Your roof tank is overflowing and wasting water — the float valve
Before you call a plumber and pay a call-out fee: most 'my tank keeps overflowing' cases are solved by adjusting or cleaning the float valve — not replacing the tank or re-plumbing anything. The whole problem is one small mechanical valve, no electricity and no high pressure, and you can set it by hand. Work through these steps in order. If they don't fix it, you'll at least know it's the pump or a bigger leak — and that is a technician's job.
The problem
You notice water constantly running from a thin pipe on the side of the tank (the overflow pipe), or a puddle and damp patch on the roof or down a wall, or the sound of water running inside the tank even when no tap is open. Your water bill creeps up, and at night you hear a steady trickle. Sometimes the tank simply fills and never stops.
Likely causes
A stuck or worn float valve that won't shut off
This is the heart of it. The float is a plastic or foam ball on an arm; as the water level rises, the ball lifts and closes the valve. If the valve is worn inside, or the arm is corroded and seized, the valve stays open, water keeps flowing in, and it spills out of the overflow pipe. This is by far the most common cause.
The float arm is set too high
The valve is fine, but it's set wrong. If the float arm sits too high, the water has to reach a level above the overflow pipe before the ball lifts far enough to shut the valve — so it spills. The fix isn't a new part, it's bending the arm down a little so it shuts earlier. Plenty of tanks overflow for this reason alone.
Debris or limescale jamming the valve
Our mains water is hard and often carries grit and scale. Over time it builds up inside the valve and stops the washer seating properly, so it keeps dribbling even when the float is fully up. A single trapped grain of sand is enough to keep a tank overflowing all day. A clean-out brings it back to life.
A perished valve washer or diaphragm
Inside the valve is a rubber washer or diaphragm that seals off the water path. Over years, with scale and age, the rubber hardens and splits and no longer seals fully. It's a very cheap part, and replacing it stops the drip for good. If you've cleaned the valve and it still weeps, the washer is usually the culprit.
A pump float switch that won't cut off (pumped systems)
If your tank is fed by a pump from below, there's an electric float switch meant to cut the pump when the tank is full. If that switch fails, the pump keeps pushing water and the tank overflows. This is an electrical fault, not the mechanical float valve — don't open it yourself; that's an electrician's job.
Fix it — step by step
- Confirm it's the overflow pipe, running from the float valveGo up to the roof and trace where the water is coming from. If it's running from a thin pipe high on the side of the tank (the overflow) and the tank is full, the valve isn't shutting off. Open the tank lid and look: is water still coming in through the feed pipe with the ball floating? That's your fault.Photograph the water level and the float position with your phone before you touch anything, so you can reset it correctly if needed.
- Shut the water off to the tankFind the feed isolation valve that lets water into the tank (usually on the feed pipe before it reaches the float) and close it. If there's no dedicated valve, shut the house main stopcock. Now you can work on the float valve with the water off.If the tank is filled by a pump, isolate the pump at the breaker too before you start — so it can't switch on suddenly.
- Adjust the float arm downwardThe first and simplest try: lower the shut-off level. If the arm is brass or metal, gently bend it down a little by hand. If it has a plastic adjustment clip or screw, move the clip down slightly. The aim is for the ball to close the valve with the level 5-10 cm below the overflow pipe.Lower it a little and test, not all at once. Drop it too far and the tank won't fill properly.
- Open the valve and clean out the gritIf adjusting the arm isn't enough, make sure the water is off, then unscrew the head of the float valve with an adjustable spanner. Out come a small piston/washer and a seal. Clean everything of sand and scale, and wipe the valve seat with a cloth. Look for any trapped grain of grit and remove it.Hold the washer and small parts in your palm, not on the roof floor — they're very easy to lose between the tiles.
- Replace the washer/diaphragm, or the whole valve if perishedInspect the rubber washer: if it's hardened, cracked or swollen, fit a new one of the same size. If the valve body itself is corroded or the thread is damaged, the cleanest fix is to replace the entire float valve — it's a cheap part sold at any plumbing shop.Take the old valve to the shop with you so you get exactly the same size and thread.
- Refit, turn the water on, and test the shut-off levelRefit the valve and tighten it (firm, not overtightened, so you don't crack the thread). Open the feed valve and restore the pump if you have one. Watch the tank fill: the valve should close fully, the water should stop, and the level should sit below the overflow pipe. Check there's no dribble once it's shut.Wait through a full fill cycle and watch. If it stops cleanly with no spill and no drip, you're done — and you've saved a plumber's fee.
Tools & parts you'll need
When to call a professional
Call a plumber or an electrician if: the tank is pump-fed and the pump never cuts off (the fault is the electric float switch or the wiring — don't open it yourself); there's a crack or a leak from the tank body itself; you need to replace or re-route the feed pipe or the overflow pipe; or the main stopcock is seized and won't close. The float valve itself is safe to adjust by hand, but anything electrical, a cracked tank, or re-plumbing pipework is a professional's job.
Need a pro? We'll point you to one.
Osta takes no commission and books nothing — we give you a technician's number and you call them directly.
Osta is not a broker or a marketplace. We take no commission on any work you agree with a technician, and we do not warrant their work — the agreement is directly between you and them.
Not sure what's broken? Take a photo.
Snap a photo with your phone — Osta identifies the appliance, reads the model plate, and tells you what's wrong and how to fix it. Free, no sign-up.
Try photo diagnosisPeople also ask
Why does my roof tank keep overflowing from that thin pipe?
That's the overflow pipe, and its job is to carry excess water safely away so the tank doesn't spill over the top. If water runs from it constantly, the float valve is no longer shutting off — either the arm is set too high, or the valve has debris in it or a perished washer. Adjusting or cleaning the float stops it.
Can I adjust the float valve myself without a plumber?
Yes — it's the easiest plumbing repair in the house. The float valve is a simple mechanical valve, no electricity and no high pressure. Most cases are solved by bending the arm down a little or clearing grit and scale from the valve. All you need is a way to shut the water off and an adjustable spanner.
I adjusted the arm and it still drips — what now?
There's usually a grain of grit or scale trapped, or the rubber washer has hardened. Shut the water off, unscrew the valve head, clean it thoroughly, and replace the washer or diaphragm — it's a very cheap part. If the valve body itself is corroded, replace the whole thing. After that it will seal fully.
How often should I check the roof tank float?
A quick check every 6 months is enough: open the lid and confirm the valve shuts fully and the level sits below the overflow. In hard-water, scaly areas like Amman and the Gulf, deposits build up faster, so an annual clean of the valve will save you from a surprise overflow.
My tank is pump-fed and the pump won't stop — same problem?
No, that's a different fault. Pumped tanks have an electric float switch that cuts the pump when the tank is full. If the pump never stops, that switch has usually failed — it's not the mechanical float valve. Don't open the pump wiring yourself; isolate it at the breaker and call an electrician.
How much water does an overflowing tank actually waste?
More than you'd think. A tank that overflows constantly can waste hundreds of litres a day — meaning a higher bill and, over time, damp damage to the roof and walls. Fixing the float valve costs next to nothing and stops the waste immediately, which is why it's worth sorting the moment you notice it.