Should You Worry About Your Bath Overflowing?

bath overflowing

“Me, worry about my bath overflowing? Its got an overflow pipe.”

“Oh! Has it?”

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.) On my bath, water would start to go down the overflow pipe when it is within 8cm of the top of the bath. The top of my bath is 8cm above the bottom of the overflow. So if it is full to the point where it will overflow the rim of the bath the head of water pushing out through the overflow will be 0.08m whereas the head of water from the water level in my header tank in the loft is 2m. Alternative forms of hot water provision can be at mains pressure which usually has a lot more than 2m head. My point is that the tap’s pipe diameter and overflow pipe diameter are similar but the input pressure is a lot higher than the output pressure. Consequently baths can fill faster than they can overflow. So eventually a system with no faults may eventually overflow the rim of the bath if the taps are full on. So DON’T LEAVE BATH TAPS RUNNING WHEN YOU LEAVE THE BATHROOM. You could forget to turn them off.

Leaking Overflows

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

bath overflowing

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.

bath overflowing

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 cup collecting water from the overflow. The cup 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 cup and a sealing washer and nut are applied. Tightening the nut presses the cup against the underside of the bath to keep the water within the cup. The bath water goes into this cup when the bath is emptied so a good seal here is a must. Over tightening the waste nut can deform the cup 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 cup and I used a sealant.

bath overflowing

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 cup 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.

A Disconnected Overflow Pipe

bath overflowing
  1. Overflow Pipe Disconnected At The Top. Above is a bath where the overflow pipe has come off the top cup (they just push on) which fits over 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 bottom cup associated with the bath plug hole. 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 cup surrounding the waste and out through the spigot onto which the overflow pipe should be attached.

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 can avoid flooding the floor below, but not 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 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.

Author: Helpful Colin

I have a background in telecommunications and a fascination with all things scientific and technical - from physics to electronics, and computing to DIY.

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