Almost every roof we are called to has already been quoted for a full replacement by somebody. Perhaps one in six needs one.
We start with a hose and a person underneath. Water is run on one section at a time, from the bottom of the slope upwards, and the person inside says when it appears. That finds the actual entry point rather than the wettest patch on the ceiling, and the two are often three metres apart.
What it usually turns out to be: fixings backed out with the metal moving in heat, a lap lifted in wind, ridge flashing never sealed, or a gutter so full the water backs under the bottom sheet. All four are a morning's work. A full replacement is three weeks and a different amount of money, and we will only tell you that you need one when you do.