Do this first
- Establish where the water is going. Into the pan, or onto the floor? That single question decides whether this is a bill problem or a building problem.
- Do the dye test. Put a few drops of food colouring into the cistern, do not flush, and come back in twenty minutes. Colour in the pan means the cistern is leaking through into it continuously.
- Lay white paper around the base and behind the pan and leave it while you do the dye test. That catches the floor-side faults at the same time.
- Isolate the cistern if water is reaching the floor: quarter turn on the small valve on the feed pipe, or the stopcock if there is no valve. Then flush to empty it.
- Read the water meter with everything else off, wait thirty minutes and read again. A passing WC is the most common reason a meter turns overnight.
- Take the cistern lid off carefully and look. Water trickling over the top of the overflow tube inside the cistern is a fill valve that is not shutting; water disappearing with the level still up is a flush valve seal.
Into the pan, or onto the floor
Everything about the urgency depends on this. Water running into the pan costs money and does no damage. Water reaching the floor is what ends up on somebody's ceiling.
| Symptom | Cause | Consequence |
|---|---|---|
| Faint trickle down the back of the pan between flushes, dye test positive | Flush valve seal or a worn flush washer | Continuous water loss on the meter |
| Cistern never stops filling, water spilling into the internal overflow | Fill valve not shutting off, or the float set too high | Continuous loss, and on older suites an external overflow dripping outside |
| Clear water on the floor at the back, present with nothing running | Supply connection: isolation valve, flexible tail, or cistern inlet | Floor damage, continuous |
| Water on the floor only just after a flush | Pan connector, or the cistern-to-pan seal on a close-coupled suite | Floor damage, intermittent |
| Discoloured water with an odour at the floor line | Waste seal or a cracked pan | Contamination. Stop using it. |
The silent one is the expensive one. A flush valve passing into the pan can waste a startling amount of water without ever making a sound you would notice, and on a metered supply it turns up as a bill you cannot explain rather than as a leak you can see. The dye test finds it in twenty minutes and costs nothing. If your meter is turning and you have not found anything else, start here: a water meter turning with everything off is very often a WC.
If the water is at floor level rather than inside the pan, a toilet leaking at the base goes into those faults specifically, including how to tell clean water from waste.
What the repair involves
Internal cistern faults are parts: a new fill valve, a new flush valve or diaphragm, and the seal between cistern and pan on a close-coupled suite, which means lifting the cistern off to get at it. On older or discontinued suites, matching the part matters more than the labour, and on a concealed cistern behind a wall or in furniture, access is the job. Supply-side faults are a new isolation valve or flexible tail and a remade joint, tested under pressure.
Where water has reached the floor, the floor comes into scope: lifting the vinyl or tiles, drying and replacing failed boarding, re-laying and resealing, and where it has gone through to a ceiling below, cutting back, drying, reboarding, skimming and redecorating. That is the full scope of bathroom and shower leak repair, and if the bill is the reason you are reading this, our water bills hub covers what a leak costs per day and what evidence an allowance claim needs.
When it is not us
- The dye test is positive and the suite is a standard modern one. A fill or flush valve is an inexpensive part and a common homeowner job. Isolate, swap, done.
- The overflow pipe outside is dripping. Same fix: the fill valve is not shutting. Worth doing promptly, because it is running to waste continuously.
- Condensation on the cistern in cold weather. Not a leak. Ventilation and warmth.
- Waste backing up out of the pan. A blocked branch or stack, which is drainage, and in a block the stack is usually communal.
- A cracked pan or a cracked cistern. Sanitaryware replacement rather than repair.
- You rent. Report it in writing. Sanitation installations are the landlord's, and a continuously running WC on a metered supply is costing whoever pays the bill.