Why the Stain Is Rarely Under the Leak
Water takes the easiest route down. It runs along the top of a ceiling, follows a joist or a pipe, and comes through wherever the plaster gives first. In a Liversedge terrace where the bathroom was moved upstairs at some point, that can put the stain in the front room and the leak at the back of the house.
So we do not begin by cutting into the stain. We establish which system the water belongs to, then which room, then which length of pipe, and only then open up where the evidence points.