Roger wrote:
The message
from Guy Dawson contains these words:

The primary function of the felt is to provide windproofing and help
avoid slates being ripped off in storms etc.
It is not (as many think) there to keep the rain out!

We have a house on the west coast of Scotland and wind is a real issue
there. If the wind can get into the loft space of a house then it will
exit any way it can. That includes blowing out roof lights and tiles.

I was under the impression that it was a long standing tradition in
Scotland to board the roof before slating. That was certainly the case
in the only bit of roofing I have ever had a hand in in Scotland. That
was back in (IIRC) 1961 near Strontian. That is too long ago for me to
actually remember if the roof got felt as well as boards but I don't
recall any.

That is certainly the very best practice in windswept areas.