A loft tank leaking or overflowing is usually a worn ball valve that will not shut off, letting water spill out of the overflow pipe. A tank dripping from its body instead points to a split seam or perished connector. Either fault can saturate loft insulation and show as a ceiling stain well away from the tank.
A cold water storage tank tucked away in the loft is easy to forget about until water starts coming through a ceiling below it. Many homes across Cornwall and Devon, particularly older cottages and bungalows with traditional vented plumbing systems, still rely on one or two loft tanks to feed the hot water cylinder and the cold taps. When a loft tank is leaking, the cause is nearly always one of a short list of fittings, and knowing which one saves a lot of guesswork before anyone goes near a ladder.
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How a loft tank normally works
A vented cold water system uses one or more tanks in the loft as a reservoir, refilled from the mains through a ball valve, similar in principle to the one inside a toilet cistern but larger. As the water level drops, a float arm drops with it, opening the valve to let more water in. When the tank refills to the set level, the rising float should push the valve fully shut. An overflow pipe, usually run out through an eave or soffit, exists purely as a safety measure in case the valve does not shut off in time, and it is designed to run externally so any failure is visible outside rather than filling the loft.
Two tanks are common in older properties: a main cold water storage tank, and a smaller feed-and-expansion (F&E) tank that tops up a sealed or semi-sealed heating system. Both work on the same ball valve principle, and both can fail in the same ways.
Ball valve failure: the most common cause
Most loft tank problems trace back to the ball valve rather than the tank itself. Over years of use, the washer inside the valve wears, grit or scale from the mains supply lodges in the valve seat, or the float arm develops a stiff pivot and stops rising cleanly. Any of these can stop the valve shutting off fully, so water keeps trickling or pouring in even once the tank is full, and the excess has nowhere to go but out through the overflow.
- Worn washer: the classic cause. The rubber or plastic washer inside the valve hardens or splits with age and stops sealing against the valve seat.
- Grit in the valve seat: sediment from the mains supply pipe lodges in the seat, holding it fractionally open.
- Stuck or bent float arm: corrosion or a knock can stiffen the arm's pivot, so it does not rise as far as it should.
- Waterlogged float ball: an older copper float ball can develop a pinhole and slowly fill with water, so it rides lower than it should and never triggers full shut-off.
A plumber can usually replace a washer or an entire valve assembly quickly and cheaply. The harder part is often just getting safe, informed access to a loft that has not been visited in years.
Split tanks, corroded joints and failed jackets
If the water is coming from the tank body itself rather than the overflow pipe, the cause is different and usually more involved. Older galvanised steel tanks corrode from the inside over decades, and eventually a seam or a pitted patch gives way. Plastic polypropylene tanks, more common from the 1980s onward, rarely split outright, but their connectors, the fittings where pipework passes through the tank wall, can perish or work loose, and joints sealed with old jointing compound can fail as it dries out and cracks.
An insulating tank jacket, common on older systems, can mask a slow leak for a long time by absorbing drips before they reach the loft floor, which is one reason a leak is sometimes well established before anyone notices.
Overflow pipe vs. a genuine tank leak
It helps to separate two different problems that get lumped together as "the loft tank is leaking".
| Symptom | Likely cause | Typical fix |
|---|---|---|
| Dripping from the external overflow pipe | Ball valve not shutting off | Replace washer, valve, or float; check float arm and adjust level |
| Slow drip from the tank body or a joint | Split seam, corroded panel, or perished connector | Reseal joint, patch, or replace the tank |
| Damp patch on the loft floor, no visible drip point | Could be either, or a nearby pipe joint unrelated to the tank | Trace the source before assuming it is the tank |
| Ceiling stain below the loft, tank looks dry | Water has tracked along joists from elsewhere | Professional leak detection to confirm the true origin |
That last row matters more than it looks. Water rarely drips straight down through a ceiling from directly above its source. It tends to run along the top of the plasterboard or along a joist until it finds a gap, so a stain in a bedroom ceiling might trace back to a tank, a pipe joint, or even a roof problem several feet away. Assuming it is the tank because that is what is visible in the loft can mean paying for a repair that does not actually stop the damage.
