Under Tile Plastic felt
I think at sometime in the past my roof tile were replaced with metal tiles there seems to be a blue plastic felt used but I've noticed that the slightest touch causes the plastic to disintegrate I'm guessing the roof tile will need to come off and a breathable felt used?
It is possible that your roof may have been fitted with one of the earlier types of underlay, possibly manufactured by icopal who also produce what used to be known as Decra Tiles. However, it's really quite difficult to confirm this without seeing an image of the product in situ, or better, by means of any visible printed name on the fabric to assist identification.
Some of the earlier membranes fitted beneath a wide range of metal roofing systems used to be non-permeable, which often results in condensation forming on the underside of the fabric, especially if the ridge itself wasn't vented.
My Metrotile contact has also asked if it is possible to obtain any images of the product which would help to identify what might be going on. Are you able to able to take any photos yourself?
There are a number of possible outcomes but we first need to determine if the entire membrane has decayed or if it's just the backing sheet. Can you see the underside of the metal tiles through the disintegrated plastic? If not, what do you actually see?
I look forward to your reply