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Repair or replace: how to decide
This is the decision that costs the most to get wrong in either direction: replacing a roof that had a decade left, or patching one that has been finished for three winters.
Signs a roof is genuinely at the end
Slipped slates appearing in different places after every gale. That is nail fatigue, and it means the fixings across the whole roof are going at roughly the same rate. Each repair is a deposit on the same problem.
Coverings that fail when handled: slate that delaminates, concrete tile that crumbles at the nib, clay that has spalled through frost. If a roofer cannot re-hang a tile because there is nothing left to hang it on, that is a whole-roof answer.
Battens so perished that fixings do not hold, or a deck that has moved enough to put a wave in the ridge line. Both mean the work is under the covering, and getting under it is most of the cost of a re-roof anyway.
Signs it is a repair, whatever you have been told
One leak, in one place, in a roof that is otherwise behaving. A stain that only appears in driving rain from one direction, which usually points at a flashing or a verge rather than the field of the roof. Damp that is worst in the coldest weeks, which is usually condensation and not a leak at all.
A handful of slipped tiles in one area, especially near a chimney or a valley where somebody has walked. That is somebody else's footprint, not the end of the roof.
The arithmetic that actually decides it
Put the repair price against the replacement price, then ask honestly how many years the repair buys. A repair at a fifth of the cost that buys five years is good value. The same repair that buys one winter, for the third year running, is not.
The other half is disruption. If the roof will need scaffolding again within two years, the scaffold cost gets paid twice, and that alone often closes the gap between the two options.
How to get an honest answer
Ask for photographs of the actual defect, from the roof, not a description. Ask what the roofer expects to find under the covering and what they would do if they are wrong. Ask them to price both options where both are viable.
A roofer who will only quote the big job, or who cannot show you what they are pointing at, is not answering the question you asked.
Questions
Does insurance ever cover it?
Storm damage, sometimes. Wear and age, no. We tell you which we think you have before you make a claim.
Can I do one slope now and the rest later?
Often yes, and on a large or complicated roof it is sometimes the sensible plan. It costs a little more overall because of the scaffolding, and we will show you that difference rather than hide it.