Water Tank Overflowing — What Causes It and How to Fix It

A water tank overflows for one of four reasons: the float valve isn’t shutting off, the supply pressure is too high for the valve, the overflow pipe can’t get rid of water fast enough, or — on pump-filled tanks — the pump doesn’t know when to stop. Each has a different fix, and fixing the wrong one wastes time and money. The checks below take about 10 minutes and tell you which one you’re dealing with.

Don’t leave it. A tank overflowing at 10 litres a minute for 8 hours loses 4,800 litres — roughly a week of normal use for a family of four. On a rooftop tank, that water also soaks the slab below, breaks down waterproofing and shows up later as ceiling stains and mould.

Quick diagnosis: match your symptom

What you see	Most likely cause	Fix
Float is sitting low in the water or half-sunk	Float ball has cracked and filled with water	Replace the float (or the whole valve)
Float is on the surface but water still trickles in	Worn washer or grit/scale on the valve seat	Clean the seat and replace the washer, or replace the valve
Water level settles above the overflow pipe inlet	Float arm or adjuster set too high	Lower the shut-off level below the overflow
Overflows mostly at night or early morning	Supply pressure rises when demand drops and forces past the valve	Fit a pressure-reducing valve, or a valve rated for your pressure
Water spills over the rim or lid, not out of the overflow pipe	Overflow pipe blocked, kinked or too small	Clear it; replace with a larger pipe if needed
Tank fills and keeps going until the pump is switched off	Failed float switch or level controller	Test and replace the float switch or controller
Rainwater tank overflows in heavy rain	Normal — or overflow too small for the roof area	Check overflow size and where it discharges

How a tank’s fill and overflow system is meant to work

Every overflow problem makes more sense once you know the four levels inside the tank, from top to bottom:

1. Rim or lid — the physical top of the tank. Water should never reach it.
2. Inlet — where the supply water comes out of the float valve. It should sit above the overflow level, so that tank water can never be drawn back into the supply pipe.
3. Overflow level — the bottom edge of the overflow pipe. If water reaches it, it drains away safely instead of spilling.
4. Shut-off level — the water level at which the float valve closes. This must sit below the overflow level, commonly by at least 25 mm (1 inch).

In a healthy tank, the float rises with the water and closes the valve at the shut-off level. The overflow pipe should stay dry. Any water coming out of the overflow pipe means something is wrong — the overflow is a safety relief, not part of normal operation.

The 10-minute diagnosis

Do these checks in order. Most people find the cause at step 1 or 2.

Step 1 — Look at the float

Open the tank lid and look at the float while the tank is full. A healthy float sits on the surface like a cork. If it’s riding low, tilted or partly under water, it has cracked and filled up — it can no longer lift the arm far enough to close the valve. Lift it out and shake it: if you hear water inside, that’s your problem.

Step 2 — Test whether the valve seals

Push the float down gently so water flows in, then lift it by hand to the top of its travel. The flow should stop completely. If water keeps trickling or hissing through with the float held up, the washer or the valve seat is worn, or there’s grit or limescale holding it open. This is the most common cause of a tank that slowly creeps up to the overflow.

Step 3 — Check where the water level stops

If the valve seals when you lift the float, let the tank fill normally and watch where the water stops. It should stop at least 25 mm below the bottom of the overflow pipe. If it reaches the overflow first, the shut-off level is simply set too high — an adjustment, not a repair.

Step 4 — Check the overflow pipe

Find where the overflow pipe discharges outside. With the inlet closed, pour a bucket of water into the tank end of the overflow pipe. It should run out the other end straight away and freely. If it backs up, trickles or doesn’t appear, the pipe is blocked — leaves, insect nests, bird nests and crushed or kinked sections are the usual culprits. If water ever came over the rim rather than out of the overflow, suspect this first.

Step 5 — Check the supply pressure

If the valve seals when tested during the day but the tank still overflows overnight, pressure is the likely cause. Screw an inexpensive pressure gauge onto an outside tap on the same supply and read it late in the evening. Then compare that reading with the pressure rating on the float valve body or its packaging. Float valves are made for different pressure ranges — a valve designed for low pressure fitted to a high-pressure mains supply often can’t close fully, or wears out quickly.

