Few sounds ruin a night’s sleep like water sheeting off the top of your tank — usually the moment municipal pressure returns after an outage. With Gauteng’s 2025–26 run of pipe bursts and 54-hour planned shutdowns, tanks are cycling from empty to full far more often, and float valves are wearing out under the workload. The good news: almost every overflow traces back to one of five causes, and the worst-case fix costs under R200 and 20 minutes.
How does a tank float valve work?
A float valve — the “ballcock” in older plumbing language — is your toilet cistern’s mechanism scaled up for a tank. A hollow float rides on the water surface; as the level rises, the float lifts an arm that presses a rubber washer against the valve seat until the inflow stops at your chosen level. Draw water, the level drops, the valve opens again. No electronics, no power — so when it fails, it fails mechanically: something is stuck, worn, cracked or set at the wrong angle. All diagnosable in your driveway.
Why is my tank overflowing? The five usual causes
- A stuck or worn float valve. Grit in the municipal supply jams the mechanism open, or the washer wears until it can’t seal against full pressure.
- The wrong valve size for your pressure. A big-bore valve on strong mains pressure may never seal cleanly and can hammer shut with a bang. Most household inlets want a 20mm or 25mm BSP valve — see JoJo tank fitting sizes explained.
- No proper overflow route. Some installations rely on water spilling from under the lid, soaking the tank base and the nearest wall every time the valve passes.
- A blocked overflow screen. Leaves and roof debris clog the mesh, so water backs up and escapes over the lid instead of the overflow pipe.
- The ballcock arm angle set too high. If the arm only closes the valve at or above the overflow outlet, the tank “overflows” while working exactly as adjusted — the free fix.
| What you see | Most likely cause | The fix |
|---|---|---|
| Overflows only when pressure returns after an outage | Worn washer passing under full pressure | Replace the float valve (R169) |
| Constant trickle from the overflow | Valve not sealing, or float waterlogged | Replace the valve or float |
| Valve seals fine by hand, tank still spills | Arm angle set too high | Adjust the arm — free |
| Water escaping under the lid, overflow pipe dry | Blocked overflow screen | Clean the mesh; fit an overflow kit (R249) |
| Loud bang when the tank finishes filling | Valve bore too big for the pressure | Fit a 20mm or 25mm valve |
How do I find the cause? The 10-minute checklist
Work through these in order.
- Lift the float arm by hand. Raise the arm fully while the tank fills. If water keeps flowing, the washer is finished — replace the valve, done.
- Check the float itself. Unscrew it and shake it. Water sloshing inside means it’s cracked and half-sunk — it can never lift the arm enough to close the valve.
- Check the arm setting. The valve should shut off with the water line a hand’s width below the overflow outlet; any higher and ripple or a pressure surge pushes water out.
- Check the overflow. Pull the screen and look for leaf litter. A blocked overflow makes a perfectly healthy tank spill from the lid.
- Watch one full fill cycle. If the valve creeps closed but never quite seals, suspect the washer — returning mains pressure after an outage is the harshest test it will face.
Honest note: many overflow complaints need nothing from our shop — a leaf in the screen or a two-minute arm adjustment fixes them free. Do the checklist before you buy anything.
How do I replace a float valve? The 20-minute fix
- Close the supply. Shut the isolation valve on the feed line to the tank.
- Don’t drain the tank. The float valve sits near the top, above the water line — work through the lid opening with the tank full.
- Unscrew the old valve. Hold the back nut inside the tank with one spanner while turning the valve body with another. Note the thread — SA fittings are BSP, most inlets 20mm or 25mm.
- Clean the seat. Wipe grit off the inlet hole and check the bulkhead washers — perished ones are an R89 spares kit, not a reason to bin the fitting.
- Fit the new valve. A washer on each side of the tank wall, hand-tight plus a quarter turn — you’re compressing rubber, not torquing a wheel nut.
- Set the level. Adjust the arm so the valve closes a hand’s width below the overflow outlet.
- Open the supply and watch a full fill. It should shut off smoothly — no drips, no hammer.
The 20-minute overflow fix
Where should the overflow water go?
Never straight down the tank wall. An overflowing tank dumps its water in one concentrated spot, usually right against your foundation. Over months that means eroded soil, damp walls and, in clay soils, cracks in plaster. Route the overflow through a screened outlet and a pipe discharging a couple of metres from the house — a flower bed, French drain or stormwater channel.
The screen matters as much as the pipe. An open overflow is a front door for mosquitoes, and light entering the tank feeds algae — if yours has already turned, see green or stale tank water. A 40mm overflow kit with mesh closes that door for R249.
When is an overflowing tank actually good news?
After a long outage — think Johannesburg’s 54-hour planned shutdowns — a brief overflow can simply mean the system worked. Pressure came back, the tank topped itself up while you slept, and the last few litres went out the overflow before the valve seated. That’s a pass, not a failure. The difference is duration: a short spill as the valve closes is fine; a tank running over for hours every time pressure returns is a worn valve. Treat the first post-outage overflow as a free diagnostic and do the hand-lift test.
FAQ
What size float valve does a JoJo tank need?
Most household tank inlets take a 20mm or 25mm BSP float valve. Don’t confuse it with the bottom outlet, which on a standard JoJo is 40mm. Measure the thread on your existing valve before ordering.
Can I just bend the ballcock arm instead of replacing the valve?
If the valve still seals when you lift the arm by hand, yes — adjusting the angle (adjuster screw on plastic arms, gentle bend on brass) is a legitimate free fix. If it passes water with the arm fully raised, no bending will help.
Brass or plastic float valve — which should I buy?
Brass (R169, 25mm) handles pressure surges and outdoor life better — our default for tank inlets. Plastic (R129, 20mm) is fine on gentler low-pressure feeds. In an outage-prone suburb, spend the extra R40.
An overflowing tank is one of the cheapest water problems you’ll ever fix — usually an R169 valve, an R249 overflow kit, or ten minutes of looking. WaterMart launches in August 2026, couriering nationwide at R85 flat (free over R950), prices including VAT. Join the waitlist on the products above and have the fix on the shelf before the next outage.