On Sun, 21 Sep 2008 02:19:39 -0700 (PDT), Adam wrote:

ended up with a roll of bitumen shed felt. Is this a reasonable
alternative?

The important bit is "damp proof" it's to stop moisture wicking up from
the ground via the support structure into the timber causing it to rot.
Traditionally slate would have been used, latterly bitumen based felts or
plastc DPM as used in walls.

Shed felt has stones on the surface and is a bit thin. The stones will
tend to push through the soft bitumen possibly making holes, so it's
probably wise to remove as many as you can (stiff brush should do it).
Then make a pad from a folded strip say 4 or 5 layers thick that is big
enough to fully support the timber over the concrete/brick/slab support.
Make all your pads the same number of layers thick or the levels will go
out as the structure settles. They still might, thats the advantage of
slate it doesn't compress.

--
Cheers
Dave.