Dave Liquorice wrote:
On Tue, 29 Jul 2008 11:44:23 +0100, The Natural Philosopher wrote:

Stops the slates lifting in the gales. That's the primary function.

Tell that to the slates on the barn roof in a gale, they rattle away
nicely and there is sarking under them. Remember we get real gales several
times a year up here and the occasional storm, not just a strong breeze
that most urbanites experience. When there is a gale or stronger blowing
one has to be careful not to step out from the shelter of the house or you
get knocked off your feet...

Rattling away is not the same as being ripped off and landing 50 yards
away..its the massive pressure drop you get in the downwind side of a
roof that creates a huge suction on the slates..the sarking to an extent
bows out and prevents that from becoming a minor explosion.