The causes in detail — and how to fix each one

1. A failed or worn float valve

This is the cause in most overflowing tanks. The float valve has three weak points:

• The float cracks with age and sunlight and fills with water. Replace it — floats are cheap and usually unscrew from the arm.
• The washer and seat wear, harden or collect grit and limescale. Close the isolating valve, take the valve apart, clean the seat and fit a new washer. In hard-water areas this is the part that fails first.
• The arm or linkage gets bent, stiff or blocked by the tank lid, so the float can’t rise far enough to close the valve.

If the valve is old, badly scaled or the body is cracked, replace the whole valve rather than individual parts. A new float valve typically costs less than a plumber’s call-out, and fitting one is a job most people can do in under half an hour with the water isolated. When buying, check two things: the connection size matches your inlet pipe, and the pressure rating covers your supply pressure.

2. Shut-off level set too high

If the valve seals properly but closes above the overflow level, adjust it down. Most modern valves have an adjusting screw or a sliding float on a vertical rod; older ball valves are adjusted by gently bending the metal arm or moving the float along it. Lower it until the water stops at least 25 mm below the overflow pipe.

Only do this when the valve is actually sealing. Lowering the float to hide a leaking valve just moves the problem — the tank holds less water and will still creep up to the overflow as the washer wears further.

3. Supply pressure too high for the valve

Water pressure in the mains usually rises overnight, when fewer people are using water. A float valve that just about seals during the day can be forced open at night. The classic sign is a tank that’s fine during the day but has overflowed by morning.

There are two proper fixes:

• Fit a float valve rated for your pressure. If your valve is a low-pressure type on a high-pressure supply, replacing it with the right type often solves the problem on its own.
• Fit a pressure-reducing valve (PRV) on the supply line to the tank, set within the float valve’s rated range. This also protects the rest of the pipework. If your mains pressure is very high, it’s worth doing regardless.

Replacing a leaking valve with an identical one won’t fix a pressure problem — it will just fail again.

4. A blocked, undersized or badly placed overflow pipe

The overflow pipe is your last line of defence. If the float valve fails and the overflow can’t cope, water goes over the rim — onto the roof, into the loft or down the walls. Three things go wrong:

• Blocked: clear it and fit a mesh screen or flap on the outlet to keep insects, birds and leaves out.
• Too small: the overflow must be able to carry away more water than the inlet can deliver. As a rule, it should be larger than the inlet pipe — at least one size up (for example, 32 mm overflow for a 25 mm inlet).
• Discharging onto the roof: route it to a drain, gutter downpipe or somewhere visible at ground level. An overflow that discharges where you’ll see it is also an early warning that the float valve is failing.

5. Pump-filled tanks: the pump doesn’t stop

Many rooftop tanks are filled by an electric pump from an underground tank or a low-pressure supply, rather than directly by mains pressure. These tanks usually have no float valve at all. Instead, the pump is switched off by a float switch or an automatic water level controller with sensors in the tank. If the tank overflows, it’s almost always because:

• the float switch is stuck, tangled on its cable or has failed;
• the controller’s sensors are coated in scale or dirt, or a wire has corroded;
• there’s no automatic control at all, and someone forgot to switch the pump off.

Test the switch by lifting it by hand while the pump runs — the pump should stop. If it doesn’t, replace the switch or have the controller checked by an electrician. If the tank is filled manually, an automatic level controller is inexpensive and prevents both overflow and the pump running dry.

6. Rainwater tanks: overflow is expected — spills are not

A rainwater tank has no float valve — the roof fills it, and during heavy rain it will overflow once it’s full. That’s normal. What isn’t normal is water spilling over the lid or around the inlet. That means the overflow is smaller than the downpipe feeding the tank, or it’s blocked by leaves. The overflow should be at least as large as the total inlet from the roof, and should discharge to a stormwater drain or soakaway away from the foundations.

If the tank overflows after almost every rain, it’s small for your roof. Use the roof catchment area calculator to see how much water your roof actually collects, and the rainwater harvesting calculator to size a tank that captures more of it.

