Cold Water Tank in the Loft Leaking or Overflowing
Reviewed by the Devon Leak Detection team. Last updated June 2026
The short answer: A loft cold water tank usually leaks or overflows because the float valve has failed, so water keeps flowing in and out through the overflow pipe. A cracked tank or perished joint can also leak directly. Turn off the supply at the mains stopcock, run the cold taps, and get the source found before the ceiling below is damaged.
If you have spotted a damp patch spreading across an upstairs ceiling, or water trickling steadily from a small pipe poking through an outside wall, the cause is often sitting quietly in the loft. Many older homes in Devon keep a cold water storage tank up there, feeding the bathroom taps and, in a good number of properties, topping up the hot water cylinder. When that tank goes wrong, the water has only one way to go, which is down. The good news is that the most common faults are well understood and, once the source is pinpointed, straightforward to put right.
This guide explains why a loft tank leaks or overflows, what the warning signs look like, and how the source is found and stopped before it turns into an expensive ceiling repair.
How a loft cold water tank is meant to work
A traditional storage tank, sometimes called a header or cistern, fills from the mains through a float valve, also known as a ballcock or ball valve. As the water level rises, a float on the end of an arm is pushed upwards. Once the tank is full, the float lifts the arm far enough to close the valve and stop more water coming in. It is a simple, reliable bit of engineering that has been doing the same job for decades.
As a backup, every tank has an overflow pipe set slightly above the normal water line. If the valve fails to shut off, the rising water reaches this pipe and is carried safely outside, usually emerging through an external wall. That dripping outside pipe is not the problem itself. It is a signal that something inside the tank has stopped working as it should.
Why the tank overflows: a failing float valve
By far the most common reason a loft tank overflows is a fault with the float valve. When the valve cannot close fully, water keeps trickling in, the level rises past the safe point, and the surplus runs out through the overflow. There are a few typical culprits, and they tend to come on gradually rather than all at once.
Common float valve faults
- A worn washer inside the valve, so it can no longer form a watertight seal when closed
- A punctured or waterlogged float that fills with water and sinks instead of rising to shut the valve off
- A stiff or seized valve mechanism, often furred up with limescale or corrosion over the years
- The float arm set too high, letting the level creep above the overflow before the valve closes
In most cases the fix is to replace the washer or fit a new float valve, which are inexpensive parts. Before any of that, the incoming supply needs to be shut off at the mains stopcock and the tank drained down by running the cold taps. If you are not comfortable working in a cramped loft around live plumbing, it is sensible to bring in someone who is.
When the tank itself is the problem
Not every loft leak comes from the valve. Older galvanised steel tanks can corrode and rust through, while plastic tanks can become brittle with age and crack, particularly around the base or where pipes connect. A hairline split may only weep when the tank is full, which makes it easy to miss at first. The connections themselves can also be at fault: a perished joint, a loose compression fitting, or a tired washer on the outlet can let water escape steadily without the overflow ever running.
There is one less obvious cause worth knowing about. If your hot water cylinder is fed from the cold tank and its internal coil fails, water from the heating side can be pushed up the feed pipe and into the cold tank, raising the level and triggering the overflow even though the float valve is perfectly healthy. Telling these scenarios apart matters, because the repair is completely different. This is exactly where pinpointing the true source first saves a lot of wasted effort.
The damage risk to the ceiling below
A loft tank can hold a substantial volume of water, often well over a hundred litres, sitting directly above your bedrooms or landing. A slow leak that goes unnoticed will soak into joists, insulation and plasterboard, leaving brown staining, bulging paint and a musty smell long before anything dramatic happens. Left for weeks, saturated plaster becomes heavy and can eventually give way, and persistent damp encourages mould and rot in the timbers.
It is worth understanding how this sits with your home insurance. Escape of water is one of the most common household claims in the UK, and many policies do cover sudden, accidental water damage. What insurers are far less likely to cover is damage that builds up gradually from a fault that was left unattended, which they may treat as a maintenance issue. Acting quickly therefore protects both your home and any potential claim. Policies vary, so it is always worth checking your own wording with your insurer before you assume what is and is not covered.
Warning signs to watch for
Tell-tale signs of a loft tank problem
- A pipe dripping or running outside, often high up on an external wall
- Damp patches, brown stains or bubbling paint on an upstairs ceiling
- The sound of water trickling or filling that never quite seems to stop
- A musty, damp smell on the landing or in the loft itself
- Soggy loft insulation or water pooling on the boards near the tank
How the source is found and stopped
The first priority with any active overflow or leak is to limit the damage. Turning off the supply at the mains stopcock and opening the cold taps will drain the tank and stop fresh water arriving while the cause is investigated. From there, the aim is to confirm exactly where the water is coming from rather than guessing.
A careful inspection of the tank will usually reveal whether the float valve is failing to seat, whether the float is waterlogged, or whether the body of the tank or one of its joints is weeping. Where the picture is less clear, for example when a ceiling is staining but the tank looks dry, non-invasive leak detection comes into its own. Using moisture meters, thermal imaging and acoustic equipment, the leak can be traced precisely without ripping up boards or cutting open ceilings on a hunch. Once the source is confirmed, the right repair follows, whether that is a new float valve, a fresh joint, or replacing a tank that has reached the end of its life.
You can read more about how we trace hidden leaks on our water leak detection page, and find more practical guides over on our articles section. Wherever you are across Devon, help is close at hand.
When to call for help
A washer or float valve is well within reach of a confident DIYer, but there are clear moments to pick up the phone. If the overflow is pouring rather than dripping, if water is already coming through a ceiling, if you cannot safely reach or work around the tank, or if you have replaced the valve and the problem returns, it is time to bring in a professional. The same applies if you suspect the tank is cracked or that the hot water cylinder coil may be involved, since those repairs are more involved.
Getting a proper diagnosis early almost always works out cheaper than dealing with a collapsed, water-stained ceiling later. If in doubt, it is better to have the source confirmed and stopped properly than to leave a slow leak quietly doing damage above your head.
Frequently Asked Questions
That pipe is almost certainly the overflow from a loft tank or a toilet cistern. It only runs when an internal float valve has failed to shut off, so water rises past the safe level and escapes outside. The dripping is a warning sign that the valve, washer or float needs attention, not the fault itself.
Yes. Turning off the supply at the mains stopcock and running the cold taps will drain the tank and stop more water arriving while you investigate. This is the quickest way to limit damage to ceilings and timbers, and it makes any repair to the float valve much safer and cleaner to carry out.
Often, yes. Replacing a worn washer or fitting a new float valve uses inexpensive parts and is a common DIY job. The catch is working safely in a cramped loft around live plumbing. If you are not confident, or the leak returns after a repair, it is sensible to call someone who can diagnose and fix it properly.
Escape of water is one of the most common UK household claims, and many policies cover sudden, accidental damage. Gradual leaks left unattended may be treated as a maintenance issue and excluded, so acting quickly helps. Cover varies between policies, so always check your own wording with your insurer before assuming what is included.
We use non-invasive methods such as moisture meters, thermal imaging and acoustic equipment to trace the source precisely. That means we can confirm where water is escaping without lifting boards or cutting open ceilings on guesswork, so any repair is targeted and disruption is kept to a minimum.
Water Coming Through the Ceiling? Let's Find the Source
If your loft tank is leaking or overflowing, our friendly Devon team can pinpoint the cause and put it right, with no unnecessary mess. Get in touch for a fast response and a free quote.