Where this starts
Every vessel filled by a float valve has an overflow, and it discharges somewhere visible on purpose. The design assumption is that a valve failing in a roof space is invisible, so the water is piped outside where somebody will notice a stain on a wall or a puddle on a path long before a ceiling comes down.
SO THE PIPE ITSELF IS THE CLUE. Trace where it comes from. A small pipe from just below an upstairs window usually serves a WC cistern. A larger one from high on a gable or under the eaves usually serves a cold water storage cistern in the loft. A small one near it may serve the feed-and-expansion tank for an open-vented heating system. Each is a different valve and a different consequence.
WHAT IS ACTUALLY FAILING is nearly always the float-operated valve: a perished washer or diaphragm so it never quite closes, a float that has taken on water and no longer rises, an arm bent or knocked out of position, or grit holding the valve off its seat. A valve that closes correctly but at too high a level is the same symptom from a different cause and is an adjustment rather than a part.
URGENCY DEPENDS ON THE VESSEL. A WC cistern overflowing wastes water and does no structural harm. A loft STORAGE cistern overflowing means a large vessel of water in a roof space is being continuously fed, and if the overflow pipe itself is blocked, undersized or has been removed during an insulation job, the water goes into the loft instead. A feed-and-expansion tank overflowing can also mean something is wrong in the heating circuit rather than in the valve.
TWO THINGS TO CHECK BEFORE ANYTHING ELSE, both free. Whether the discharge is continuous or intermittent — continuous is a valve not closing, intermittent is a level set marginally too high. And whether it began after work was done in the loft, because a knocked float arm is a very common outcome of laying insulation.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A float valve washer or diaphragm that no longer seals
Most commonA continuous trickle or steady run, with the vessel's level sitting at or above the overflow, and the valve audibly still passing water.
How to check: Whether it is continuous. A valve that never quite closes discharges without stopping.
The level set too high
Most commonIntermittent discharge after each refill, with the valve otherwise shutting off cleanly and the level just above the overflow.
How to check: Where the level stops relative to the overflow. A valve working correctly at the wrong setting behaves like a failed one.
A waterlogged or displaced float
CommonA float sitting low in the water or partly full when lifted, or an arm bent or knocked aside, often after work in the loft.
How to check: Lift the float by hand. If that stops the flow, the float or its arm is the problem rather than the valve.
Grit holding the valve off its seat
CommonDischarge that started after work on the water supply or after the water was off, sometimes stopping by itself and returning.
How to check: Whether it is intermittent and recent. A particle in the seat is not a worn valve and replacing the part is not the fix.
Supply pressure higher than the valve can close against
Less commonMore than one vessel overflowing, or a valve that closes at low demand and passes at night when mains pressure rises.
How to check: Whether it is worse at night. Mains pressure commonly rises when demand falls, and a marginal valve passes then.
A heating system fault pushing water into the feed tank
Less commonThe small feed-and-expansion tank overflowing, often with the tank hot, bubbling or its level rising when the heating runs.
How to check: Whether the tank is warm or the level rises with the heating. That is the circuit, not the valve.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
A WC cistern, a loft storage cistern and a feed-and-expansion tank fail for different reasons and carry very different consequences.
- 2
Continuous is a valve that never closes; intermittent after each refill is a level set marginally too high, which is an adjustment rather than a part.
- 3
It separates a failed valve from a failed float or a knocked arm in one movement, and the second is far more common after loft work.
- 4
An overflow that has been blocked, shortened or disconnected during insulation or a conversion turns a visible warning into a loft full of water.
- 5
Laying insulation, running a cable or boarding a loft knocks float arms and displaces lids, and the timing usually identifies the cause outright.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the overflow serves a loft storage cistern and the discharge is heavy, since a large vessel being continuously fed in a roof space is a real risk if anything about the overflow is compromised.
- Where the feed-and-expansion tank is overflowing and the tank is warm or the level rises with the heating, which points at the heating circuit rather than the valve.
- Where more than one vessel is overflowing, indicating supply pressure rather than parts.
- Where there is any sign that water has been entering the loft rather than leaving through the overflow — staining on a ceiling below, damp insulation, a musty smell.
- Where the system is unvented or sealed and a discharge is coming from a pressure relief rather than an overflow, which is a safety device reporting something.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.