An overflow pipe, sometimes known as a warning pipe is attached to the cisterns in your home. Water will only flow through this pipe if the level of water in the cistern exceeds its predetermined level.
Some common components requiring an overflow pipe are:
The overflow will normally exit adjacent to the toilet directly to outside; the pipe can be of various materials including, plastic, copper or lead.
Cold water storage cistern
This is generally found in a loft space or attic, it is a large tank that will supply water to your baths, basins and sinks for cold water, and supply water to your cylinder for hot water to baths, basins and sinks. The overflow will normally exit to outside through eaves and soffits. The pipe can be of various materials including, plastic, copper or lead.
Feed and expansion cistern (F&E or Header tank), this is a smaller tank used to supply cold water to your central heating and allow water to expand safely within the system. The overflow will normally exit to outside through eaves and soffits. The pipe can be of various materials including, plastic, copper or lead.
Dripping overflow pipe
When your overflow pipe is dripping water, it may indicate you have a problem with the float valve. The float valve is usually a plastic ball at the end of a metal arm in the cisterns; it controls the water level of the tank at a predetermined level. If there's dripping water it may need adjusting or a washer/ complete float assembly replaced.
Check the cisterns on a regular basis, and contact a qualified plumber if you are concerned in any way.