Yesterday I was fitting some eaves protector to a small section of roof where I recently noticed water was pooling and leaked into the home when it was heavy. This part of the job was fairly straightforward but I noticed on of the end tiles the “sheet” that holds the mortar was loose and broken. What are these sheets called? I can find reference to undercloaking but this seems to be long sheets and not small, individual tile like sheets. Can anyone advise as I would like to replace this broken one. I have attached a photo for information; the grey sheets are the pieces I refer to. Thanks.