Nag, nag, nag Wolfie, it's like having two wives!
:-D
But thanks for the pm Wolfie, I thought you would answer this as it's a wood question.........
Well, I've lost my copy of the Bulgarian Building Regs (I lent it to a mate....) but as far as I can see your main problem will be getting the battens 'flat' for interlocking tiles.
Interlocking tiles don't like uneven roofs
As far as the breathable felt is concerned, what does everyone else out there use? It's approved and ok for the UK but who knows what rules they have over there?
I don't understand your statement
"I have been told that it is possible to get hold of a breathable membrane (similar to tyvek i would assume) but it will be difficult to lay it on such an eneven roof, even if i double up the battens." Breathable felt couldn't care less how uneven your roof is, it just lays there (like 'er indoors :-D). It's the battens that don't like an uneven roof.
Possibly the only way would be to find the highest point of the existing rafters and pack out all the battens to suit that high point. A slow and tedious job.
Perhaps that's why the east europeans use the 'half pipe' tiles?
Can you post a picture?