The house is old (built 1904) and I don't know what work has been done on the roof or when. I can see some small remnants of what looks like a lath and plaster ceiling, which has since been replaced with plasterboard. In between the joists (and raising to a few inches above them) is yellow fibreglass insulation, and it's on top of this that the debris sits. It looks a bit like broken up plaster, but I can't tell for sure. The underside of the roof itself is covered with felt and there's no damp or any obvious signs of damage.
Can anybody shed any light on what this might be? Obviously, it looks like I'm just going to have to do the dirty job of getting rid of it all but knowing where it's come from would be helpful.
Also, any general tips on roof boarding in an old house would be great. Thanks in advance.