Overflow pipe dripping outside

The short answer

An overflow pipe is a safety device, not a drain. Water coming out of it means the cistern is filling past its normal level, and the only thing that controls that level is the fill valve.

Both faults can exist on the same toilet at once, and a single dye test finds both if you remember to look outside as well as in the pan.

How this works in practice

The cistern fills until a float tells the fill valve to shut off. If the valve does not shut fully, the level keeps rising, and the overflow pipe carries the excess outside instead of letting it spill onto the floor.

That is why the fault is so easy to miss from inside. Everything downstairs looks normal, the toilet flushes properly, there is no damp and no noise beyond a faint hiss, and the only sign is water leaving a pipe on an outside wall that nobody has reason to look at.

• Look at the outside wall behind or beside the bathroom, usually a short plastic pipe emerging just above ground floor or at the level of the cistern.
• Look for the mark as well as the water. Staining, moss or a wet patch below the pipe shows it has been running even if it is dry when you check.
• Check at different times. An intermittent fill valve passes when mains pressure is highest, which is commonly overnight.
• Confirm which cistern it is with dye if the property has more than one toilet.

The dye test finds this fault at the pipe rather than in the pan, so it is the same half hour of work either way. the dye test and what a negative result rules out covers both results.

What changes the answer

Three things, and the first is why the band is wide rather than a number.

• How far the valve is passing. A fill valve that is nearly closing dribbles; one that has failed open runs at close to the full mains flow. Those differ by more than an order of magnitude, and nothing visible from outside distinguishes them.
• Mains pressure, which decides how much passes through a partly open valve and which varies through the day. That is the mechanism behind an intermittent overflow.
• Whether the cistern has an internal overflow instead. Many modern cisterns discharge the excess into the pan rather than through an external pipe, which means the same fault produces no external sign at all and shows only as a passing flush valve would.

That third case is worth knowing about, because it means a property with no external overflow pipes is not a property that cannot have this fault. It is a property where this fault looks like the other one.

Assumptions and sources

The band boundary this page sits above has no published source, in common with the other three on this site. It is a practical classification, it renders as an editable input, and the only measured figure anywhere near it is the residential average across all leaking toilets, which sits at a lower boundary.

Everything else here is mechanism rather than measurement. A cistern fills until a valve shuts off, an overflow carries the excess outside, and mains pressure varies through the day. None of those is a statistic and none is cited as one.

What to do next

Three things, and the second is the one that gets skipped.

• Identify the cistern with dye, so you are certain which toilet the pipe belongs to before anything is opened up.
• Check the other toilets too. A fill valve wearing out is a function of age and water quality, so toilets fitted at the same time in the same house tend to fail at similar times.
• Confirm the rate at the meter. An overflow is the largest toilet fault, so it is the one most likely to be large enough for the meter to register clearly, which turns the band into a measured number.

That last step is worth doing before a repair rather than after. confirm the rate of loss at the meter gives a figure the band cannot, and a before and after pair of readings is what proves the repair worked.

Common questions