The Natural Philosopher wrote:

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.

Guy
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Guy Dawson I.T. Manager Crossflight Ltd