Mistakes that make overflow worse

• Bending the float arm down to hide a leaking valve. The tank stops overflowing for a while but holds less water — a 1,000-litre tank set to shut off at 80% is now an 800-litre tank — and the worn washer keeps getting worse.
• Treating a running overflow as normal. A dribbling overflow pipe on a mains- or pump-filled tank is a fault, not a sign the tank is full. Small continuous leaks waste a surprising amount of water over a year.
• Replacing a valve with the same type when pressure is the problem. Check the pressure first, then buy a valve rated for it or fit a PRV.
• Never checking the overflow pipe. A blocked overflow does nothing until the day the float valve fails — and then the water goes over the rim. Check it once a year when you clean or inspect the tank.
• Letting overflow run onto a flat roof. Constant wetting breaks down waterproofing and leads to leaks, damp and corroded reinforcement in concrete slabs. On a rooftop tank, also make sure the roof can carry the tank when it’s full — see whether your roof can hold a full water tank.

When to call a plumber

• There’s no isolating valve and you’d need to shut off the whole property to work on the tank.
• Your supply pressure is very high and a pressure-reducing valve needs fitting.
• The overflow pipe needs rerouting or enlarging through a wall or roof.
• The tank is hard to reach safely, such as on a steep or fragile roof.
• Water has already damaged a ceiling or slab — have the structure checked as well as the tank.

Related calculators

• Rooftop load bearing calculator — check that a full rooftop tank is within what your roof can carry.
• Tank refill time calculator — see how fast your tank should fill, which helps judge whether the overflow can keep up with the inlet.
• Water pressure calculator — for gravity-fed systems, works out the pressure your tank delivers to taps below it. For mains pressure, use a gauge on an outside tap.
• Roof catchment area calculator — for rainwater tanks that overflow in every storm.

Having the opposite problem? If your tank fills too slowly instead, see why a water tank takes so long to refill. For pressure units, see water pressure explained: PSI, bar and kPa.

Frequently asked questions

Why is my tank overflowing when it doesn’t look full?

Water only comes out of the overflow pipe once it reaches the overflow level, so the tank is at its designed “full” level even if there seems to be space above. That’s normal — the gap between the overflow and the rim is meant to stay empty. The real question is why the water rose past the shut-off level. Test the float valve as in steps 1–3 above. If the valve is working, check whether the overflow pipe was fitted lower than the shut-off level; if so, lower the shut-off level or raise the overflow.

How do I stop my water tank overflowing at night?

Overnight overflow is usually a pressure problem: mains pressure rises when demand drops, and it pushes past a valve that just about seals during the day. Measure the pressure late in the evening with a gauge on an outside tap. Then either fit a float valve rated for that pressure, or fit a pressure-reducing valve on the tank supply. A timer-controlled valve that closes the inlet overnight also works, but it treats the symptom rather than the cause.

How far below the overflow should the water stop?

Set the float valve so the water stops at least 25 mm (about 1 inch) below the bottom of the overflow pipe. Many installers leave more. The point where water comes out of the float valve should sit above the overflow, so tank water can never be drawn back into the supply pipe.

What size should a water tank overflow pipe be?

Larger than the inlet — at least one pipe size up, so it can carry away water faster than it can arrive. For a 25 mm inlet, use at least 32 mm. On rainwater tanks, the overflow should be at least as large as the downpipes feeding the tank. If you’re not sure, go bigger: an oversized overflow costs little, an undersized one lets water over the rim.

How long does a float valve last?

It depends mostly on water quality and pressure. In soft water at moderate pressure, a good valve can run for many years with just an occasional washer change. In hard water, limescale on the seat can stop a valve sealing within a few years, and high pressure wears washers faster. Brass valves generally outlast plastic ones. If an old valve is leaking and scaled up, replacing the whole valve is usually quicker and more reliable than repairing it.

Can an overflowing tank damage my roof?

Yes. Water discharged onto a flat roof keeps the slab wet, breaks down the waterproofing and, over months, leads to damp patches, peeling plaster and corrosion of the steel in concrete slabs. Route the overflow to a drain or downpipe, fix the cause of the overflow, and have any stained or cracked slab looked at.

My pump-filled rooftop tank overflows — what should I check first?

The float switch or level controller. Lift the float switch by hand while the pump is running — the pump should stop within a second or two. If it doesn’t, the switch or its wiring has failed. With electronic controllers, check that the sensors in the tank are clean and the wires aren’t corroded. Until it’s fixed, switch the pump off manually once the tank is full.