Verne, You are a font of knowledge; thanks for replying! The mortar stuff seems to be all over the top of the insulation, and there are bits hanging from the inside of the roof, waiting to drop. So my impression is that the inside of the roof used to be covered in this mortar stuff, presumably to hold the tiles on to the batons. It would appear, as things stand, that the roof is doing ok without this mortar stuff in place, but I don't want to tempt fate. I'm not sure why there is so much of it in that corner; perhaps the same person who rolled back the insulation from that rectangular strip ch