But if the slates are laid conventionally the copper rivets (probably
actually nails) of a lower row of tiles will be covered by the overlap
of the row above and no nails should be exposed hence no leaching of
copper salts down the roof ! If the nails (rivets) are exposed, then
so are the holes they go though, and the roof will leak like a sieve.

AWEM

Further to answer to Meow. Slates have 3 holes, two about 2/3 way up
at the sides which you can nail some copper nails in and one at the
bottom in the middle. With that one a large rivet is inverted and slid
up between the two slates below and the wire part of the rivet enters
the hole from below and its bent down to secure the tile.
Look like this:
http://www.rylandswire.com/discrivets.htm
I dont know what they are talking about with "penultimate row", as
each tile has one.

The slates you refer to with three holes, are fibre cement man made slates,
known as Eternite or Fasnet. The two upper holes are for the fixing nails
and the bottom hole is for the copper rivet. This rivet is there to stop
the slate from curling up in hot weather and wind up lift, as these slates
are not ridged as natural slate. They are noted for attracting the growth of
moss in between the side joints on a low pitched roof. This is caused by the
ingress of dirt holding in the side joint.