The leak may be some distance away
Rainwater can travel along roofing felt or timber before it shows up inside the loft. The rafter that looks wet may not be directly beneath the broken tile, damaged flashing or failed valley.
Condensation creates a different pattern
A local leak normally produces a local wet area. Condensation is more likely to affect many rafters, nail heads or areas of underlay at once, particularly after cold weather.
Purlins and wall plates deserve special care
These members can carry substantial roof loads. If decay has reduced their section, the repair should be designed around how strong the timber still is rather than simply treating the surface.
Think about bats before treatment
Historic England warns that timber treatments in roof spaces may affect bats. Where bats may be present, appropriate ecological advice may be needed before treatment or disruptive work begins.
Useful technical sources
These links provide background guidance. A particular building still needs its own inspection.