>>34404
>OP pic is borked.
Yes it cannot be fixed, since corrupted.
I would replaster and give it a whitewash. Or maybe whitewash first since it has some disinfecting properties. The plaster also should stick since it's the same material (Ca) in it's core. But maybe I'm wrong, never tried.
The place needs constant heating and airing to dry out I think.
>used some sort of tar to stick tiles to the slate floor
It was customary here to tar below the wooden tiles. It prevents shit coming up and rotting from below. If the upper side is kept clean and dry, it's fine for a lifetime and more.
Now we use laminated shit and plastic crap under it. I'm fairly sure a couple of decades and it has to be replaced.