How Do I Fix a Dripping Overflow Pipe from a Roof-Mounted Tank?

A dripping overflow pipe from a roof-mounted cold water storage tank is one of the more common household plumbing issues in New Zealand, particularly in older homes that still rely on a gravity-fed header tank. In most cases the culprit is a faulty or misadjusted ball valve (also called a ballcock or float valve) that is failing to shut off the water supply once the tank reaches the correct level. The good news is that diagnosing the problem is straightforward, and some of the fixes are well within a confident DIYer's reach — though others are legally restricted to a licensed tradesperson.

Understanding Why the Overflow Drips

Your roof tank is fitted with an overflow pipe as a safety measure. If the water level rises too high — because the ball valve isn't closing properly — water exits through the overflow rather than flooding your ceiling. A constant or intermittent drip from that pipe is the tank telling you something is wrong. Common causes include:

• Worn or corroded ball valve washer — the most frequent cause; the rubber seat wears out over time and no longer seals.
• Float set too high — the arm holding the float may be bent upward, allowing the water level to rise above the shutoff point.
• Cracked or waterlogged float — if the plastic float has cracked, it fills with water and can no longer rise high enough to close the valve.
• Scale or debris jamming the valve — mineral deposits (common in hard-water areas) can prevent the valve from seating correctly.
• Faulty or end-of-life valve — older brass valves eventually fail and need full replacement.

Safety First

Before you do anything, be aware that working on a roof or in a roof space carries real risks. Always use a stable ladder with someone footing it, wear appropriate footwear, and never work on a wet or slippery roof. If the tank is in a confined ceiling space, watch for exposed nails, insulation (which may contain older hazardous materials in pre-1980s homes), and limited ventilation. Review safe working guidance from WorkSafe NZ before heading up.

What You Can Do Yourself

Adjusting or replacing a ball valve in a cold water storage tank is not restricted plumbing work under New Zealand law, provided you are working on the tank itself and not on the mains supply pipework. However, always confirm the scope of any work with a licensed plumber if you are unsure.

Step 1 — Isolate the Water Supply

Locate the mains stopcock (usually under the kitchen sink or at the boundary toby) and turn it off. Alternatively, if there is an isolating valve on the supply line feeding the roof tank, close that instead. Flush a tap downstairs to confirm flow has stopped.

Step 2 — Inspect the Float

Lift the float by hand. If the overflow stops dripping immediately, the float arm simply needs bending slightly downward so the valve closes at a lower water level. Gently bend the arm (on older brass types) or adjust the adjustment screw/clip on modern plastic valves. The water surface should sit roughly 25 mm below the overflow pipe inlet.

If the float feels heavy or you can hear water sloshing inside it, the float has cracked and needs replacing. Floats are inexpensive and available at Bunnings or Mitre 10.

Step 3 — Replace the Washer or Valve

If adjusting the float arm doesn't stop the drip, the valve washer is likely worn. With the water isolated:

1. Unscrew the end cap of the ball valve.
2. Remove the piston or plunger.
3. Prise out the old rubber washer and fit a new one of the same size.
4. Reassemble, restore the water supply, and check the level.

If the valve body is corroded, cracked, or the mechanism is beyond a simple washer swap, replace the entire ball valve. Match the valve size (typically 15 mm or 20 mm) and type. Portsmouth-type and diaphragm (Torbeck-style) valves are both widely available at PlaceMakers.

Step 4 — Check the Overflow Pipe Itself

While you're up there, confirm the overflow pipe is properly directed to discharge safely outside the building — not into the ceiling cavity. It should terminate in a visible location so you can spot future drips easily. This is consistent with good practice under the NZ Building Code clause G12 (Water Supplies).

When to Call a Professional

Call a licensed plumber (registered with the Plumbers, Gasfitters and Drainlayers Board) if:

• The problem involves the mains supply pipework connecting to the tank — any work on pressurised supply pipes is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006 and must be carried out by a licensed tradesperson.
• You discover the tank itself is cracked, corroded, or leaking from the base or sides.
• The overflow pipe is incorrectly routed or discharging into the roof space.
• You're not comfortable working at height or in a confined ceiling space.
• The tank is very old (pre-1980s galvanised steel tanks may contain sediment and should be assessed professionally).

A qualified local roofer can also help if the overflow pipe penetration through the roof cladding is showing signs of leaking or deterioration. You can find licensed plumbers and roofers in your area on tradedirectory.co.nz.

A Note on Healthy Homes and Rental Properties

If this tank serves a rental property, landlords have obligations under the Tenancy Services Healthy Homes standards to maintain the property in a reasonable state of repair. A persistently dripping overflow that causes dampness or water damage should be addressed promptly.

Sources & Further Reading

• MBIE Building Performance — NZ Building Code
• Plumbers, Gasfitters and Drainlayers Board — Licensed Work
• WorkSafe NZ — Working at Height
• Tenancy Services — Healthy Homes Standards
• BRANZ — Building Research
• Bunnings — Plumbing Supplies