On Sat, 22 Jun 2019 19:00:34 +0100, The Natural Philosopher wrote:

Sarking is ther to reduce wind speed and stop tiles being sucked off

Thought it was more as a secondary waterproof layer for when a
tile/slate slips/breaks or the wind is in the "wrong" direction and
blows water up and under the tiles/slates.

Also keeps the atmospheric fall out out of the loft.

--
Cheers
Dave.