Something in the Rafters!

[anthony1289]

this autumn a quarter of the roof (1930 tiles straight onto rafters, no felt) has developed what I'm hoping is mould, rather than dry rot?
or could it be Salt condensation deposits?
it brushes off fine, not wet, no obvious leaks.
do people think it IS mould?

 

[Notch7]

white spot mould I would say -caused by high humidity