Rotten Wood Replacement
Rest assured, if you have a leak, there is more than likely a good chance you will find rotten wood where the leak is present. There are many different types and sizes in the thickness of the decking (decking is what the roof is applied to). Older homes are decked with 1x1 decking which is 3/4 of an inch thick. Newer built homes are decked with OSB (oriental strand board), or CDX 4 ply, and it is usually 1/2 of an inch thick.
Homes are usually framed with 2x6, and the fascia general matches. Roof trusses and decking go hand and hand, so again if there is a leak present, then your may have issues that you cant see. There is never a way for any roofer, to estimate how much damage has been done, without inspecting the roof decking, from either the top or bottom, of the decking itself.
It is in your best in best interest to inspect any decking while a roofing repair, or new install in going on, do to the fact that is it is much harder to replace after the fact.
Wood Destroying Organisms or (wdo reports) usually find these issues when your buying a new home. If left untreated a leak can spread to your truss, and fascia bard, and do a lot more damage under roofing system. Decking should be changed over at least 4 feet span. (touching three trusses), to give you adequate strength.