Like for like replacement of a mono-roof?

[idontknowwhatiamdoing]

My mono-roof needs new underlay, replacement of rotten sarking and one rotten truss.

I am planning to strip off the whole roof carefully and replace the rotten wood and put a new underlay, batons and the original tiles.

I will augment what the previous builders did with DPC and other protective measures on the roof so it is belt and braces.

My question is, the roof is your typical 70s monoroof, what am I liable for here?

Does it count as a new roof if I strip everything back to the structural parts, or am I still being judged by 1970s standards even if I add modern materials?

Edited March 1, 2021 by idontknowwhatiamdoing