What to do when you find it
- Check safely first. Only go into the loft if you can do so safely, with good lighting and solid footing across joists, not just insulation.
- Identify the drip point. Is it coming from the overflow pipe, the tank body, a pipe joint nearby, or is there just a damp patch with no clear source?
- Isolate if you can. If there is a dedicated isolation valve on the tank's feed, or you can safely reach the mains stopcock, closing it stops the inflow while you get help.
- Protect the ceiling below. If a ceiling is bulging or actively dripping, a controlled release into a bucket, done carefully, causes far less damage than a ceiling collapsing under trapped water.
- Get the source confirmed, not guessed. Especially where the drip point in the loft is not obvious, or where a ceiling stain has appeared with a tank that looks dry, professional plumbing leak detection finds the actual source non-invasively, rather than opening up plasterboard on a guess.
If your home insurer is involved because of ceiling or loft damage, a documented finding matters. Trace and access cover, according to price-comparison service Confused.com, typically pays to find the leak and make good the damage caused by the search itself, commonly up to around £5,000, though it does not usually cover the cost of repairing the pipe or valve once found. Getting a clear, written diagnosis supports that claim from the outset.
Loft tanks in Cornwall and Devon housing stock
A large share of the older housing across Cornwall and Devon, granite and cob cottages, farmhouse conversions, and post-war bungalows alike, was built with vented plumbing systems that still rely on a loft tank today, even where the boiler or cylinder has since been upgraded. Coastal exposure and damp loft conditions can accelerate corrosion on older galvanised tanks, and holiday lets that stand empty between bookings are especially exposed, since a slow ball valve fault can run unnoticed for weeks with nobody checking the loft. We see a steady run of these calls every summer as holiday cottages are opened up after a quiet spell and owners find a ceiling stain that started long before anyone was there to notice the drip.
Frequently asked questions
Why is my loft water tank overflowing?
Almost always a worn or stuck ball valve that no longer shuts off when the tank is full. The washer wears, the arm sticks, or grit jams the valve seat, so water keeps entering after the tank should have stopped filling, and it exits through the overflow pipe instead.
Is a dripping loft tank overflow pipe serious?
It is worth fixing promptly. A dripping overflow wastes water and often means the ball valve is already failing, which can progress to a fuller, more constant overflow. Left unattended it can also stain external walls or, if the pipe discharges awkwardly, feed water back towards the loft space.
How do I know if my loft tank is actually split, not just overflowing?
An overflow only ever drips from the designated overflow pipe. A split tank or perished joint drips or trickles from the tank body itself, often onto the loft floor away from any pipe, and the water is usually a slow, low-level constant rather than a valve-related surge.
Can a leaking loft tank cause ceiling damage?
Yes. Loft insulation and joists sit directly beneath most cold water tanks, so a leak or overflow saturates the insulation first, then tracks along joists before it shows as a ceiling stain, often some distance from the tank itself, which makes the source hard to guess from the stain alone.
Should I turn off my loft tank if it is leaking?
If you can safely reach the loft, closing the mains stopcock or the tank's own isolation valve (if fitted) stops further inflow while you assess the problem. If the leak is heavy, if the ceiling below is bulging, or if you are not confident accessing the loft safely, treat it as urgent and get help rather than climbing up regardless.
Do old loft tanks need replacing rather than repairing?
Older galvanised steel tanks corrode from the inside and eventually pit or split, at which point patching is a short-term measure at best. Plastic tanks rarely split outright but their fittings and washers still wear. A plumber can usually judge on site whether a repair or a full tank swap is the sensible option.
Why does my loft tank only overflow sometimes?
Intermittent overflowing often points to a ball valve that is borderline, closing most of the time but occasionally sticking, or to mains pressure fluctuations that briefly push the water level above the valve's shut-off point. It is also worth checking after any nearby plumbing work disturbs the supply.
Loft tank leaking and not sure why?
If a ceiling stain has appeared or a loft tank will not stop overflowing, we can confirm whether it is the valve, the tank, or something else entirely, and document it for your insurer if needed, across Cornwall & Devon.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)