A leaking downpipe has caused a dry rot outbreak. The science of this (and more importantly how to deal with it) is the same as the science of collections care. The pipe is already repaired so the next job is to remove the fungus and affected wood, plaster and deafening. We may need to reinforce the part-digested joists and will certainly need the services of a plasterer once a dehumidifier has dried the area out fully. As usual it’s about controlling relative humidity, in this case reducing it to a point where the dry rot fungus can’t survive.