Shut the mains stopcock, then run the bathroom cold taps to empty the tank. Switch off the boiler and immersion heater. A drip from the overflow outside means the float valve is not closing, a quick repair. Water on the tank body or in the insulation means a split seam, a failed joint or a nearby pipe. Find that today.
A brown ring is spreading on the landing ceiling, or you have heard water trickling above the bedroom for the third night running, or a neighbour has pointed out the pipe dribbling under your eaves. In an older Cornish house with a gravity hot water system, the cold water tank in the loft is the usual suspect, and because nobody goes up there from one year to the next, the fault often has a head start. Most loft tank problems come down to the float valve, the tank and its joints, or frost. Here is how to stop the water, tell those apart, and know what is safe to do yourself.
Stop the water before you diagnose anything
Everything in the tank sits above a plasterboard ceiling, so the first job is to get the water out of the loft through the taps rather than the ceiling.
- Turn off the mains stopcock. It stops the tank refilling. If you cannot find it, our guide to where the stopcock hides in older houses covers the usual places.
- Run the bathroom cold taps and flush the toilets. In a gravity system these draw from the loft tank, so every litre through a tap is a litre that cannot come through the ceiling. Keep going until the flow dies.
- Switch off the boiler and the immersion heater. The hot water cylinder refills from the same tank. Heating a cylinder that can no longer top itself up is a bad idea, so leave the heat off until everything is refilled.
- Protect the room below. Move furniture and anything electrical clear. If a ceiling is bulging, keep everyone out of that room and do not poke it.
Overflow dripping outside: the float valve
The overflow is a pipe fitted near the top of the tank that runs straight through the wall to open air. When water comes out of it, the tank is overfilling and the overflow is doing its job. The fault is the float valve (the arm with a ball on the end) that is supposed to shut the supply off once the tank is full. The usual reasons it fails are a worn washer inside the valve, a punctured float that half fills with water and sits low, a valve seized with age, or an arm set so high that the level reaches the overflow before the valve closes.
Any plumber can replace the washer, the float or the whole valve, and the parts cost very little. In summer a dribbling overflow is a job for this week rather than tonight. In winter it changes character. If the overflow pipe freezes or the outlet blocks, the excess water has nowhere to go except over the rim of the tank and into the loft insulation, and that is how a washer that never got changed turns into a collapsed ceiling.
Water on the tank itself: seams, joints and old metal
If the overflow is dry but the loft is wet, the tank or its connections are letting go. Old galvanised steel tanks rust through, usually at the base or a corner. Plastic tanks go brittle with age and crack around the outlet fittings, and a hairline split may only weep when the tank is full. The joints are just as likely: a perished washer on the outlet, a compression nut that has worked loose, or a tank connector that has never sat quite square.
| What you notice | Most likely cause | What to do |
|---|---|---|
| Steady drip from the overflow pipe outside | Float valve not shutting off | Replace the washer, float or valve this week |
| Damp around the pipe connections | Weeping joint or tired washer | Nip the nut gently, or have the joint remade |
| Water running down the outside of the tank body | Split seam or crack | Drain it and replace the tank |
| Wet insulation but the tank and joints are dry | A pipe run nearby, or the overflow has slipped off | Trace the leak before it stains a ceiling |
| Tank water is warm and the level keeps rising | Failed coil in the hot water cylinder | Have the cylinder checked, not the valve |
A sheet of dry kitchen roll is the best tool you have. Run it along the base seam, each corner and every joint. Damp shows instantly, and it tells you where the water is coming from rather than where it has run to.
The one that fools people: warm water in a cold tank
If the water in the loft tank feels warm and the level keeps creeping up even though the float valve looks fine, the fault may be two floors down. In a vented system the hot water cylinder is fed from the loft tank, and if the coil inside the cylinder fails, heated water can be pushed back up the feed pipe. The tank fills past its normal level, the overflow runs, and a new float valve makes no difference. Our guide to a hot water cylinder dripping or leaking explains what else to look for on the cylinder.
