Hello all, need some advice, just moved house, while putting stuff in the attic I noticed that you can see daylight though many of the tiles, the roof is tiled but has no felt, it has a mortar type bedding on the edges, a lot of this mortar has fallen off, I am going to refil the missing mortar, my question is what is this mortar made up from, it looks like a plaster type mixture, can anyone help with this?? your advice would be greatfully recieved..