Ample, you ought to see the Tyvec flapping about under our slate
roofs. Once you re-new the covering and fit a sarking layer then you
need to worry about ventilation.

Excellent, I'd hoped so. I seem to recall some make of felt that is
breathable that can be used instead of the standard stuff. Am I imagining
it, or does the stuff exist, and does it mean that when I replace the roof,
I don't need to install ventilation? It is an Edwardian property and I'd
rather have a totally invisible installation.

With what you propose I'd be more worried about driven rain/snow
getting in and running down the back of the celotex. Where is that
water going to go?

I'll think about it when I install it. In any case, the water have just
landed on the lath and plaster below, so whatever I decide will probably be
an improvement. A bit of guttering sending it to an overflow will probably
do the trick.

Christian.