On 30/06/15 12:19, Tim Watts wrote:

If the sarking is old style and not breathable, you'll need a 50mm gap
and enough vents (in soffit and ridge, or near ridge) to ensure a flow
of air on the cold side of the celotex (cold in winter that is).


Oh - and also if you have an impermeable weather layer - eg felt or
metal instead of tiles.