The supporting timbers on this outbuilding roof are probably “under-sized”. This has caused pronounced sagging and deflection.
The clay tiles themselves have been coated with a bitumen based substance to stop them slipping, stick them together and prevent rainwater leaks.
The sealant restricts ventilation of the roof timbers, can cause condensation and result in rot occuring. It also means that when re-reoofing is undertaken the best of the original tiles cannot be re-used, significantly adding to the cost of the work.
This roof is near the end of its useful life and will need renewing soon.