Water running from the overflow pipe even when nobody is using a tap? Here is why the float valve stops shutting, what to check and how it is fixed.
If water keeps running out of the small overflow pipe from your roof tank, the float valve (ball cock) on the inlet is almost always failing to shut. The float rises with the water and closes the valve when the tank is full; a worn washer, grit in the seat, a sticking arm or a waterlogged float lets water keep coming in, and the surplus leaves through the overflow. Close the stop valve on the tank inlet if you can reach it safely, then have the valve adjusted, re-washered or replaced. Our tank cost guide puts a float valve replacement at RM80–RM200 including labour.
An overflowing tank rarely announces itself indoors. The overflow pipe is designed to carry surplus water out of the building, so the first clue is usually outside: a thin pipe high on a side or back wall that drips or runs when nobody is using water. Other clues follow from that.
Our high water bill guide explains the meter test that confirms it: a ball valve that never closes is one of the largest and least visible leaks a home can have, because the water goes exactly where the builder sent it — outside.
Mains water enters the tank through a valve near the top. A float, traditionally a hollow ball on a metal or plastic arm, sits on the water surface. As the tank fills, the float rises and the arm pushes a plunger with a rubber washer against the valve seat, closing the inlet. When you use water, the level drops, the float falls and the valve opens again. That is the whole mechanism: a lever that turns the water level into a closing force.
Newer valves use a compact float that slides on a vertical stem instead of a long arm, which suits smaller tanks and tight lids. The principle is the same. Either way, the valve must close fully below the level of the overflow pipe, so that the overflow only ever carries water when something has gone wrong.
Our roof water tank guide notes that the rules for storage tanks require a float valve on the inlet and a lid with an opening of at least 300 mm over the float valve for maintenance, so the valve can be reached without taking the tank apart.
Most overflow is the float valve, but check before buying parts. These are the usual patterns.
| What you see | Most likely cause | How to check |
|---|---|---|
| Overflow runs constantly, day and night | Float valve not closing | Look into the tank: water above the valve's shut-off level, or visibly passing the valve |
| Overflow runs only after a water cut | Grit in the valve, or a tired valve at high pressure | Watch the overflow once the supply is back to normal; a valve that stops after a while may need cleaning |
| Ceiling wet below the tank, overflow pipe dry | Tank, joint or pipe leak, not overflow | Check the tank body, the outlet joint and the overflow pipe itself for cracks |
| Water on the roof or down a wall, no pipe visible | Overflow pipe missing, broken or discharging onto the roof | Trace the overflow from the tank to its outlet |
| Meter turns, overflow dry, tank level normal | A leak elsewhere in the house | Isolate the tank inlet and repeat the meter test |
Our emergency plumber guide covers what to do first in any plumbing emergency, including where the main stop valve usually is.
A float valve is a small, standard part, and the choice is usually quick once someone has looked at it.
Replacement means isolating the supply, draining the water below the valve, fitting the new valve with fresh seals, then refilling and watching it close at the right level. It is a good moment to check the overflow pipe, the lid and the inside of the tank at the same time.
The overflow is a safety route, not a fault. When a float valve fails, the overflow is what stops the water rising over the rim and into the roof space. That only works if the overflow is clear, big enough, and discharges somewhere visible and harmless.
Water running down the same wall for months leaves stains and can soak into the wall. If that has happened, the valve repair comes first, then the wall can be assessed.
Many older terrace houses have the tank inside the roof space, above a bedroom or the kitchen. Here an overflow fault is more serious: if the overflow pipe is blocked, loose or leaking at its joint, the water ends up on the ceiling boards. Sagging, brown rings or a wet light fitting under the tank are signs to stop the inlet and look at the tank before anything else.
If the tank body or its joints are leaking rather than overflowing, that is a separate job; our tank cleaning guide notes that a drained tank is a good moment to inspect it for cracks, and leak detection is the way to find a leak that is not obvious.
In a strata building you do not usually have your own roof tank. The roof tanks, suction tank and pumps are common property, maintained by the JMB or MC, so an overflowing building tank is reported to management. In a landed home, our roof water tank guide notes that the owner maintains the tank, the float valve, the overflow and the pipes. In a rented house, a float valve worn out by age is normally treated as the landlord’s repair; our landlord and tenant repairs guide explains how that split usually works.
These are the figures published on ClickBina’s own guides. A float valve is a small job; the price moves with access to the tank and whether other parts need attention.
| Item | Indicative cost | Notes |
|---|---|---|
| Float valve replacement | RM80–RM200 | Including labour; from our tank cost guide, which suggests replacement every 3–5 years |
| Site visit | RM150 | Waived if you go ahead with the job |
| Leak detection visit, if the cause is not the valve | RM300–RM800 | Per visit, charged separately from any repair |
WhatsApp us your job details for a quote — we reply within the hour.
💬 WhatsApp for a quoteAccess changes the job more than the part does. A tank on an open stand is simple; a tank deep in a low roof space may need boards laid to work safely, and a tank that has not been opened for years may need its lid, overflow or outlet joint attended to at the same time.
WhatsApp us your job details for a quote: where the tank is (on a stand, in the roof space, on a flat roof), whether the overflow runs all the time or only after a water cut, and whether any ceiling is wet. We reply within the hour. The price we agree is a flat price with no hidden charges, and a site visit is RM150, waived if you go ahead. If the tank is only one part of a wider job, such as ceiling repair, repainting or a bathroom, ClickBina handles whole-house refurbishment and coordinates the trades on one job.
Pricing note: the figures above are indicative Klang Valley ranges from ClickBina’s published guides. The price for your tank is confirmed in writing after the access and the valve have been checked.
WhatsApp us your job details for a quote — we reply within the hour.
💬 WhatsApp for a quoteTell us what you need — we reply within the hour.