How to treat an untraceable leak in a block of flats

Hi, I wouldn't recommend installing a permanent capture pipe to divert the water into a waste pipe. While it may seem like a practical solution, it only hides the problem and could allow further damage to the structure, ceilings, insulation, or electrical installations over time.

The best approach is to identify the source of the leak. In blocks of flats, water can travel a considerable distance before becoming visible, making the leak appear to come from a different location. A leak detection specialist can often trace intermittent leaks using moisture meters, thermal imaging, pressure testing, and acoustic equipment without excessive disruption.

As the leak is affecting a flat, it's also worth involving the freeholder or managing agent if you haven't already, as the source may be within a communal service or another property. Once the cause has been identified, the repair can be carried out properly rather than simply managing the symptoms.

If you can provide more details on where the leak is appearing (ceiling, wall, bathroom, kitchen etc.) and whether the flats are leasehold or managed by a housing association, we may be able to offer more specific advice.

Always happy to pop over and provide a quotation, All the best.

Diverting an untraceable leak into a waste pipe is generally not recommended because it hides a problem that will inevitably worsen, potentially leading to structural rot, mold, or a sudden burst pipe. In a block of flats, "untraceable" intermittent leaks are often caused by thermal expansion in old pipes or rainwater ingress that only occurs during specific wind directions.

At Plumbers Glasgow Ltd, we specialize in non-destructive leak detection using thermal imaging and acoustic sensors to find these hidden drips without tearing down your ceilings. Before considering a "capture" system, we recommend a pressure test of the communal risers and internal pipework to confirm if the leak is from the supply or a failing waste seal. We provide 24/7 emergency support across Glasgow and can help you coordinate with your factors or neighbors to locate the source and provide a permanent, insured repair.

Yes, it is possible to do this, but it is only a temporary solution. I recommend having a professional inspect the origin of the leak and repair the main pipe to prevent further damage.

Diverting it isn’t the right fix — it just hides the problem and can lead to rot, mould, and structural damage.

Intermittent leaks are often from:
• Pipe joints expanding/contracting
• Waste pipes (only leak when used)
• Heating pipes under pressure cycles

Answer to post:

You shouldn’t try to “capture and divert” the leak — that’s only masking it and can cause bigger issues long term.

The correct approach is to trace and fix the source, even if it means opening up sections. Start with:
• Pressure testing pipework
• Checking waste pipes from flats above
• Using a leak detection specialist (thermal camera/tracer gas)

In blocks of flats, this should also be raised with management/freeholder, as it may be a communal pipe.

It will be better to isolate the main supply first and use the moisture detector to find the actual area. It can be checked dedicated isolations of individual flat if available to reach exact point.

Notify freeholder / management company

Hi, thanks for your question. In a block of flats, a very slow and intermittent leak can be difficult to trace because water often travels along joists, pipework, or concrete before it finally shows through the ceiling. Intermittent leaks are often linked to waste pipes rather than pressurised supply pipes, because waste pipes only leak when in use. However, old pipework, heating pipe joints, seals, condensate lines, or even rainwater ingress can also be the cause.

I would not recommend installing a permanent "capturing" pipe in the ceiling void and diverting the water into a waste pipe as the solution. That may hide the symptom, but it leaves the underlying defect active, which can continue to damage ceilings, timber, fixings, insulation, and adjacent flats. It can also make future diagnosis much harder. Water regulations and industry guidance both emphasise that concealed fittings should remain accessible for maintenance and repair, rather than allowing hidden defects to persist.

The right approach is usually:

identify whether it is mains pressure, heating, waste, condensate, or external ingress;
isolate supplies in a controlled way, flat by flat or branch by branch if possible;
monitor the pattern of dripping;
then carry out targeted leak detection before opening up the structure too far.
In blocks, management or the freeholder should normally be involved early, because the source may be in another flat or in a communal service run.

A practical first test is to track exactly when the drip appears:

If it worsens after showers, sinks, washing machines, or toilets are used, suspect waste pipework
If it appears even when nobody is using water, suspect a pressurised pipe or heating line
If it is weather-related, consider roof, flashing, or rainwater ingress

A temporary catch tray may sometimes be used as an emergency damage-limitation measure while the source is being investigated, but it should only be temporary, visible, and accessible. It is not a proper repair.

If you are in Hull or the surrounding area, HJZ Plumbing can help assess intermittent leaks, isolate likely pipework, and advise on the safest next step before unnecessary damage is done. For reliable leak investigation in Hull, contact HJZ Plumbing.

Find the source: get a leak detection plumber (thermal camera / tracer gas / acoustic) and check waste/soil stacks as well as water pipes.
• Isolate/test: turn off the flat’s stop tap and see if it stops; also do timed tests (no water use vs. water use).
• In a block: report to freeholder/managing agent and the flat above—many leaks are in communal/stack pipework and need access.

Temporary measure only: catch in a tray and protect electrics until it’s traced.

I am afraid you or your plumber will have to find the leak and make access to repair as per current standards
Kevin

1. First: things that are still worth trying (even for very slow, intermittent leaks)
Before committing to diversion, there are a few low-disruption diagnostic steps that often work for old buildings:
A. Pressure and isolation testing
Temporarily isolate:
Cold water
Hot water
Heating circuit (if applicable)
Leave each off for 24–48 hours and observe.
If dripping stops when a circuit is isolated ? you’ve found the system involved.
A plumber can do a pressure drop test that detects leaks too small to see.
B. Dye testing (very effective for intermittent drips)
Add food dye or fluorescent tracer to suspected systems one at a time.
Check the drip point over several days.
This can positively identify which pipe run is leaking.
C. Thermal / acoustic leak detection
Specialist leak detection companies use:
Thermal imaging
Acoustic microphones
Tracer gas
These often locate leaks without opening ceilings and are commonly accepted by insurers and management companies.
2. Responsibility & legal considerations (very important in flats)
Before altering anything:
Check:
Who owns the pipe?
Communal supply pipes are often the freeholder / management company’s responsibility.
Altering them without permission can cause serious issues.
Building insurance:
Many policies require repair, not diversion.
A “managed leak” could invalidate future claims if damage worsens.
If the pipe is communal, document everything in writing and push the managing agent to act.
3. About installing a “capturing” or diversion system
Is it technically possible?
Yes. People have done variations of this:
A small drip tray or funnel beneath the leak
Connected to:
A condensate line
A waste pipe with a proper trap

A “catch pipe” or drain diversion should only ever be used as a very short-term emergency measure, not as a permanent fix.

Best option would be to bring a professional leak detector to source the leak as this will only become worse over time, diverting it doesn’t solve the problem

Presume it’s above could there be a bathroom above? If so could be when someone is using the bath/shower