All of the exterior parts of it - facias/soffits etc have to come off, also enough tiles to get access, and then the wooden framework underneath (which usually looks like a "ladder" can be assessed and replaced where it`s needed.
I`m not a roofer so some of my terms may be wrong but I`m just telling you what I saw from my own roof being done. The wood on mine was absolutely knackered, it just crumbled in my fingers. Not a particularly cheap repair either so shop around
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