What the big symptoms mean
A sagging roofline is the one we never ignore — it points to overloaded or water-weakened timbers, and it doesn't improve on its own. Daylight in the loft means the underlay has failed or coverings have moved; if light gets in, so does wind-driven rain. Grit in the gutters is concrete tiles shedding their protective surface — a slow-motion signal that the roof is entering its final decade.
The cheap habits that add years
Clear the gutters twice a year. Glance at the roof from the pavement after every named storm — you're looking for anything that's moved. Keep moss off before it establishes. And get any single slipped tile fixed quickly: one tile costs very little, but the water it lets in quietly destroys felt, battens and plaster for months before you see a stain.