The short answer
A dripping outside overflow pipe almost always means a valve has stopped shutting off the water somewhere indoors. The usual culprit is a worn or stuck float valve in a toilet cistern, a loft cold water tank, or the heating feed tank. The pipe’s height and material tell you which one to check.
You step outside, glance up at the wall, and there it is: a thin stream of water trickling from a small pipe poking through the brickwork. It might be a steady drip, or it might be running freely down the render. Either way, an overflow pipe doing its job is a sign that something inside the house has stopped behaving.
Overflow pipes are not faults in themselves. They are safety outlets. Every cistern and storage tank in a typical home has a fill valve that is supposed to shut the water off at the right level. If that valve fails to close, the water keeps rising. The overflow pipe carries it safely outside instead of letting it spill across the floor or ceiling. So when you see water coming out, the pipe is working perfectly. The valve behind it is not.
The good news is that the cause is usually a small, inexpensive part. The slightly trickier bit is working out which tank or cistern the water is coming from, because most homes have more than one. Below we walk through how to read the clues, what tends to go wrong, and when it is worth bringing someone in to look.
Why a dripping overflow pipe matters
It is tempting to ignore a slow drip, especially if it is not causing any obvious damage. But there are good reasons to deal with it sooner rather than later.
First, it wastes water. A valve stuck part open can let a surprising volume through over a week, and on a metered supply that lands on your bill. Second, that constant trickle keeps the external wall wet. Over time, damp brickwork can lead to staining, frost damage in cold spells, and in some cases penetrating damp inside. Third, a fill valve that has failed once tends to keep failing, and the next time the water might not find the overflow so neatly. A cistern that overflows internally, or a loft tank that backs up, can cause far more trouble than the dripping pipe itself.
There is also a simpler point. The drip is your early warning. It is telling you a part has worn out before anything worse happens. Acting on it now is the cheap, calm version of the problem.
How to work out which overflow it is
The single most useful clue is where the pipe comes out of the wall. Stand back and look at the height it exits, then think about what sits behind it.
A pipe high up, at roof or eaves level
If the pipe exits near the top of the wall, under the eaves or close to the roofline, it almost certainly runs from a tank in the loft. In most older properties that means either the cold water storage tank, which feeds the taps and toilets, or the smaller feed and expansion tank that tops up the central heating. Both have a float valve, and both can overflow when that valve sticks.
A pipe at first-floor level or behind a bathroom
A pipe coming out lower down, roughly level with an upstairs window or directly behind where a toilet sits, usually points to a toilet cistern. This is the most common cause of all. The cistern’s fill valve has stopped closing, the level has crept up past the internal overflow, and the water is running out through the wall.
A metal pipe, or water from a tundish
If the pipe is copper or metal rather than white plastic, it is more likely linked to a boiler or an unvented hot water cylinder. These systems discharge through a metal pipe for a reason: the water can be hot. With an unvented cylinder, a small amount of discharge while the water is heating can be normal, as the heated water expands. A constant flow, though, is not normal and usually points to a pressure or expansion problem that needs a qualified engineer.
If you are still unsure, there is a low-tech test. Flush each toilet and watch the outside pipe for a minute or two afterwards. Then, with the heating off, check again. By a process of elimination you can usually narrow it down without going near the loft.
Quick signs to check before you call anyone:
- Note the height the pipe exits the wall (high up, mid-level, or low)
- Check whether the pipe is white plastic or copper/metal
- Listen for a toilet that hisses or keeps refilling on its own
- Look for a trickle running into the toilet pan between flushes
- If you can reach the loft safely, see which tank is overfilling
- Note whether the drip is constant or only happens while heating water
What actually goes wrong with the valve
A float valve is a simple thing. A ball or float rides on the surface of the water, and as the level rises it pushes a lever that closes the inlet. When everything is healthy the water stops at the set level and the valve sits closed. A dripping overflow means the valve is no longer reaching that closed position. A few common reasons sit behind that.
The most frequent is a worn washer or seal inside the valve. Rubber perishes with age and hard water deposits build up, so the valve can no longer make a watertight seal even when fully shut. Water keeps seeping past and the level slowly climbs.
