A bath overflow leaking almost always traces back to a worn overflow washer or a loose banjo fitting, and it typically only leaks once filled close to the overflow opening, which is why it gets missed for so long. Filling the bath right up and checking underneath usually confirms it, before damage builds up unnoticed behind the panel.
Of all the fittings under a bath, the overflow is the one most easily forgotten, because most baths, most of the time, are never filled anywhere near it. A bath overflow leaking is a genuinely common fault that hides in plain sight: everything seems fine for weeks, then a deep bath produces a stain on the ceiling below, and nobody can work out why the leak seems to come and go.
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Why the overflow gets forgotten
Every bath has an overflow opening near the top, connected via internal pipework to the waste system, there specifically to prevent flooding if a tap is left running. In everyday use, most people bathe well below that level, so the overflow fitting sits dry and untested for long stretches. It only comes under any real pressure on the rare occasion the bath is filled deep, for a long soak, for a child's bath topped up generously, or when a tap is accidentally left on. That rarity is exactly why a failing overflow can go unnoticed far longer than a leak on a fitting used every single day.
How the overflow washer fails
The seal at the heart of most overflow leaks is a simple rubber washer, compressed between the overflow fitting and the underside of the bath to keep water in the pipe rather than let it weep out around the joint. Like every rubber seal in a plumbing system, it hardens and loses flexibility over years of use, especially in a bathroom subject to regular temperature swings between hot baths and a cold room. Once it hardens, it no longer compresses evenly against the bath surface, and water finds its way past under pressure, precisely the pressure a nearly-full bath provides and a shallow one does not.
Banjo fittings: the usual suspect
On most baths, the overflow washer is actually part of a banjo fitting, the connector beneath the bath that joins both the overflow and the main waste into a single pipe run. It is a compact, awkward-to-reach assembly, usually accessible only via a bath panel, and it is where the majority of "overflow leak" reports actually originate. A hardened washer, a fitting that was never fully tightened during installation, or one knocked slightly out of alignment during other bathroom work can all produce the same symptom: dry for weeks, then a leak the moment the bath fills deep.
The only-leaks-when-full pattern
This pattern is the single most useful diagnostic clue with a bath overflow. If water only ever appears after a particularly deep bath, and never during a normal, shallower one, the overflow fitting is almost always the culprit, since it is the only part of the system that behaves differently at that fill level. Compare that against a supply or waste leak, which typically shows up on every use regardless of depth.
| When the leak appears | Likely fitting |
|---|---|
| Only when the bath is filled close to the top | Overflow washer or banjo fitting |
| Every time the bath is used, any depth | Waste, trap, or supply connection |
| Only while taps are running, stops once still | Supply pipework or tap connections |
| Only after the plug is pulled and water drains | Waste outlet or trap seal |
Testing and confirming the source
- Fill deliberately close to the overflow, further than a normal bath, and hold that level for several minutes.
- Check underneath while it holds, via a removable panel if fitted, or watch the ceiling below if that is the only access.
- Repeat with a shallower fill on a separate occasion to confirm the leak genuinely does not appear below overflow height.
- If the pattern is not clear-cut, or water appears at more than one stage, treat it as a job for proper diagnosis rather than a straightforward washer swap.
Where the leak has already caused a stain or damp patch and the source is not obvious from a simple test, non-invasive plumbing leak detection can confirm exactly which fitting is responsible before anyone reaches for a spanner or a tile spacer.
Getting it fixed properly, once
Because the overflow only leaks under specific conditions, it is easy to replace a washer, test with a shallow bath, declare it fixed, and then find the same stain reappearing months later after the next deep soak. Confirming the fault under the actual condition that triggers it, a proper deep fill and hold, is what separates a lasting fix from a repeat call-out. If the damage has been significant enough to involve your insurer, a documented diagnosis also supports any trace and access claim, which price-comparison service Confused.com notes typically covers finding the leak and making good the search damage, commonly up to around £5,000, rather than the repair itself.
Bath overflows in Cornwall and Devon homes
In older Cornish and Devon bathrooms, particularly where a bath has been retrofitted into a period property with limited under-bath access, a leaking overflow can sit undiscovered against a solid floor or boxed-in void for a long time, since there is often no ceiling below to show a stain quickly. Holiday lets add another layer: a deep bath run by one set of guests can start a leak that nobody notices until the next changeover, or later. We see this fitting come up regularly in call-outs where a homeowner is confident "nothing's changed" and is surprised the culprit was the overflow all along, simply because their usual bathing habits never tested it.
Frequently asked questions
Why does my bath overflow leak only when the bath is full?
The overflow fitting and its seals only come into contact with water once the bath is filled close to the overflow opening. A worn washer or a loose banjo fitting can sit perfectly dry and undetected through weeks of shallower baths, then leak the moment someone fills it right up.
What is the overflow washer and why does it fail?
It is the rubber seal, usually part of the banjo fitting, that seals the overflow connection against the underside of the bath. Like most rubber plumbing seals, it hardens and loses flexibility with age, or was compressed unevenly at installation, and once it hardens it stops sealing reliably under water pressure.
Can a bath overflow leak cause damage if it only happens occasionally?
Yes. Even an occasional leak, if it happens every time the bath is filled close to the top, adds up over months, and because it is hidden beneath the bath it often causes floor or ceiling damage before it is ever noticed. Infrequent does not mean harmless when the water has nowhere obvious to show itself.
How do I test if my bath overflow is leaking?
Fill the bath close to the overflow opening, well beyond a normal bathing level, and hold it there for several minutes while checking underneath, ideally via a removable bath panel or the ceiling below. If water only appears at this stage and not during a normal fill or drain, the overflow fitting is the likely cause.
Is a leaking bath overflow an easy fix?
Often yes, once the source is confirmed. Replacing a worn overflow washer or reseating a banjo fitting is usually straightforward for a plumber with panel access. The harder part is often confirming the leak is genuinely the overflow and not a different fitting nearby that happens to sit close to it.
Why did my bath overflow leak come back after it was fixed?
A repeat leak usually means the original repair addressed a symptom rather than the confirmed cause, for example replacing a washer when the real issue was a hairline crack in the banjo fitting itself, or a leak from a nearby joint being mistaken for the overflow. Confirming the exact source before repairing again avoids repeating the same fix.
Bath only leaks when it's full? Let's confirm why
We can test and confirm whether a leak is coming from the overflow, the banjo fitting, or something else entirely, across Cornwall & Devon, before any repair work begins.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)