Rotten roof joist how do I remedy it

[MangoPete]

Hi folks,
i have an 1850's house which I am renovating.
The roofer replaced loose slates but did not do the ridge tiles . i found some were loose and want to bed them in but I have found the main roof joist on top of the roof under the ridge has 1/2 an inch of wet rot along it. It is about 8 inches thick but the rot is not good.
DO I cut it out and splice in new wood
treat it with preservative or 5 star wood treatment ?
Treat it then cover it with resin them rebed the ridge tiles.
I assume new mortar will stop the water getting in.
Many thanks
Peter

[AlBags]

1/2" in 8" is not so bad. Is the timber 8x2"?
You could dress off the rot and nail/coachbolt a new piece along to restore it. Probably better spending all the time doing that rather than filling it.
If you really dont want to do that .. expose .. chop off the rot, treat twice, allow to dry, and use strongish mortar to rebed the ridge.