An overflow pipe dripping outside your house is trying to tell you something. It’s a safety mechanism — it redirects excess water outside rather than flooding your home. The drip itself isn’t the problem; it’s a symptom of an underlying issue. The fix depends entirely on which overflow pipe is dripping.
Identifying Which Overflow Is Dripping
First, work out where the dripping pipe comes from. Follow the pipe back through the wall to find its source:
| Location of Pipe | Likely Source | Common Cause |
|---|---|---|
| Low down, near ground floor bathroom | Toilet cistern | Faulty fill valve or float |
| High up, near roof/loft level | Cold water storage tank | Faulty ball valve |
| Near the boiler (any height) | Boiler pressure relief valve | High system pressure or faulty PRV |
| Above bathroom window | Header/expansion tank | Faulty ball valve or system issue |
Fix 1: Toilet Cistern Overflow
This is the most common overflow issue in UK homes. The toilet cistern overfills because the fill valve (ball valve or fill valve assembly) isn’t shutting off properly.
Step-by-Step Fix
- Lift the cistern lid and check the water level. It should be about 25mm (1 inch) below the overflow pipe inside the cistern
- Check the float — if you have an older ball-cock valve with a float arm and ball, check whether the ball has water inside it (cracked float) or whether the float arm needs adjusting. Bend the arm slightly downward to lower the water level
- Modern fill valves — most modern toilets use a compact fill valve with an adjustable float. Turn the adjustment screw to lower the shut-off point. Usually a quarter-turn is enough
- Check the valve seal — if adjusting the float doesn’t stop the filling, the valve’s internal washer or diaphragm is worn. Turn off the water supply, remove the valve top, and replace the washer. Replacement washers cost about 50p from any hardware shop
- If all else fails — replace the entire fill valve. Modern fill valve assemblies cost £5–£10 and install in 10 minutes. The B&Q toilet repair guide covers this step by step
Fix 2: Cold Water Tank Overflow (Loft)
If the overflow from your loft tank is dripping, the ball valve in the tank isn’t shutting off:
- Go up to the loft and check the water level in the cold water storage tank. The water should be about 50mm below the overflow pipe
- Lift the float arm — if the water stops running, the float height needs adjusting. For a traditional ball-cock, bend the arm downward slightly. For a Part 2 valve with an adjustable float, use the adjustment mechanism
- Check the float — unscrew the float ball and shake it. If there’s water inside, it’s cracked and isn’t floating properly. Replace it (a new float ball costs about £3)
- Replace the washer — turn off the mains water supply, open a cold tap downstairs to drain the pipe, then remove the valve cap and replace the internal washer. Over time, these washers harden and no longer seal properly
If the tank continues to overfill after replacing the washer, the valve seat may be scored or corroded. At this point, replacing the entire ball valve is easier and more reliable than trying to re-seat an old valve. A new Part 2 ball valve costs about £10–£15.
Fix 3: Boiler Overflow (Pressure Relief Valve)
A dripping boiler overflow pipe is more serious and shouldn’t be ignored. The pressure relief valve (PRV) releases water when the system pressure exceeds a safe level (usually 3 bar).
Common Causes
- System pressure too high — check the boiler pressure gauge. Normal operating pressure is 1.0–1.5 bar when cold. If it’s above 2.5 bar, you need to bleed some pressure off
- Faulty expansion vessel — the expansion vessel absorbs the pressure increase when water heats up. If it loses its air charge, the pressure spikes every time the heating runs. This needs professional attention
- Faulty PRV — the valve itself might be stuck partially open. A qualified gas engineer should replace this
- Filling loop left open — someone filled the system and forgot to close the filling loop. Check both valves on the loop are fully closed
What You Can Safely Do
- Check the pressure gauge — if above 2 bar (cold), bleed a radiator to release some water and bring the pressure down to about 1.2 bar
- Check the filling loop is fully closed
- If the pressure keeps rising when the heating runs, call a qualified Gas Safe registered engineer — the expansion vessel likely needs recharging or replacing
Important: Never attempt to repair or replace a boiler’s pressure relief valve yourself unless you are Gas Safe registered. This is both a legal requirement and a safety issue. The Gas Safe Register lets you verify whether an engineer is properly registered.
When to Call a Plumber
- Boiler-related overflows — always get a Gas Safe engineer for boiler issues
- The overflow keeps dripping after you’ve replaced the valve washer and adjusted the float
- You’re not comfortable working in the loft or with water supplies
- There’s significant water damage or the overflow has been dripping for a long time (check for rot or damp around the exit point on the outside wall)
Preventing Future Overflow Issues
- Check your boiler pressure monthly — a 30-second glance at the gauge catches problems early
- Listen for your toilet cistern refilling when nobody has flushed — this indicates a leaking flapper valve or fill valve issue
- If you have a loft tank, check it annually when you’re up there for other maintenance
- Keep the filling loop valves closed after topping up your heating system
