Woland wrote:

Plastic: UV tends to degrade many plastic materials with time so I
wouldn't put a bet on it lasting 20 years... unless it's explicitly
certified to be UV-resistant.

Wood seems to be the longest latsting option unless water gets its
way over, under, around... So that's the material I chose in the end
(also because it's the one you can more easily modify).

On the boat, the epoxy coat deteriorated where covered with varnish, but
where covered with paint it has been fine. Not sure if "resin" of shed
fame behaves in the same way. Perhaps I could paint it if it looked
dodgy after a year or so?

Wood wouuld be ideal, but I have had problems with the deterioration of
the felt roof, even though we used good felt. The big wooden shed here
is laid on a concrete base with a plastic membrane, then expanded
polystyrene (?) sheet, then a ply floor. Then old carpet. This has been
fine and warm. But that is a working rather than storage shed.

My reason for looking at the "resin" shed was that I hoped the supplied
ventilated plastic floor would remove the need for damp proof membreanes
etc. The shed is only for garden tools, power washer etc, and won't have
any need for power or anything. Drainage is OK, and we don't have
standing water or anything.

The outcome of all this ruminating on personal and other people's
observations is that putting the joists (even if pressure treated)
directly on the soil ( levelled, graveleld, etc it's still soil) is
pretty bad. What is always recommended is something along

- level the ground
-put damp proof barrier (thick black polyethylene sheed) on ground (or
below if you lay cement)
-put slabs on barrier
-now you can put joists on, although I would further recommend you lay
joists on bricks to make sure joists are not directly touching the
slabs and getting any stagnating water for long time

Now your shed floor would be seriously safe...

Good luck

In my last house the shed was on bricks with an air space below. We had
some problems with cats and other animals under the shed. They would
pull away the wire netting every few weeks.

Thanks for the thoughts

Bill