I have seen one property loft and seen plastic sheet covering on top,I cannot remove because it’s been glued and ask vendor and they told me it’s there since they bought it.
From what I can see, the roof looks very old and will be dirty
The lining is probably an attempt to reduce dirt penetration, but might be to catch water leakage, or possibly an ill-advised attempt to reduce heat loss.
Its quite common for people to do something like that on old roofs to try and limit the muck that makes its way through the unfelted roof, and all the old torching that drops off the back of the tiles.
This. You can see the underside of the slates? on the left in the pic.
You do get lots of dust and rubbish dropping on the inside on that type of roof.
Have a careful look at the timbers (and the dust) looking for any signs of water coming in.
And have a good look at the outside of the roof (binoculars or drone survey)- looking for cracked or missing slates, lumps and bumps
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