On 02/02/2004 The Natural Philosopher a wrote :
Don't boher. it is only there to stop pressure build up blowing slates/tiles
off...

That may be one reason, but I think another reason was so that roofs
could be made lighter/cheaper, with less overlap needed if there was a
layer of felt underneath.

--

Regards,
Harry (M1BYT) (Lap)
http://www.ukradioamateur.org