if the timbers are not to badly rotton you might be able to salvage them by using something like probor and drilling the timbers and injecting the gel into the heart of the timber to protect it etc,if there are any signs of hyphe on the plaster/walls then the plaster will more then likly need to come off and the walls treated with a fungcide solution,
we used to do the diamond drilling way,but i do belive the modern version is just to do a perimeter drill and treat,this way it stops it from migrating past the treatment.
the trouble with dry rot is,even if you have stopped the initial moisture once established the rot will create its own invoiroment,so will continue to spread.
i have over the years been invovled with some big dry rot jobs and have seen moisture droplets on the spores etc,
i would suggest to put your mind at rest is to get a preservation surveyor out to inspect the area,but be alert that there not going to take your pants down.
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