Once its wet, everything is ruined. And it traps even smallest moisture up against the back of the tin. Works great until it doesn't :BangHead:
And it meant reworking the facia trim. I've seen works. An unnamed historic house and museum had a whole additional layer of lath added because the architect didn't want the slate attached to the original. I was working as a sub so didn;t have to deal with the trim issues or talk to the architect!