A supply leak comes from pressurised clean water and usually shows up on your water meter or as falling pressure. A drain leak comes from gravity-fed waste water and does not touch your meter, tending to smell foul rather than musty. The two need different investigations, so working out which one you have first saves time and money.
Water damage looks much the same whatever caused it: a damp patch, a smell, a patch of ground that never quite dries out. But a drain leak or supply leak are genuinely different problems under the surface, with different pipework, different pressures and different specialists needed to fix them. Get the diagnosis wrong and you can pay for the wrong investigation entirely. This guide walks through the practical differences and how each one is confirmed. For supply-side leaks, see our guide to underground water leak detection.
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The core difference: pressurised vs gravity
The single most useful thing to understand is that a supply pipe and a drain behave completely differently under normal conditions. Your supply pipe carries clean water under mains pressure, day and night, whether or not you are using it. A crack or joint failure in a supply pipe leaks constantly, because the pressure is always there pushing water out.
Your drain, by contrast, only carries water when something is actively being flushed, drained or discharged, and it relies on gravity rather than pressure. A drain leak usually only shows itself, or gets worse, when water is running through that specific pipe, such as after a bath empties or a washing machine discharges. This single distinction, constant pressure versus intermittent gravity flow, explains most of the other differences below.
Signs that point to a supply leak
- Your water meter keeps ticking over when every tap and appliance in the house is switched off.
- Mains water pressure has dropped noticeably at taps and showers.
- Your water bill has risen without any change in household usage.
- A damp patch is near a known supply run, a stopcock, or where a pipe enters the property.
- The water involved looks and smells clean, not foul.
These are the classic fingerprints of a leak in the pressurised system, the pipework covered by our guide to plumbing leak detection.
Signs that point to a drain leak
- A foul, sewage-like smell, especially near a manhole, gully or where a drain run is known to pass.
- Dampness or a wet patch that appears mainly after water has been used, such as after a shower, a toilet flush or the washing machine emptying, then seems to settle.
- Your water meter is completely unaffected and mains pressure is normal.
- Subsidence-like settlement, a sunken patch of lawn or path, or vegetation growing unusually well over one specific line in the garden.
- Gurgling, slow-draining fixtures elsewhere in the house alongside the damp patch, hinting at a blockage or collapse in the same run.
Smell and timing: the fastest clues
If you only have two clues to go on, smell and timing get you a long way. Foul, sewage-like odours point strongly towards a drain, since clean supply water rarely smells of anything beyond ordinary mustiness once it has sat for a while. Timing matters just as much: a supply leak is there all the time, a drain leak often needs water actively flowing through that section of pipe to show itself, so it can seem to come and go depending on what the household is doing.
Supply leak vs drain leak at a glance
| Feature | Supply leak | Drain leak |
|---|---|---|
| Water type | Clean, pressurised mains or heating water | Waste water, gravity fed |
| Water meter | Often keeps moving with everything off | Unaffected |
| Smell | Little to none, or general mustiness | Foul, sewage-like |
| Timing | Constant, present whether used or not | Often tied to active use of that fixture |
| Typical detection method | Acoustic, thermal imaging, tracer gas | CCTV drain survey |
| Usual specialist | Leak detection engineer, then a plumber | Drainage specialist |
How each one gets found
Because the two systems behave so differently, they are investigated with different tools:
- Supply leak detection: acoustic sensors and correlators listen for the distinctive sound a pressurised leak makes, thermal imaging highlights surface temperature changes where water is escaping, and tracer gas, a safe mix of 5% hydrogen and 95% nitrogen, pinpoints the exact spot once the area is narrowed down.
- CCTV drain survey: a waterproof camera is fed through the drain run itself, recording the inside of the pipe in real time to show cracks, root ingress, collapses or misaligned joints directly, rather than inferring the location from the outside.
Trying to run a CCTV camera through a clean supply pipe, or trying to acoustically trace a gravity drain that only leaks intermittently, generally will not give you a useful answer. Matching the method to the system is what makes the investigation quick rather than a process of elimination by trial and error.
Who fixes what
Once the leak is located, the two paths usually split again. A confirmed supply leak is generally handed to a plumber, working from a report that shows exactly where to open up. A confirmed drain leak typically goes to a drainage specialist, who can use the CCTV survey footage to plan a repair, a relining, or an excavation, depending on what the camera has shown. Where supply and drainage pipework run close together, which is common under floors and in gardens, it is worth ruling out both systems before repair work starts, so you are not paying twice because the wrong pipe was fixed first.
Cornwall and Devon: older combined systems
Across Cornwall and Devon, older properties, granite cottages and converted farm buildings in particular, sometimes have supply and drainage pipework routed unusually close together, laid decades apart with little record of exactly where. We see this most weeks on rural properties and older terraces, where a damp patch near an outside wall could plausibly be either system, and going straight to the wrong specialist wastes a visit. Working through the pressure, smell and timing clues first, before booking either a CCTV survey or a leak detection sweep, is the quickest way to the right answer on these older layouts.
Frequently asked questions
How do I know if it’s a drain leak or a supply leak?
A supply leak usually shows through a moving water meter, a pressure drop, or clean water appearing where it shouldn’t. A drain leak tends to smell of foul waste water, appears near a drain run rather than a supply pipe, and does not affect your water meter or mains pressure at all.
Does a drain leak affect my water bill?
No. A drain carries waste water away under gravity after you have already used and paid for it, so a drain leak does not register on your water meter. If your bill has risen without an obvious cause, that points towards a supply-side leak instead.
What smell suggests a drain leak rather than a supply leak?
A foul, sewage-like smell usually points to a drain leak, since waste water and, in some cases, sewer gas is escaping. A supply leak from clean mains or heating water rarely smells of anything beyond a general mustiness as damp material sits for a while.
Who fixes a supply leak versus a drain leak?
A supply leak is usually repaired by a plumber once it has been located, often working from a leak detection report. A drain leak is typically addressed by a drainage specialist, frequently guided by a CCTV drain survey that shows the exact defect inside the pipe.
Can a drain leak and a supply leak happen in the same spot?
It happens more often than you’d think, particularly where supply and drainage pipework run close together underground or under a floor. This is why it helps to rule out both systems methodically rather than assuming a single cause once dampness appears.
Do I need a CCTV drain survey or leak detection?
If the water meter is moving, pressure has dropped, or clean water is involved, leak detection using acoustic, thermal and tracer gas methods is the right starting point. If the smell and evidence point to waste water and gravity pipework, a CCTV drain survey is usually the more direct route.
Is a leaking drain covered by insurance?
Escape of water cover under a standard buildings insurance policy can apply to drain leaks as well as supply leaks, and the Association of British Insurers notes that escape of water costs insurers about £1.8 million in payouts every day. Check your specific policy wording, since drainage cover sometimes sits under a separate section.
Not sure if it’s your drain or your supply pipe? Let’s find out
Across Cornwall & Devon we investigate both supply and drainage systems, matching the method, leak detection or CCTV survey, to the evidence, so you only pay for the right specialist once.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)