What is safe to touch, and what is not
Lifting the lid, watching the float valve close, feeling the joints with kitchen roll and putting a bucket under a drip are all fine. A few things are not.
- Do not walk on the ceiling. Step only on joists or boards. A foot through wet plasterboard is a far bigger job than the tank.
- Do not touch wet electrics. Loft lighting cables and the immersion heater feed often run near the tank. If water is on or near a cable or a light fitting below, switch that circuit off at the consumer unit and leave it off until it has been checked.
- Do not force a seized float valve or stopcock. Old brass snaps.
The stain on the ceiling below
A slow leak from a loft tank soaks into insulation first, which holds the water against the plasterboard like a sponge. By the time a stain shows on the ceiling, the board above it may have been wet for weeks. Bubbling paint, a soft patch or a sag are signs the board is saturated. Lift the wet insulation away from the ceiling if you can do so safely, and let air get to the board.
One caution from our side of the trade. Water runs along joists and pipe runs before it drops, so the stain and the source are often rooms apart. If the tank has been fixed and the mark keeps growing, or the ceiling is stained but the loft looks dry, the real leak may be a pipe hidden in the floor void. Our article on a ceiling stain when the loft is dry covers that situation, and our water leak detection survey uses moisture mapping, thermal imaging and acoustic listening to find the source without opening the ceiling on a guess.
On insurance: most buildings policies cover sudden escape of water damage, so a ruined ceiling and decorations are usually claimable. The worn-out valve or tank itself is normally treated as wear and tear and is not. Photograph everything before you start mopping, and keep the failed part.
Frost, empty houses and older Cornish lofts
The loft is the coldest space in the house, above the insulation and outside the heated envelope. On higher ground around Bodmin Moor and Launceston a hard frost will freeze an unlagged loft pipe, and the split only shows when the thaw comes. Lag every pipe up there, fit a jacket around the sides and top of the tank, and leave the loft floor directly beneath the tank uninsulated so a little warmth rises from the rooms below. If the house is a holiday let or a second home that stands empty in winter, keep the heating ticking over on a low setting or drain down properly before you leave.
If the tank is old galvanised steel, or a plastic one with more than one repair on it, replacement is routine. If the boiler is due for renewal anyway, converting to a combi or an unvented cylinder removes the loft tank altogether. And if you are not sure what is leaking, we provide leak detection across Cornwall and would rather find it before the ceiling comes down than after.
Frequently asked questions
Why is the pipe outside my house dripping water?
That pipe is almost certainly the overflow from the loft tank or a toilet cistern. It only runs when the float valve inside has failed to shut off, so the water level rises past the safe point and escapes outside. The dripping is the warning, not the fault. The valve, washer or float needs attention.
Is a leaking loft tank an emergency?
A drip from the overflow pipe outside is not, though it should be fixed before winter. Water from the tank body, its joints or soaking into the insulation is urgent, because the whole tank sits above your ceilings. Shut the stopcock, drain the tank through the bathroom taps and get it looked at the same day.
Can I fix a leaking float valve myself?
Often, yes. Replacing the washer or fitting a new float valve uses cheap parts and is a common job. The catch is working in a cramped loft around live plumbing and cables. If you are not confident, or the overflow keeps running after the repair, have a plumber do it and check the level at the same time.
Will home insurance cover damage from a loft tank leak?
Most buildings policies cover damage caused by a sudden escape of water, so a stained or collapsed ceiling is usually claimable. The worn valve or corroded tank itself is generally treated as wear and tear and excluded. Policies vary, so check your wording, photograph the damage before repairs and keep the failed part.
Ceiling stained and the loft looks dry?
If the tank has been checked and water is still finding its way down, we will trace where it is really coming from without opening ceilings on a guess, and give you a report your insurer can work from.
