acoustic listening had narrowed the location precisely enough that excavation was targeted and limited in scope.

The fitting was removed and replaced with new 32mm and 25mm fittings and 32mm pipework. Following the repair, a full pressure test was carried out with pressure holding, and the external water meter was checked and confirmed stationary — ending the loss of approximately one litre every five minutes that had been registering before work began. Water staining observed on the joists within the void confirmed that the leak had been active for some time prior to detection.

For a water mains leak detection case like this one, where the supply runs through a suspended void and beneath a solid concrete threshold, the result demonstrates how gas tracing and acoustic equipment together can avoid unnecessary widespread excavation. The concrete step reinstatement remains outstanding and will require a separate visit, but the mains supply has been repaired and is currently holding. The engineer also recommended that while the area remains accessible, all fittings in the zone should be upgraded to Philmac fittings transitioning from 35mm to 22mm copper, which are better suited to mains supply conditions.