Next is a float that has stopped floating. The plastic ball can develop a hairline crack and slowly fill with water. A waterlogged float sits too low to ever close the valve, so the inlet stays open and the tank overfills. The fix is usually just a new float.
A valve can also simply stick, jammed open by limescale or grit, which is common in harder water areas. And in some homes the incoming mains pressure is high enough that an older or worn valve cannot hold against it, letting water creep past even when the float has lifted.
Finally, sometimes nothing has failed at all and the valve is just set too high, so the normal water level already sits at the overflow. Adjusting the float down a little can be enough to stop it.
Fixing it, or knowing when to get it checked
If the source is a toilet cistern, this is often a confident DIY job. Turn off the water at the isolation valve on the supply pipe, lift the cistern lid, and have a look. Sometimes gently bending or adjusting the float arm down resets the level. If the washer has perished or the float is waterlogged, replacement parts are cheap and widely available, and a fill valve can usually be swapped out in under an hour with basic tools.
A loft tank is a step up. The float valve works the same way, but you are dealing with a larger volume of water, awkward access, and the risk of a slip in a dark loft. If you are comfortable and the tank is easy to reach, the same principles apply. If not, there is no shame in leaving it to someone who does it every day.
Anything involving a boiler, an unvented hot water cylinder, or a metal discharge pipe is different. These are sealed, pressurised systems, and a constant discharge can point to a failed expansion vessel or a pressure relief valve that needs attention. This is not a DIY area. It should be looked at by a qualified heating engineer, and unvented cylinders in particular must be worked on by someone suitably certified.
There is also one situation worth flagging. If you have traced the drip but cannot find an obvious overfilling tank or cistern, or if the damp seems worse than a single dripping pipe should explain, the water may not be coming from where you think. A hidden pipe leak inside a wall or under a floor can show up in unexpected places. That is the point where a proper water leak detection survey earns its keep, because it pinpoints the source without tearing the house apart.
When to bring in a professional
As a rule of thumb, a dripping plastic overflow from a toilet or accessible tank is a manageable repair. Call someone in when the pipe is metal, when the discharge is constant from a heating system, when you cannot safely reach the source, or when fixing the obvious valve does not stop the water. And if the moisture appears alongside unexplained damp, rising water bills, or the sound of running water with no tap on, it is worth getting the property checked properly rather than guessing.
We cover homes right across Devon, and a clear diagnosis usually saves money in the long run by avoiding unnecessary work. You can see the areas we work in on our Devon page, or browse more guides like this one in our articles.
Frequently Asked Questions
Usually not an emergency, but it should not be left indefinitely. A slow drip from a plastic overflow can typically wait a few days for a repair. It does waste water and keep the wall wet, so the sooner the valve is sorted, the better. A constant flow, or hot water discharging from a metal pipe, is more pressing and should be looked at promptly.
The pipe’s height is the best clue. A pipe exiting low down or behind a bathroom usually comes from a toilet cistern, while one near the eaves or roofline comes from a loft tank. You can confirm a toilet by listening for hissing or a trickle running into the pan between flushes. If you can reach the loft safely, look for the tank that is overfilling.
With an unvented cylinder, a small amount of discharge while the water heats can be normal, because heated water expands and a little may pass through the relief valve. A steady or constant flow is different and often points to a failed expansion vessel or a pressure issue. That needs a qualified heating engineer rather than a DIY fix.
Often, yes. Turn off the supply at the isolation valve, lift the cistern lid, and check the float and washer. Adjusting the float down, replacing a perished washer, or fitting a new fill valve are all common DIY jobs with inexpensive parts. If the source is a boiler or unvented cylinder, leave it to a professional.
It can over time. A constant trickle keeps the external wall saturated, which may lead to staining, frost damage, or penetrating damp. If you notice internal damp that seems out of proportion to a single dripping pipe, the water may be coming from a hidden leak elsewhere, which is worth investigating properly.
It varies with how far the valve is stuck open, but even a modest drip adds up over days and weeks. On a metered supply this shows on your bill. Because the volume is so variable, the practical answer is simply to fix it once you notice it, rather than leaving it running.
Not sure where the water is coming from?
If the overflow has you stumped or the damp does not add up, we can trace the true source quickly and accurately across Devon. Same-week appointments often available.