Flying Freehold: Does anyone have any experience of purchasing

There is a default in ITalian law for both access and maintenance, but the access should be written into your act of sale.  As regards maintenance, it may be that the roof (for example) is 50/50, but it doesnt mean your neighbour will fork out the money if needed.    However, its a very common situation in Italy.   Obviously if you have a water leak and it damages the downstairs property they will want you to pay for the damage, so insurance is always a good idea.