On Tue, 20 May 2008 07:11:46 -0700 (PDT), RobertL wrote:

One simple solution is to use some wooden 4"x2" props with carpenter's
wedges to adjust the height. But I am worried about creating a damp-
conducting path up to the joists even if I stand the base on damp
proofing membrane.

The normal solution would ne to build a sleeper wall under the joists and
pack up to the underside of the joists with a DPC (a bit of slate is
strong in compression). Props will work but you'll have to think of a way
of stopping the wedges working loose over time and possibly some means of
spreading the load on the soil so they don't sink in.

--
Cheers
Dave.