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@Vesalius what is shown in those photos is not dry rot a lot of insect activity which is a good thing not bad. Very unusual to get dry rot in the roof except under valley gutters because despite the name dry rot needs a constantly damp and humid environment to develop. From the photographs the roof looks about 40 -50 years old the timbers are properly sized and there is little distress in them. If you want more info PM me.
One page from the survey demonstrating the dry rot
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