There is a lot said about roofing felt - we don't like felt for the simple reason that if the roof leaks, having felt in the way means that water is displaced from the leak, and you often don't know about the leak until too late. If you ARE going to use a felt, it must be breathable. The reason for this is simple - hot air rises, and more importantly, water vapour rises, being the lighter of Oxygen, Nitrogen and Water in the air. So it ends up in the roof. If the air in the roof space is warm, and contains water - when the roof gets cold in winter, water condenses on the underside of the roof. Now, if you have breathable roofing felt moisture finds it hard to condense on the felt, and gets through and wicks away. If you have that old bitumen felting - the black stuff - water condenses and runs down it - soaking the battens and rafters - and in bad cases, rotting them. So - if you are told by an architect or Building Regulations that you must have roofing felt - make sure it is a breathable roof membrane such as Monarfil, or similar - it looks a bit like linen cloth - usually blue or sometimes grey in colour.
ANY roof should be well ventilated - if you see white stains on the rafters, it could well be condensation. If you have the black tarry stuff, watch it carefully - sometimes you will see water droplets on it -which are not good at all - if you have those, your roof badly needs ventilating.
