bath overflowing

Should You Worry About Your Bath Overflowing?


Bragging bath owner, “Me, worry about my bath overflowing? It’s got an overflow pipe. I shan’t worry.”

Plumber, “Oh! Has it? Well I would still worry.”

Why you DO NEED to worry about your bath overflowing?

I know someone who has had a bad experience with a bath overflowing, but it wasn’t their bath overflowing. It was in a flat two floors above. Interestingly the flat in between on the floor immediately below the bath didn’t get wet. How can that happen?

Well lets start with the overflowing bath. The person, whose bath it was, admitted it had overflowed but not very much. I suspect this means that the water did actually run over the rim of the bath. That means it rose above the bath overflow outlet (assuming there was one.)

Why An Overflow Pipe Won’t Stop A Bath Overflowing

In my bath water starts to go down the overflow when it’s within 8cm of the top of the bath. So if it fills to the point where it will flow over the rim the head of water pushing out through the overflow pipe will be 0.08m. At the same time the head of water driving my hot tap which comes from my hot water header tank in the loft is 2m.

Alternative forms of hot water provision, e.g. from a combi boiler, may be at mains pressure which usually has significantly more than a 2m head.

In both cases the cold water is at mains pressure too.

With a local hot water header tank the pipe feeding the hot tap is about the same diameter as the overflow pipe. When mains pressure supplies the hot water tap the feed pipe might be smaller, but the extra pressure from the mains more than makes up for it.

My Point Is This:

The volume of water entering from the hot tap alone (which may have the least pressure) could exceed that leaving through the overflow pipe. The total supply would be even greater should the cold tap be on too.

Consequently a bath with these conditions (where the hot tap is full on and the cold on too) can fill faster than excess water can flow away down the overflow pipe. Eventually it will fill to the rim and overflow onto the floor.

The Lesson Here Is:

DON’T LEAVE BATH TAPS RUNNING WHEN YOU LEAVE THE BATHROOM. You may not come back as soon as you expect.

Problems With Bath Overflow Pipes

Leaking Overflows

Even if the water is just allowed to run down the overflow there can be problems. See the diagrams below:

Water dripping off a ‘P’ trap having run down from a leaking overflow.
Water dripping off a ‘P’ trap having run down from a leaking overflow.

Above is a bath with a badly fitted overflow. Normally this isn’t a problem because the level of water in the bath never reaches the overflow, but when it does it dribbles out of the gap because the two parts of the overflow are not tightened up sufficiently…..

Cross Section of A Bath Overflow Outlet
Cross Section of A Bath Overflow Outlet

NOTE: The silver cover you see on the inside of the bath needs to be sealed to the bath with a synthetic rubber washer and the screw thread must be made watertight. The water pressure at this point is always low so it isn’t difficult to get it sealed by tightening it.

A different method is used to seal the bath waste to the Coupling Box collecting water from the overflow. The Coupling Box is required to fit flat against the bottom of the bath to make a seal. The bath waste passes through the bath from the inside like a giant bolt. It then passes through a big hole in the overflow receiving Coupling Box and a sealing washer and nut are applied. Tightening the nut presses the Coupling Box against the underside of the bath to keep the water within it. The bath water goes into this Coupling Box when the bath is emptied so a good seal here is a must. Over tightening the waste nut can deform the Coupling Box increasing the likelihood of a leak. I once had to apply polyester resin to a bath waste hole to strengthen it. I then had to sand it very flat to seal with the Coupling Box, and I used a sealant.

Cross Section of A Bath Waste With Overflow Connection
Cross Section of A Bath Waste With Overflow Connection

The bath waste must be sealed to the bottom of the bath on the inside. A thin rubber washer or silicon sealant can be used for this. There’s no need for excess sealant as a small leak here will not make any mess. Any water leaking around the waste will arrive in the Coupling Box underneath just like the rest of the bath water can when it is being emptied. However such a leak will let the bath empty slowly while in use with the bath plug in situ.

