First identify which overflow is actually dripping
An overflow pipe is there to give unwanted water somewhere safe to escape, so the drip is a symptom rather than the original fault. The useful first step is to work out where the pipe comes from. In many homes an overflow may be connected to a toilet cistern, cold-water storage cistern, feed-and-expansion tank or another small storage vessel. Look at the position outside and compare it with the rooms or loft space behind it. A pipe high under the eaves suggests a different source from one leaving a bathroom wall. Do not climb onto roofs or into awkward loft spaces simply to follow it; the outside position alone can often narrow things down.
A cistern that is filling too high is a common cause
If a float-operated inlet valve does not shut off at the correct level, water continues to enter until it reaches the overflow. Older ball valves can stick or develop worn washers, while modern compact fill valves can become scaled or fail internally. Sometimes the float has been adjusted badly rather than the valve itself being worn out. The important clue is whether the dripping continues steadily even when no taps or toilets are being used. If the source is a toilet cistern, you may also hear it filling repeatedly. In a loft tank, the problem can be quieter and may only become obvious from the wet patch outside.
Heating tanks need a little more caution
A small feed-and-expansion cistern serving a traditional heating system can overflow for reasons that are not limited to a faulty ball valve. Hot water may be pushing back into the tank, the water level may be set too high, or another fault may be causing the system to expand abnormally. If the overflow water feels warm or the drip becomes worse when the heating is running, mention that when arranging help. Do not start altering valves or tying up floats in a heating cistern just to stop the overflow. That can hide the warning while leaving the underlying cause in place.
What you can observe without dismantling anything
Note when the drip starts, whether it is constant and whether it changes after flushing a toilet, running hot water or using the heating. If you can safely see the relevant cistern from a boarded loft or normal bathroom position, check whether water is close to the overflow level. You do not need to remove valves or reach across an unboarded loft. A short video of the outside drip and a photograph showing where the pipe exits the wall can be surprisingly useful to a plumber. The pattern often tells them which tank or cistern deserves attention first.
Why it is worth dealing with rather than ignoring
A small overflow may seem harmless because the water is already outside, but it can waste a large amount over time and leave staining on brickwork or render. In cold weather, continuous dripping can also create slippery surfaces or ice below. More importantly, an overflow is telling you that a control inside the system is not doing its job. If a loft cistern is very close to spilling, a blocked or poorly positioned overflow can turn a minor valve fault into internal water damage. Stopping the source is better than accepting the overflow as normal.
What to tell a plumber before they visit
Say where the pipe exits the building, roughly how high it is and whether the water is cold, warm or only appears at certain times. Mention if you have a conventional hot-water cylinder, loft tanks or an older heating system, because that helps identify the likely source. If the drip began after recent plumbing work, include that detail. A plumber may be able to arrive with the right type of fill valve or washer if the arrangement is clear in advance, but they may still need to inspect the actual cistern before confirming the repair.
What the repair should leave you with
Once the fault is repaired, the overflow should become dry during normal use and the cistern should stop at a sensible level without constant topping-up. Ask the plumber to let the system refill fully before finishing, because a valve that appears quiet for the first minute may still creep upwards. If the overflow came from a loft cistern, it is also worth checking that the overflow pipe itself is secure and unobstructed. The repair is not about removing the warning pipe; it is about restoring the control that prevents water reaching it. If staining has developed on render or brickwork, wait until the pipe has stayed dry before deciding whether any cleaning or decoration is needed. A fresh mark immediately after the repair would be useful evidence that the source has not been completely resolved.
This page is general guidance about why is my overflow pipe dripping; the cause in an individual property depends on the actual plumbing arrangement.