Andy Dingley coughed up some electrons that declared:

On 3 Apr, 01:14, Tim S wrote:

why not just remove the sarking and let the roof breath through the
tiling?

Breathing isn't the same as (or the opposite of) windproofing.
Breathing allows a low flow rate with only a small pressure
differential, windproofing prevents the flow rate becoming excessive
even when the pressure difference is high (i.e. the wind is blowing).
A breathable roof still shouldn't be draughty. Although an unsarked
roof would be breathable, it will also be unacceptably draughty.

Ah, but the celotex and taped joints would prevent the wind from entering
the loft/room space, surely. Not discounting what others have said
abou****er blowing in, but this was my original line of thought...

Be careful with ventilation above roof insulation. Condensation in
that insulation layer will cause rot, rot you don't notice, and by the
time you do the repair costs get pretty serious.

Indeed. Given the abysmal lack of effort to ventilate anything there right
now, I'm surprised I haven't more rot than there is... But a roof rafter
would be very difficult to replace.

When I eventually get round to this (it's in phase 2, much later...) I will
put some little probe holes (like a short bit of 8mm pipe glued in) and
plugs in various locations in the ceiling so I can stick an RH probe in for
occasional monitoring.

Cheers

Tim