It looks as though it's in a roof, but I'm not certain from the pics. If this
is wrong then disregard the rest of this message.
This fluffy stuff is fairly common in roofs. It is NOT a fungus. From the
photographs it is difficult to tell which, but it is either crystallisation of
salts, possibly from chemicals in old preservation treatments (favourite), or
else surface mould of some kind, although this is normally black in colour.
Both situations are due to surface dampness on the timbers, most likely
condensation. The "fluff" indicates the dampness has dried and it can just be
brushed off and forgotten.
We've been through the causes and cures for roof condensation several times
recently so there's bound to be either a current or archived thread you can look
at. Warm houses and cold roofs this time of year encourage condensation and
it's not really a major problem unless the timbers remain wet for weeks, when
wet rot might become a problem. Dry rot almost never occurs in roofs.
Things to do:
Look for any roof leaks after a rainy day and make sure they are cured
Increase ventilation if possible, both in the roof and in the rooms below
Try to keep vapour production to a minimum.
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