Disconnected Overflow Pipes

Water leaking from an overflow when the overflow pipe is disconnected at the top.
Water leaking from an overflow when the overflow pipe is disconnected at the top.
  1. Overflow Pipe Disconnected At The Top. Above is a bath where the overflow pipe has come off the Overflow Outlet (it just pushes onto the Spigot). The Overflow Outlet is fitted into the overflow hole in the bath. Again there is no problem most of the time except when an overflow occurs.
  2. Overflow Pipe Disconnected At The Bottom. Worse still the overflow pipe can come off the Coupling Box associated with the bath plug hole (the bath waste). If this occurs water can gush out onto the floor under the bath every time the bath is emptied. While draining, the bath water passes out through the holes in the waste which are there to allow overflow water to come into the waste pipe. The water flows into the Coupling Box surrounding the waste and out through the spigot and onto the floor because the overflow pipe has come off it.

Consequences Of A Bath Overflowing

In both cases depicted above water will pour onto the floor under the bath without the user knowing. If it is a few dribbles each time and it lands on floor boards it will probably dry up without ill effect, but it might go mouldy and smell. If there is sufficient it is bound to go through any holes it finds in the floor boards where pipes/cables pass through and land on the top surface of the ceiling plaster-board of the room below. It often finds holes in that plaster-board above light fittings and pours out there where those investigating the problem risk the DANGER of ELECTRIC SHOCK.

A bath overflowing may not flood the floor below, but may flood the floor below that.

I suspect in the case to which I initially referred, the water ran along the ceiling to the wall and ran down the wall of the room on the floor below between the brickwork and dry lining boards. Dry lining boards would likely be fixed to the wall with a small gap between the board and bricks. The ceiling of the room two floors below probably had its ceiling going all the way to the brick wall so that it could catch the water and direct it along that ceiling to a light fitting where it came out. See the diagram below:

bath overflowing

In the case I know of, the person on the first floor reported little or no water penetration so maybe the hole in their ceiling was away from the water flow between other floor joists.

This particular water leak drenched a mains powered smoke detector. It failed but caused a permanent alarm at another detector it was linked to. The only way to stop that alarm was to pull the fuse for the lighting circuit. Well it made the lights safe during the wet period but the flat was left without a working smoke alarm.

My Recommendations To The I.E.T. (Institution of Engineering and Technology)

  1. I recommend that the building/wiring regulations be amended requiring smoke alarms to be water tight so that they aren’t affected by ingress of leaking water from above and can maintain their ability to keep people safe under adverse conditions. They could have an upper surface that shed water like an umbrella, with screw holes on the outside, but had vents underneath to take in the smoke and give out the sound.
  2. Permanently wired alarms should be connected to a fused spur of the socket supply so that they might still be powered when the lights alone are turned off. Leaking water very often affects lighting which then requires the lighting supply to be turned off while drying or repairing the lights. Socket supplies may be unaffected.



4 responses to “Should You Worry About Your Bath Overflowing?”

  1. Becci avatar

    I’m glad my nightmarish experience has been blogged about… Hopefully others can learn from this.

  2. plumbing installation red avatar

    Thanks for this blog. I learned some tips about this blog. I had experienced these problems last week thanks to my friend who help me with my problem. I hope you post more blog helps us. So that we can learn more. Thanks for the ideas you shared with us. It is a big help not just for me but for all the readers who don’t have any ideas about this kind of tips.

    1. Helpful Colin avatar


      Thanks for commenting. As a consequence I have read the post again and improved some drawings and edited the post.

      Regards, Colin.

  3. James avatar

    Interesting to read your blog Colin.
    Have you come across the Nova-Flo device that automatically shuts the water off when water goes through the overflow outlet?
    You described the difference in head between the overflow and rim of the bath vs the head of the tank affects the drainage rate considerably. The dimensions for overflows were defined when we used to have more basic plumbing and a low flow rate through the taps. These days with modern flow rates and shower pumps etc, the overflow just can’t cope. To simply make an enormous overflow would be considered hugely wasteful (water is purified, heated and pumped which all require energy).

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