Toilet Overflow Pipe Leaking Outside

Is the dripping pipe really from the toilet?

The small plastic pipes poking through an outside wall are not all toilet overflows. Before touching the toilet, work out which pipe is dripping and trace it back inside. A house can have four or five of them, and each one points to a different fault.

A toilet overflow usually leaves the wall at roughly the height of the cistern, close to where the toilet sits on the other side. It is a thin pipe, typically 19mm or 22mm across. A pipe coming out higher up, near the eaves, is more likely from the cold water storage tank in the loft, and that is a loft tank ballcock problem, not a toilet one. A pipe from the small heating header tank points to a header tank overflowing. A copper pipe from near the boiler is the pressure relief discharge, which is covered in boiler overflow pipe dripping outside.

The quickest proof is to go into the bathroom, take the lid off the cistern and look. If water is trickling into the overflow tube or the water level sits right at its rim, you have found the source. You can also turn the isolation valve on the toilet feed off for ten minutes. If the dripping outside stops, it is the toilet.

Why a toilet has an overflow pipe at all

Under the Water Supply (Water Fittings) Regulations 1999, every ordinary flushing cistern must have a warning pipe or a device that is no less effective. The Regulations define a warning pipe as an overflow whose outlet sits where the discharge can readily be seen. That is the point of the pipe outside: it is meant to drip where you will notice it, so a faulty valve gets fixed instead of wasting water unseen.

Many newer toilets have no outside pipe. Their cisterns overflow internally, down through the flush valve and into the pan, which counts as the "no less effective device". If your toilet is modern and still has an outside pipe, it may have been kept from an older installation. Either way the cause of the overflow is the same.

Stop the drip tonight in three steps

None of this needs tools beyond a flat screwdriver, and none of it involves undoing a joint.

• Lift the float gently by hand. If the water stops hissing into the cistern, the valve itself works and the float is simply set too high. Adjust it down, as below, and the drip should stop within a flush or two.
• If lifting the float does not stop the flow, the valve washer or seal is not closing. Turn the isolation valve on the feed pipe a quarter turn with a flat screwdriver, so the slot sits across the pipe. The cistern will stop filling.
• With the valve off, flush when you need to and then open the isolation valve briefly to refill, closing it again once the water is at the right level. It is fiddly, but it stops the waste and the wet wall until the valve is replaced.

If the isolation valve will not turn or starts dripping itself, do not force it. Turn the water off at the main stopcock instead and see how to turn off a stopcock.

Setting the water level correctly

Most cisterns have a water line moulded or printed on the inside. With no mark, aim for the water to stop about 25mm below the top of the overflow tube or the overflow outlet. How you lower it depends on the valve.

On an older side-entry ball valve with a brass arm, the arm is bent slightly downwards so the float closes the valve sooner. With a plastic arm, there is usually an adjusting screw at the valve end, or the float slides along the arm and locks. On a modern bottom-entry fill valve, where the valve stands upright from the cistern floor, the float is a cup or cylinder that slides on the valve body. You release a clip or turn a threaded rod on top to move it down. Fluidmaster and Siamp valves work this way, while a Torbeck uses a small float on a short arm with its own adjuster. Each is set slightly differently, so check the name printed on the valve before guessing.

Lower the level a little, flush and watch the refill. If the water now stops short of the overflow and the outside pipe dries out within an hour, that was the whole fault.

When the valve itself has failed

If the water keeps creeping up with the float fully down, the valve is letting water past. On a ball valve, the rubber washer that the arm presses shut hardens and grooves over time. On a diaphragm valve, the diaphragm splits or scale builds on its seat. On fill valves, a speck of grit or limescale can hold the seal open, which is common after mains work in the street stirs up sediment.

Some valves can be serviced. Torbeck valves are designed so the cap unscrews and the internals come out for cleaning, and many diaphragm ball valves take a replacement diaphragm. Flushing the valve clean with the cap off and the supply briefly opened into a cup can clear grit. But a valve that is years old and scaled up is usually better replaced. A new fill valve typically costs somewhere between £8 and £30 at a plumbers' merchant, depending on whether it is side or bottom entry and the brand.

High mains pressure can also push water past a tired valve. If more than one cistern in the house is overflowing, or the pipes bang when a tap is shut, the pressure may be the real cause. Our guide to a toilet inlet valve leaking covers the valve faults in more detail.

Why you should not leave it dripping

A slow drip from an overflow looks harmless, but water is running through the cistern all day and night. On a water meter you pay for all of it, and a steady overflow can be enough to make the meter move with every tap off, which is one of the checks in a water meter running with no taps on.

Outside, constant water on brickwork stains the wall, washes out mortar and in winter freezes into an icicle that can block the pipe end. A blocked overflow is the dangerous version, because the cistern then has nowhere to go but over its own rim onto the bathroom floor. If you see water coming over the top of the cistern, that is a toilet cistern overflowing and it needs the supply turned off now.

What an engineer does, and typical costs

An engineer confirms which pipe is running, checks the water level and the float, and tests the valve with the float held down. If the valve is at fault they isolate the supply, remove the old valve, fit a new one of the right type and inlet position, set the water level and check that the overflow pipe runs freely to the outside. They also check the isolation valve, because an old one that has not been turned in years often weeps once it is used.

Across the London market, replacing a toilet fill valve usually falls somewhere between £80 and £150 including a standard valve, with more for concealed cisterns behind panels. With us, the visit is a £60 call-out at any hour, and the repair price is agreed with you on site before any work starts. Book a toilet repair if the valve will not close or you cannot reach the cistern.