What could the white be?

[petrho]

This is in an old outbuilding. There is a bit of white on the inner ceiling. Looks like lime or salt. What could it be? Does it need to be addressed? How?

 

[Åsa Lund]

That is a moisture and rot damage. Is it a cold wall that conducts moisture, the gray you see behind?

 

[petrho]

It is a slag stone wall. Can it be easily repaired by myself? I think it is an old damage. No moisture there now as far as I can see.

 

[Åsa Lund]

Test with a moisture meter to see if there is any moisture left. How difficult it becomes depends on what is on top of the roof.