A suspected leak under the bath is best confirmed with a fill, drain and overflow test before any tiles come off. Water at each stage points to a different fitting: supply, overflow, or waste and trap. A removable panel often gives enough access alone, and detection can pinpoint the source first, so any tiling opened is targeted.
A stain creeping across a ceiling below the bathroom, or a patch of soft flooring near the bath panel, is one of the more stressful things to find in a home, mostly because the instinctive next step, ripping off tiles to see what is going on, is expensive and often unnecessary. A leak under the bath nearly always comes from one of a handful of fittings, and a bit of careful testing beforehand tells you far more than guessing ever will.
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Where baths actually leak from
Under most baths sits a compact cluster of fittings: the hot and cold supply connections, the waste outlet at the bottom of the bath, the overflow that links to the same waste via a banjo fitting, and often a section of pipe boxed in beneath the bath itself. Any one of these can fail independently, and each has a slightly different leak pattern.
- Supply connections: the pipework feeding the taps, which can develop a slow drip at a joint or fitting.
- Waste and trap: the outlet and U-bend beneath the plughole, prone to perished seals or a loose connection.
- Overflow and banjo fitting: the fitting joining the overflow to the waste, which only leaks once water reaches overflow height.
- Sealant around the bath edge: not a plumbing leak as such, but failed silicone lets shower or splash water track down behind the bath panel and can look identical to a pipework leak.
The fill, drain and overflow test
Before assuming the worst, a simple staged test narrows down which fitting is responsible, because each stage puts a different part of the system under pressure.
- Run the taps with the plug out. Let water run through and straight down the waste for a few minutes without filling the bath. This tests the supply connections and the waste/trap under running flow.
- Fill and hold. Put the plug in, fill to a normal bathing level, and leave it standing for 15 to 20 minutes. This isolates the waste and trap seal under static pressure, separate from the running-water test.
- Fill to overflow height. Carefully fill further, close to the overflow opening (without flooding the room), to test the overflow and banjo fitting specifically.
- Check for water after each stage, not just at the end, ideally with someone watching underneath or in the room below if access allows, so you know which stage produced the leak rather than just that a leak exists.
Banjo fittings and why they fail
The banjo fitting, named for its rough resemblance to the instrument, is the connector beneath most baths that joins the overflow pipe into the main waste pipe. It relies on rubber washers compressed against the underside of the bath to seal both connections. Over time those washers harden, shrink slightly, or simply were not seated correctly during the original installation, and the fitting starts to weep, usually only once the bath is filled close to overflow level, since that is when the fitting is actually under water and pressure. Because it sits low down, boxed in and rarely disturbed, a leaking banjo fitting can run slowly for months before it is ever noticed above floor level.
Access options before you touch a tile
| Access route | What it allows | When it is enough |
|---|---|---|
| Removable bath panel | Full view of waste, trap, banjo fitting and supply joints | Most modern baths with a panel rather than a tiled front |
| Access hatch in an adjoining room or ceiling below | View of pipe runs beneath the bath from below | Where the bathroom is on a suspended floor with an accessible void below |
| Small, targeted tile removal | Access to a specific confirmed leak point | Only once the source has been confirmed, not as a first step |
| Full tile or panel removal | Complete visual access to the whole area | Last resort, when the leak point cannot be confirmed any other way |
A removable panel is the access route worth trying first in most homes, since it often reveals the fitting responsible without any damage at all. Where the bath front is fully tiled with no panel, that is exactly the situation where confirming the leak source before opening anything up matters most.
Why detection-first beats exploratory tiling
Removing tiles on a guess, hoping to spot an obvious drip, is a common instinct but a costly one when the guess is wrong, and it often is, since water travels along joists and pipe runs before it ever shows itself. Non-invasive plumbing leak detection uses acoustic sensors, thermal imaging and tracer gas to locate the actual leaking joint or pipe run before anything is opened up. Where full trace and access work is needed, our dedicated trace and access leak detection service combines that diagnosis with the careful, minimal-damage access work itself, so tiling comes off only where it needs to, not everywhere it might.
Trace and access cover for the work
If a hidden leak under the bath has caused ceiling or floor damage, your buildings insurance may include trace and access cover. As price-comparison service Confused.com notes, this typically pays to find the leak and make good the damage caused specifically by the search for it, commonly up to around £5,000, though it does not usually extend to repairing the leaking pipe or fitting itself once found. A clear, documented diagnosis from the outset, rather than speculative tile removal, tends to sit much better with insurers reviewing a claim.
Bath leaks in Cornwall and Devon bathrooms
Older cottages and barn conversions across Cornwall and Devon often have baths retrofitted into spaces that were never designed around them, with pipework boxed in tightly and little room for a proper access panel. Holiday lets are a particular risk here, since a slow banjo fitting leak beneath a boxed-in bath can go unnoticed between guest changeovers for weeks. We see this pattern most in older stone-built properties, where limited access beneath the bath makes confirming the source before opening anything up especially worthwhile.
Frequently asked questions
How do I know if a leak is coming from under my bath?
Typical signs are a stain on the ceiling below the bathroom, a soft or discoloured patch on a nearby floor, or a musty smell close to the bath panel. Timing helps too: water appearing only when the bath is run, draining, or overflowing points to different fittings and narrows down the likely source.
Do I have to remove tiles to fix a leak under the bath?
Not always, and not as a first step. Many baths have a removable panel that gives useful access without touching a single tile. Professional leak detection can often confirm the exact source first, so any tile or panel removal that is needed is targeted rather than exploratory.
What is the fill, drain and overflow test?
It is a simple way to narrow down a bath leak by testing each function separately: run the taps without the plug in, fill and hold the water without running taps, then fill to the overflow level, checking for signs of water each time. Whichever stage produces a leak points to that specific fitting.
Will my insurance cover access to find a leak under the bath?
Many buildings insurance policies include trace and access cover, which price-comparison service Confused.com notes typically pays to find the leak and make good the damage caused by that search, commonly up to around £5,000, though it does not usually cover repairing the leaking fitting itself.
Why does my bath only leak when it's full?
A leak that only appears near a full bath usually points to the overflow fitting or a joint that sits higher up the bath body, since it is only submerged or under pressure once the water reaches that level. A leak present from the moment water enters is more likely to be the waste, trap or a supply connection.
Can leak detection find a leak under a bath without lifting the floor?
Yes, in most cases. Acoustic sensors, thermal imaging and tracer gas can pinpoint a leaking joint or pipe run beneath a bath or floor without opening it up first, which is the point of non-invasive detection: confirming exactly where to open up, rather than lifting boards or tiles to go looking.
What is a banjo fitting and why does it leak?
A banjo fitting connects the bath's overflow and main waste into a single pipe below the bath. Its seals are compressed rubber washers that harden and shrink with age, and because the fitting sits low and out of sight, a slow leak from a perished banjo seal can run for a long time before anyone notices.
Leak under the bath and don't want the tiles off unnecessarily?
We can confirm exactly what is leaking and where, using non-invasive detection, before any tiling or panels come off, across Cornwall & Devon.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)